116 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015
RESIDUES AND TRACE ELEMENTS
Modified Screen-Printed Ion Selective Electrodes for Potentiometric Determination of Sodium Dodecylsulfate in Different Samples Tamer Awad Ali
Egyptian Petroleum Research Institute (EPRI), 11727, Cairo, Egypt
Gehad G. Mohamed
Cairo University, Chemistry Department, Faculty of Science, 12613, Giza, Egypt
Fabrication and general performance characteristics of novel screen-printed sensors for potentiometric determination of sodium dodecylsulfate (SDS) are described. The sensors are based on the use of ionassociation complexes of SDS with cetylpyridinium chloride (electrode I) and cetyltrimethylammonium bromide (electrode II) as exchange sites in a screen-printed electrode matrix. Electrodes (I) and (II) show fast, stable, and near-Nernstian response for the mono-charge anion of SDS over the concentration range of 1 × 10–2 – 5.8 × 10–7 and 1 × 10–2 – 6.3 × 10–7 mol/L at 25°C and the pH range of 2.0–9.0 and 2.0–8.0 with anionic slope of 57.32 ± 0.81 and 56.58 ± 0.65 mV/decade, respectively. Electrodes (I) and (II) have lower LODs of 5.8 × 10−7 and 6.3 × 10−7 mol/L and response times of about 8 and 13 s, respectively. Shelf life of 5 months for both electrodes is adequate. Selectivity coefficients of SDS related to a number of interfering cations, and some inorganic compounds were investigated. There were negligible interferences caused by most of the investigated species. The direct determination of 0.10–13.50 mg of SDS by electrodes (I) and (II) shows average recoveries of 99.96 and 99.85%, and mean RSDs of 0.83 and 1.04%, respectively. In the present investigation, both electrodes were used successfully as end point indicators for determination of SDS in pure pharmaceutical preparations and real spiked water samples. The results obtained using the proposed sensors to determine SDS in solution compared favorably with those obtained by the standard addition method.
S
odium dodecylsulfate (SDS) is a highly effective surfactant used in a variety of tasks requiring the removal of oily materials and residues. It is a component in industrial products, including engine degreasers, floor cleaners, and car wash soaps (1). It is used in lower concentrations in toothpastes, shampoos, and shaving foams. It is an important component in bubble bath formulations for its thickening effect and its ability to create lather. In medicine, SDS (Figure 1) is used rectally as a laxative in Received October 16, 2012. Accepted by AK March 12, 2013. Corresponding author’s e-mail: [email protected] DOI: 10.5740/jaoacint.12-388
enemas and as an excipient on some dissolvable aspirins and other fiber therapy caplets (2). Wide application of surfactants may result in the pollution of surface waters. Their determination—as intermediates, in final formulations, and in the environment—has been the focus of many analytical approaches, including titrimetry (3, 4), spectrophotometry (5, 6), spectrofluorimetry (7, 8), and chromatography (9, 10), as well as electrochemical methods incorporating ion-selective electrodes (ISEs; 11–13). However, some of these methods, such as spectrofluorimetry and chromatography, need expensive equipment and special pretreatments. Ionic surfactants have usually been determined by a two-phase titration method (14, 15). The main disadvantages of this method are the limited application to strongly colored and turbid samples, the toxicity of organic chlorinated solvent used, the formation of emulsion during titration which can disturb visual end point detection, and the numerous matrix interferences, etc. Most of these limitations can be overcome by using ISEs as indicators in potentiometric surfactant titration. All surfactant titrations are based on antagonist reaction, whereby an ionic surfactant reacts with an oppositely charged ion, usually of another surfactant, to form a water-insoluble salt (ion-pair). The first such electrodes used were made of coated wire and based on ion-association complexes of quaternary ammonium cation and surfactant anion (16–18). The screenprinting technique may be one of the most promising approaches in producing simple, rapid, and inexpensive biosensors (19–21). The biosensors based on screen-printed electrodes have been extensively used for detection of biomolecules, pesticides, antigens, and anions (22, 23). Electrochemical biosensors based on screen-printed electrodes (SPEs) meet the requirements of in situ screening devices in that all the equipment needed for electrochemical analysis is portable. SPEs have all the major performance characteristics of biosensors: sample preparation is minimal; they are quick, cheap, and easy to use; and they are small and can be miniaturized with new technology (24–26). In this study, modified SPEs were fabricated for rapid and sensitive quantification of SDS in both pure pharmaceutical preparations and water samples. The new fabricated SPEs, as potentiometric SDS sensors using home-made printing carbon ink, are developed based on the use of ion-pair compounds of SDS with cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB) as the electroactive substances, and dibutylphthalate (DBP) as plasticizer. This study investigated the general performance
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 117 University, using a PerkinElmer 2400 CHN Vario Elemental Analyzer (Santa Clara, CA). Preparation of SDS-Modified Screen-Printed Electrodes Figure 1. Structure of sodium dodecylsulfate.
characteristics of the fabricated electrodes using the respective ion-pair compounds, Dodecylsulfate-cetypyridinium (DS-CP, electrode I) and Dodecylsulfate-cetyltrimethyl-ammonium (DS-CTA, electrode II). It was found that both electrodes can be successfully applied for determination of pure SDS in pharmaceutical preparations and spiked water samples. Experimental Reagents All reagents were of analytical grade and bidistilled water was used throughout the experiments. Tricresylphosphate (TCP) from Alfa Aesar (Ward Hill, MA) was used for preparation of the sensors. Other types of plasticizers, namely dioctylphthalate (DOP), DBP, o-nitrophenyloctylether (o-NPOE), and dioctyl sebacate (DOS), were purchased from Sigma-Aldrich (St. Louis, MO), Merck (Whitehouse Station, NJ), Fluka (St. Louis, MO), and Merck, respectively. Relatively high MW polyvinyl chloride (PVC; Sigma-Aldrich) and graphite powder (synthetic 1–2 µm; Sigma-Aldrich) was used for the fabrication of different electrodes. CPC, C21H38NCl, 339.99 g/mol; CTAB, C19H42BrN, 364.5 g/mol; and SDS, C12H25SO4Na, 288.38 g/mol, were purchased from Sigma-Aldrich, Fluka, and BDH (Radnor, PA), respectively. Pharmaceutical formulations containing surfactants, such as Leaders Care solution (Leader’s Pharm Co., for Multipharma; each 120 mL contains 4 g SDS), was used. Anionic surfactants in wastewater sample (Aga area, Dakahlia, Egypt) and seawater sample (Marsa Matrouh, Egypt, seawater 1; and Baltim area, Kafr El Sheikh, Egypt, seawater 2) from the Mediterranean Sea were collected. Formation water sample were also collected (Karama, Al-Wahhat Al-Bahhriyah, Qarun Petroleum Co., Egypt). Apparatus Laboratory potential measurements were performed using a HANNA pH meter, Model 211 (Hanna Instruments, Județul Cluj, Romania). Silver-silver chloride double-junction reference electrode, 6.0726.100 (Metrohm USA, Riverview, FL) in conjugation with different ion selective electrodes, was used. Commercial surfactant electrodes, Cationic Surfactant Electrode, 6.0507.120, (Metrohm USA) were used as a second sensing electrode in comparing the results for determination of cationic surfactants. A digital burette (Cole-Parmer, Vernon Hills, IL) was used for the measurement of surfactants under investigation. Microanalysis for carbon, hydrogen, nitrogen, and sulfur were carried out at the Microanalytical Centers, Cairo
Modified SPEs were printed in arrays of six couples consisting of the working and the reference electrodes (each 5 × 35 mm) following the procedures previously described (24–28). A polyvinyl chloride flexible sheet (0.2 mm) was used as a substrate, not affected by the curing temperature or the ink solvent and easily cut by scissors. A pseudo silver-silver chloride electrode was printed using self-made PVC ink that contains silver-silver chloride (65 + 35%) and is cured 30 min at 60°C. The working electrodes were then fabricated. They were printed using self-made carbon ink prepared by mixing 2.5–19 mg DS-CP or DS-CTA ion pairs, 450 mg DBP, 1.25 g PVC 8%, 0.75 g carbon powder, and were cured 30 min at 50°C. A layer of insulator was then placed onto the printed electrodes, leaving a defined rectangular-shaped (5 × 5 mm) working area and a similar area (for the electrical contact) on the other side. Fabricated electrodes were stored at 4°C and used directly in the potentiometric measurements. Preparation of Ion Exchangers The ion exchangers of SDS with DS-CTA and DS-CP –2 were prepared by slow addition of 15 mL of 10 mol/L SDS –2 to CTAB or CPC solutions (10 mol/L). The mixtures were stirred 10 min, the precipitates were filtered off through No. 42 Whatman filter paper, washed with double-distilled water, dried at room temperature, and finely ground to fine powder (28, 29). Calibration of Modified SPEs The modified SPEs were calibrated by immersion, together with a reference electrode, into a 25 mL beaker containing 2.0 mL of acetate buffer solution at pH 3. Then, a 5 mL aliquot of SDS solution was added with continuous stirring to give final SDS concentration ranging from 1 × 10–2 to 1 × 10–7 mol/L, and the potential was recorded after stabilization to ±0.4 mV. A calibration graph was then fabricated by plotting the recorded potentials as a function of –log [DS–]. The resulting graph was used for subsequent determination of unknown SDS concentration using the same procedure (25, 28). Preparation of Surfactant Solutions Stock solutions (1 × 10–2 mol/L) of SDS, CTAB, and CPC were prepared by dissolving 0.228, 0.3645, and 0.3399 g in doubledistilled water (1 L) with continuous stirring, respectively. Lower concentrations were prepared by accurately diluting from the stock solution. The adsorption of surfactant on the inner surface of vessels was eliminated, as previously reported (25, 27, 30). Determination of Potentiometric Selectivity Coefficient Potentiometric selectivity coefficient (KSDS,jpot) was evaluated according to International Union of Pure and Applied Chemistry (IUPAC) guidelines using the separate solutions method (SSM;
118 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 (a)
Determination of SDS in water samples.––In a 10 mL beaker, a 3 mL aliquot of water sample was transferred, definite concentration of SDS was added, and a 2.0 mL citrate buffer of pH 3.0 solution was added. SDS was quantified in the water samples by using potentiometric titration with CPC and CTAB and SPEs and commercial surfactant electrodes, in addition to the two-phase titration method (14, 15, 25, 27, 34). Potentiometric Titration
(b)
For the potentiometric titration method, an aliquot of the sample solution containing 0.100–13.5 mg SDS was titrated –2 against 10 mol/L CTAB solution. The equivalence points were estimated from the first derivative of the S-shape titration curves (25, 27). Results and Discussion
Figure 2. Effect of IP content on calibration of modified SPEs using (a) electrode (I), (b) electrode (II).
27, 28, 31, 32), in which the potential of the cell comprising the electrodes (I) and (II) modified SPEs and a reference electrode is measured with two separate solutions, one containing the SDS of the activity aSDS (but no j) and the other containing the interfering ion j of the same activity aSDS = aj (but no SDS). Different interfering anions and cations of a concentration of –3 1 × 10 mol/L at pH 3.0 are used, and the results are obtained by using the following equation: pot
log K SDS, j
= ((E1-E2)/S)
+ (1+ (Z1/Z2))log a)
pot
where KSDS,j is the potentiometric selectivity coefficient, E1 is −3 the potential measured in 1 × 10 mol/L SDS, E2 is the potential −3 measured in 1 × 10 mol/L of the interfering compound (j), Z1 and Z2 are the charges of the SDS and interfering species j, respectively, and S is slope of the electrode calibration plot. Sample Analysis Potentiometric titration of SDS in pure pharmaceutical samples.—Bidistilled water was added to a known volume of Leader’s Care solution to make up 25 mol in a calibrated volumetric flask. A 1 mL aliquot of the dilute solution was transferred to a 10 mL beaker containing a 2 mL citrate buffer of pH 3.0. SDS was quantified in the pharmaceutical preparations by using potentiometric titration with CPC and CTAB standard solutions (25, 27) and screen-printed electrodes. The results obtained were compared with those previously reported using the standard addition method (25, 27, 33).
The sparingly soluble ion-pairs of DS-CP and DS-CTA, which formed instantaneously upon the addition of SDS solution to CPC and CTAB solutions, were used for fabrication of modified SDS ion-selective electrodes. Using elemental analysis, the composition of the ion pairs formed is 1:1 ion pair (found (calculated): C = 67.35% (67.76%), H = 12.47% (12.20%), N = 2.56% (2.55%), and S = 5.72% (5.82%) for DS-CTA (C31H67NSO4) ion pair; and C = 69.15% (69.60%), H = 11.53% (11.07%), N = 2.34% (2.46%), and S = 5.64% (5.62%) for DS-CP (C33H63NO4S) ion pair. Optimization of Electrode Performance Under Batch Condition The modified electrodes were prepared and optimized by testing the nature and content of the modifier, effect of soaking time, type of plasticizer, pH, temperature, response time, durability of sensors, and application. Effect of Ion-Pair Content Nine electrodes were prepared. The amounts of carbon powder and DBP were held constant in each electrode. The proportions of modifier in these nine electrodes were 2.5, 5.0, 10.0, 12.5, 14.0, 15.0, 16.0, 17.0, and 19.0 mg of DS-CP or DS-CTA ion pairs while the other constituents were unchanged. The Nernstian slopes for the resulting calibration graphs and correlation coefficients were 49.81 (0.985), 50.70 (0.989), 53.53 (0.990), 55.90 (0.995), 56.40 (0.997), 57.32 (0.999), 56.95 (0.998), 50.45 (0.979), 46.95 (0.970), and 44.38 (0.983), 45.88 (0.985), 49.24 (0.987), 52.23 (0.989), 53.87 (0.990), 54.35 (0.992), 55.00 (0.995), 56.58 (0.998),and 55.90 (0.996) mV/decade for 2.5, 5.0, 10.0, 12.5, 14.0, 15.0, 16.0, 17.0, and 19.0 mg of ion pairs for electrode I and electrode II, respectively. Figure 2 shows that –7 –2 the linear ranges of the electrodes are 5.8 × 10 – 1.0 × 10 and –7 –2 6.3 × 10 – 1.0 × 10 mol/L for electrodes I and II, respectively. These results show that electrodes (I) and (II), which contain 15.0 and 17.0 mg ion pairs, have a higher Nernstian slope and a wide range of linearity. The 15.0 and 17.0 mg ion pair was chosen as the optimum amount for the fabrication of the SDS
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 119 (a)
Table 1. Response characteristics of SDS-modified screen-printed electrode Parameter Slope, mV/decade
Electrode I
Electrode II
57.32 ± 0.81
56.58 ± 0.65
0.999
0.998
Correlation coefficient, r Lower detection limit, mol/L
5.8×10
Response time, s Working pH range Usable range, mol/L Intercept, mV
RSD% a
6.3×10−7
8
13
2-9
2-8
5.8 ×10−7 – 1.0 ×10–2 6.3 ×10–7 – 1.0 ×10–2 243.19 ± 0.27
231.74 ± 0.92
5
4–5
0.83
1.04
Life time, months
(b)
−7
a
Average of four determinations.
solutions (35, 36). The SPEs should be kept dry in an opaque closed vessel and stored in a refrigerator while not in use. Influence of Plasticizer Composition
Figure 3. Effect of plasticizer type on calibration of modified SPEs using (a) electrode (I) and (b) electrode (II).
electrodes. A non-curve response was observed with electrodes containing lower or higher ratios of the modifier. Effect of Soaking Time Freshly prepared SPEs must be soaked in DS-CP and DS-CTA ion pairs suspended solution to activate the surface of the screenprinted layer and form an infinitesimally thin gel layer at which ion exchange occurs. How much time this preconditioning process requires depends on diffusion and equilibration at the electrode-test solution interface; if equilibrium can be established quickly, then a fast response is possible (35). Thus, the performance characteristics of the DS ion-selective SPE was investigated as a function of soaking time. For this purpose, the SPEs [electrode (I) and (II)] were soaked in aqueous suspension of DS-CP and DS-CTA ion pairs, and the titration curves were plotted, from which the total potential changes were recorded after 0, 5, 10, 15, 20, 25, 30, 45, 60, 120, 180, and 360 min. The optimum soaking time was 15 min, when the slope of the calibration curves was 57.32 and 56.58 mV/decade at 25°C, for electrodes (I) and (II), respectively. It is recommended that soaking time be no more than 10 and 15 min for electrodes (I) and (II), respectively, to avoid leaching, although very little of the electroactive species does leach into the bathing
The effect of plasticizer composition was studied on the characteristic performances of electrodes (I) and (II). SPE sensors were prepared using different plasticizers, namely, DBP, DOS, DOP, o-NPOE, and TCP. Sensors incorporated paste plasticized with DBP, TCP, and DOP showed less response [slope 57.32, 55.85, 54.65, and 56.58, 56.12, 51.98 mV/decade for electrodes (I) and (II), respectively] for SDS. This response probably resulted from poor solubility of ion associate in the plasticizer. In the case of DOS [slope 44.95 and 40.24 mV/decade for electrodes (I) and (II), respectively], a negligible or very poor response was obtained. The solubility of the complex was examined by its ability to form transparent solution in the plasticizer. In the case of o-NPOE plasticizer (slope 57.96 and 57.84 mV/decade for electrode (I) and (II), respectively), a clear transparent solution was formed. Whereas, in the case of TCP, DOP, and DOS plasticizers, turbid and suspended solutions were obtained, which indicates that the complex solubility increases in the order of o-NPOE > DBP > TCP > DOP >DOS (Figure 3). o-NPOE may improve the SPE selectivity due to its high dielectric constant (ε = 24). It affects considerable dissolution of ion-association within the SPE; consequently enhances its partition coefficient in the SPE and also provides suitable mechanical property of the SPE compared with less permittivity plasticizers. Performance of Sensors The potentiometric performance characteristics of the anionic surfactant sensors based on use of DS-CP and DS-CTA ion pairs as electroactive materials and DBP as a plasticizer in SPE were evaluated according to IUPAC recommendations (37, 38), and the results are given in Table 1. In citrate buffer of pH 3.0, the sensors –2 –7 displayed linear and stable response for 1 × 10 – 5.8 × 10 and –2 –7 1 × 10 – 6.3 × 10 mol/L SDS at 25 °C with anionic slope of −7 57.32 ± 0.81 and 56.58 ± 0.65 mV/decade, and LODs of 5.8 × 10
120 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 on the number of times used (39). After 6 months, the electrode response deteriorated, which may be attributed to the aging of the SPE matrixes, ion pair, as well as plasticizer (40). Repeatability/Reproducibility
Figure 4. Dynamic response time of (a) electrode (I) and (b) electrode (II).
and 6.3 × 10−7 mol/L, and response times of about 8 and 13 s for electrodes (I) and (II), respectively. Response Time The static response time of the electrodes was measured after successive immersion of the electrodes in a series of SDS solutions, in each of which the SDS concentration increased −7 −3 10-fold, from 1.0 × 10 to 1.0 × 10 mol/L. The static response −3 time thus obtained was less than 8 and 13 s for 1.0 × 10 mol/L SDS using electrodes (I) and (II), respectively. At lower concentrations, however, the response time was a little longer and reached 10 and 14 s for electrodes (I) and (II), respectively. The actual potential versus time traces is shown in Figure 4. The potentials remained constant for approximately 3 min, after which a very slow change within the resolution of the meter was recorded. The sensing behavior of the SPE did not depend on whether the potentials were recorded from low to high concentrations, or vice versa. The electrodes were found to have fast response for 45 days without any significant change observed in the working concentration range, slope, or response time (25, 27). Lifetime of Electrodes The electrodes were used for 4–5 months without any significant effect on the paste potential. The lifetime of the electrodes was determined by reading the potential values of the calibration solutions and plotting the calibration curves for the 5 month period. The slopes of the electrodes (57.32 ± 0.81 and 56.58 ± 0.56 mV/decade change) showed a gradual decrease after 105 and 95 days. After 135 and 120 days, the slopes reached approximately 54.85 ± 1.25 and 52.45 ± 2.06 mV/decade change. It can therefore be concluded that the lifetime of the proposed electrodes prepared by using electrodes (I) and (II) is at least 5 months, which is much better than previously reported ISEs (27, 28, 39). After this period, the slight change in the observed slopes could be corrected by conditioning the SPE with 0.01 mol/L SDS solution for 1–3 days. With this treatment, the assembly could be used for 2 months more. The lifetime of SPEs mainly depends on the type of ion pair and plasticizers used, and
The reproducibility of the electrodes were examined by preparing six different electrodes [electrode (I) and (II)] with the same paste composition and determining the slope of the calibration curve obtained for each electrode. The data reveal that the average slopes obtained for four replicates was 57.75 ± 0.88 and 56.21 ± 1.32 mV/decade with RSDs of 1.12 and 2.15% for electrodes (I) and (II), respectively. The repeatability of the method was examined by measuring the potential response of different concentrations of SDS over a wide time interval of 3 weeks. The average slopes found with SD were 57.13 ± 1.25 and 56.05 ± 2.15 mV/decade (RSD = 0.72 and 1.25%) for electrodes (I) and (II), respectively. The LOD, which is evaluated according to IUPAC recommendations (37), was 5.8 × 10−7 and 6.3 × 10−7 mol/L for electrodes (I) and (II), respectively. Influence of pH To investigate the pH effect on the potential response of the electrodes, the potentials were measured for a fixed concentration of SDS solution (1.0 × 10−3 – 1.0 × 10−4 mol/L) having different pH values. The pH varied from 1 to 11 by addition of HNO3 or NaOH. The potential variation as a function of pH is plotted in Figure 5. The composition of the electrodes was kept constant during all experiments. The results showed that the potential of electrodes is constant between pH 2–9 and 2–8 for electrodes (I) and (II), respectively. Thus, the electrodes work satisfactorily in the pH range 2–9, as no interference from H+ or OH− is observed in this range. Fluctuations above pH 9.0 might be justified by the formation of the soluble and insoluble hydroxyl complexes of dodecylsulfate in the solution. Fluctuations below pH 2.0 were attributed to the partial protonation of the SDS used (27). Effect of Test Solution Temperature Thermal stability of the electrodes was tested by fabricating calibration graphs for the electrode potential, Eelect, vs p[SDS] at different test solution temperatures (20, 25, 30, 35, 40, 45, 50, 55, and 60°C). The electrode potentials from these graphs at p[SDS] = 0 were obtained and plotted versus (t-25), where t is the temperature of the solution in °C (Figure 6). A straight-line plot is obtained by using Antropov’s equation (41): E° = E°(25) + (dE°/dt)(t-25) where E°(25) is the standard electrode potential at 25°C. The slope of the straight-line obtained represents the isothermal coefficient of the electrodes [0.00157 and 0.00113 V/°C for electrodes (I) and (II), respectively]. The value of the obtained isothermal coefficient of the electrodes indicates that they have fairly high thermal stability within the investigated temperature
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 121
Figure 6. Variation of the cell electromotive force with the temperature for (a) electrode (I) and (b) electrode (II).
Figure 5. Effect of pH of the test solution on the potential readings of (a) electrode (I) and (b) electrode (II).
range and show no deviation from the theoretical Nernstian behavior. Selectivity and Interference The most important characteristic of any ion-sensitive sensor is its response to the primary ion in the presence of other ions in solution, which is expressed in terms of the potentiometric selectivity coefficient. Potentiometric selectivity coefficient pot (KSDS,j ) describes the preference of the SPE for an interfering − ion j relative to SDS determined by SSM. The selectivity data so obtained are summarized in Table 2. The selectivity data show that most of the secondary anions are carbonate, nitrate, chloride, acetate, sulfate, thiocynate, oxalate, benzoate, phthalate, dodecylbenzenesulfonate, and dioctylsulfosuccinate. There was no significant interference from most of the tested substances, with the exception of dodecylbenzenesulfonate anion. The selectivity studies showed that the DBP plasticized electrode (I) sensor gave better results than the electrode (II) sensor under the same experimental conditions (Table 2). Analytical Applications The prepared electrodes were successfully used for determination of SDS in aqueous solutions and in pharmaceutical preparations (i.e., Leaders Care) using the standard addition and potentiometric titration methods. The results are summarized in Table 3. Using the potentiometric titration method, different –3 –3 volumes of 1.0 × 10 mol/L of pure SDS and 1.0 × 10 of pharmaceutical preparations were taken, completed to 20 mL with double-distilled water, and the pH was adjusted to 3 using
citrate buffer. The SD-selective sensors in conjunction with a silver-silver chloride reference electrode were immersed in the solution and titrations were carried out with standard 1.0 × 10–3 mol/L CTAB and CPC. The electromotive force was recorded after potential stabilization. The end point was determined by plotting potential versus added titrant volume. The standard addition method was also applied, and the change in potential readings (at a constant temperature of 25°C) was recorded after each addition and used to calculate the concentration of SDS sample solutions (29) by using the following equation: Cs X Cx
Vs
=
E/S - Vx
(Vx + Vs ) 10
where Cx and Vx are the concentration and volume of the unknown sample, respectively. Cs and Vs are the concentration and the volume of the standard, respectively. S is the slope of the calibration graph and ∆E is the change in mV due to the addition of the standard. Determination of the concentration depends mainly on ∆E; therefore, noticeable ∆E was obtained Table 2. Potentiometric selectivity coefficients of various interfering anions using SDS sensors K Interferent, J
(Electrode I)
pot SDS, J (Electrode II)
5.6 × 10
–3
4.5 × 10–3
NO2–
5.3 × 10
–3
3.2 × 10–3
Cl–
6.9 × 10–4
5.7 × 10–4
Acetate
–3
4.7 × 10
3.0 × 10–3
SO42–
6.8 × 10–4
4.5 × 10–4
Oxalate
–3
5.5 × 10
4.4 × 10–3
SCN–
3.5 × 10–3
1.9 × 10–3
Benzoate
–3
3.7 × 10
2.8 × 10–3
Phthalate
2.8 × 10–4
1.7 × 10–4
NO3
6.0 × 10
–3
5.2 × 10–3
CO32–
9.0 × 10–4
7.2 × 10–4
0.55
0.70
HCO3–
–
Dodecylbenzenesulfonate Dioctylsulfosuccinate
6.5 × 10
–3
4.9 × 10–3
122 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 Table 3. Determination of SDS in pharmaceutical preparations (Leaders Care) using potentiometric titration and standard addition methods Pure solution
Leaders Care
Electrode I Taken, mg
Electrode II a
Found, Recovery, RSD , mg % %
Found, mg
Recovery, %
0.490
98.00
Electrode I a
RSD , %
Taken, mg
Found, Recovery, mg %
Electrode II a
RSD , %
Found, Recovery, RSDa, mg % %
Standard addition method 0.500
0.495
99.00
4.35
2.68
5.00
4.88
97.60
1.56
4.90
98.00
2.68
1.000
0.998
99.80
3.55
0.992
99.20
3.15
10.00
10.05
100.0
2.78
9.93
99.30
3.02
1.500
1.521
101.4
2.75
1.489
99.26
1.59
20.00
19.92
99.60
3.23
19.94
99.70
4.36
0.500
0.498
99.60
2.57
0.496
99.20
4.92
98.40
3.26
5.04
100.8
4.02
1.000
0.999
99.90
1.23
1.03
1.500
1.532
102.1
0.99
1.512
Potentiometric titration method
a
4.06
5.00
103.0
2.98
10.00
9.89
98.90
2.98
9.96
99.60
3.28
100.8
3.76
20.00
20.03
100.2
1.95
19.93
99.65
2.66
Average of five determinations.
Table 4. Determination of SDS in spiked seawater and wastewater samples by potentiometric titration with 10–3 mol/L CPC and CTAB using proposed SPEs SPEs Electrode I Samples
Found, mg
RSD, %
Seawater 1
20.23
Seawater 2
19.55
Electrode II a
Found, mg
RSD, %
0.88
20.10
1.20
19.35
Two-phase method a
Commercial electrode Found, mg
RSD, %a
1.96
20.00
1.40
1.58
19.20
2.10
Found, mg
RSD, %
1.22
19.85
0.98
18.99
a
Wastewater
14.60
0.95
14.51
1.12
14.00
2.00
14.33
1.75
Formation waterb
10.46
1.24
10.32
1.52
10.15
2.13
10.22
1.88
a
Average of five determinations.
b
Formation water sample was collected from Qarun Petroleum Co., Egypt (Karama, al-Wahhat,-al-Bahhriyah).
upon using high concentration of the standard. Electrodes (I) and (II) were also applied for determination of SDS in different real spiked water samples (Table 4). The results obtained using the two modified SPEs were compared with those obtained by the commercial surfactant electrode and the two-phase titration method (Table 4). The data clearly indicate satisfactory agreement between the surfactant contents in different samples determined by the proposed sensors and the standard addition method. Conclusions The modified SPEs developed in the present work exhibited good reproducibility for about 5 months in the concentration range of 5.8 × 10−7 – 1.0 × 10–2 and 6.3 × 10–7 – 1 × 10–2 mol/L, and the pH range of the proposed sensors was 2.0–9.0 and 2.0–8.0. Dodecylsulfate-screen-printed electrodes based on the ion-pair compound of DS-CTA, DS-CP, and DBP as plasticizer were developed. The proposed electrodes were successfully applied to the determination of SDS in pharmaceutical preparations and in real spiked water samples. The analytical method proposed proved to be simple, rapid, and accurate.
References (1) Duarte, M., Giordani, R.B., De Carli, G.A., Zuanazzi, J.A., Macedo, A.J., & Tasca, T. (2009) Exp. Parasitol. 123, 195–198. http://dx.doi.org/10.1016/j.exppara.2009.07.002 (2) Abe, M., & Scamehorn, J.F. (2004) Mixed Surfactant Systems, 2nd Ed., CRC Press, Boca Raton, FL. http://dx.doi. org/10.1201/9781420031010 (3) Kelly, M.A., Altria, K.D., & Clark, B.J. (1997) J. Chromatogr. A 78l, 67–71. http://dx.doi.org/10.1016/S0021-9673(97)00501-3 (4) Erdem, B., Sully, Y., Sudol, E.D., Dimonie, V.L., & El-Aasser, M.S. (2000) Langmuir 16, 4890–4895. http://dx.doi. org/10.1021/la991547y (5) Ghoreishi, S.M., Behpour, M., & Ghafari Farsani, A. (2007) Dyes Pigments 74, 585–589. http://dx.doi.org/10.1016/j. dyepig.2006.03.023 (6) Santalad, A., Srijaranai, S., Burakham, R., Sakai, T., & Deming, R.L. (2008) Microchem. J. 90, 50–55. http://dx.doi. org/10.1016/j.microc.2008.03.007 (7) Culzoni, M.J. Aucelio, R.Q., & Escandar, G.M. (2010) Talanta 82, 325–332. http://dx.doi.org/10.1016/j.talanta.2010.04.043 (8) Zhu, X., Bao, L., Guo, R., & Wu, J. (2004) Anal. Chim. Acta 523, 43–48. http://dx.doi.org/10.1016/j.aca.2004.06.067 (9) Vasanth Kumar, K., & Rocha, F. (2009) J. Chromatogr. A 1216, 8528–8534. http://dx.doi.org/10.1016/j.chroma.2009.09.077 (10) Moliner-Martínez, Y., Cárdenas, S., & Valcárcel, M. (2007) J. Chromatogr. A 1167, 210–216. http://dx.doi.org/10.1016/j. chroma.2007.07.076
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 123 (11) Ghoreishi, S.M., Behpour, M., & Farsani, A.G. (2007) Dyes Pigments, 74, 585–589. http://dx.doi.org/10.1016/j. dyepig.2006.03.023 (12) Rastegari, A.A., Bordbar, A.-K., & Taheri-Kafrani, A. (2009) Colloids. Surfaces. B 73, 132–139. http://dx.doi.org/10.1016/j. colsurfb.2009.05.010 (13) Gerlache, M., Sentürk, Z., Viré, J.C., & Kauffmann, J.M. (1997) Anal. Chim. Acta 349, 59–65. http://dx.doi.org/10.1016/S00032670(97)00277-8 (14) Tsubouchi, M., & Tanaka, Y. (1984) Talanta 31, 633–634. http:// dx.doi.org/10.1016/0039-9140(84)80186-1 (15) Fischer, I., & Franzreb, M. (2011) Colloids. Surfaces A 377, 97–102. http://dx.doi.org/10.1016/j.colsurfa.2010.12.030 (16) Matesic-Puac, R., Sak-Bosnar, M., Bilic, M., & Grabaric, B.S. (2005) Sens. Actuators B 106, 221–228. http://dx.doi. org/10.1016/j.snb.2004.08.001 (17) Kovács, B., Csóka, B., Nagy, G., & Ivaska, A. (2001) Anal. Chim. Acta 437, 67–76. http://dx.doi.org/10.1016/S00032670(01)00987-4 (18) Shafiee-Dastjerdi, L., & Alizadeh, N. (2004) Anal. Chim. Acta 505, 195–200. http://dx.doi.org/10.1016/j.aca.2003.10.062 (19) Albareda-Sirvent, M., Merkoçi, A., & Alegret, A. (2001) Sens. Actuators, B 79, 48–57. http://dx.doi.org/10.1016/S09254005(01)00847-4 (20) Waibel, M., Schulze, H., Huber, N., & Bachmann, T.T. (2006) Biosens. Bioelectron. 21, 1132–1140. http://dx.doi. org/10.1016/j.bios.2005.04.010 (21) Martinez, N.A., Messina, G.A., Bertolino, F.A., Salinas, E., & Raba, J. (2008) Sens. Actuators, B 133, 256–262. http://dx.doi. org/10.1016/j.snb.2008.02.025 (22) Albareda-Sirvent, M., Merkoci, A., & Alegret, S. (2000) Sens. Actuators, B 69, 153–163. http://dx.doi.org/10.1016/S09254005(00)00536-0 (23) Kandimalla, V.B., Tripathi, V.S., & Ju, H. (2008) Electrochemical Sensors, Biosensors and their Biomedical Applications, Elsevier, New York, NY, Chapter 16, pp 503–529 (24) Khaled, E., Mohamed, G.G., & Awad, T. (2008) Sens. Actuators, B 135, 74–80. http://dx.doi.org/10.1016/j. snb.2008.07.027 (25) Mohamed, G.G., Ali, T.A., El-Shahat, M.F., Al-Sabagh, A.M., Migahed, M.A., & Khaled, E. (2010) Anal. Chim. Acta 673, 79–87. http://dx.doi.org/10.1016/j.aca.2010.05.016
(26) Frag, E.Y.Z., Ali, T.A., Mohamed, G.G., & Awad, Y.H.H. (2012) Int. J. Electrochem. Sci. 7, 4443–4464 (27) Mohamed, G.G., Ali, T.A., El-Shahat, M.F., Al-Sabagh, A.M., & Migahed, M.A. (2010) Electroanalysis 22, 2587–2599. http:// dx.doi.org/10.1002/elan.201000096 (28) Mohamed, G.G., El-Shahat, M.F., Al-Sabagh, A.M., Migahed, M.A., & Ali, T.A. (2011) Analyst 136, 1488–1495. http://dx.doi. org/10.1039/c0an00662a (29) Arvand-Barmchia, M., Mousavia, M.F., Zanjanchib, M.A., & Shamsipurc, M. (2003) Microchem. J. 74, 149–156. http:// dx.doi.org/10.1016/S0026-265X(02)00182-0 (30) Raulf, M., Buschmann, N., & Sommer, D. (1995) Fresenius J. Anal. Chem. 351, 526–529. http://dx.doi.org/10.1007/ BF00322727 (31) Ma, T.S., & Hassan, S.S.M. (1982) Organic Analysis Using Ion Selective Electrodes, Academic Press, London, UK (32) Hassan, S.S.M., Mahmoud, W.H., & Othman, A.M. (1997) Talanta 44, 1087–1094. http://dx.doi.org/10.1016/S00399140(96)02199-6 (33) British Pharmacopoeia (2010) The British Pharmacopoeia Secretariat, London, UK (34) Reid, V.W., Longman, G.F., & Heinerth, E. (1967) Tenside Surfact. Deterg. 4, 292–304 (35) Ghoreishi, S.M., Behpour, M., & Nabi, M. (2006) Sens. Actuators, B 113, 963–969. http://dx.doi.org/10.1016/j. snb.2005.04.003 (36) Alizadeh, N., & Mahmodian, M. (2000) Electroanalysis 12, 509–512. http://dx.doi.org/10.1002/(SICI)15214109(200005)12:73.0.CO;2-U (37) Buck, R.P., & Lindner, E. (1994) Pure Appl. Chem. 66, 2527–2536. http://dx.doi.org/10.1351/pac199466122527 (38) Gerlache, M., Sentiirk, Z., Vire, J.C., & Kauffmann, J.M. (1997) Anal. Chim. Acta 349, 59–65. http://dx.doi.org/10.1016/S00032670(97)00277-8 (39) Sadeghi, S., Dashti, G.R., & Shamsipur, M. (2002) Sens. Actuators, B Chem. 81, 223–228. http://dx.doi.org/10.1016/ S0925-4005(01)00956-X (40) Oesch, U., & Simon, W. (1980) Anal. Chem. 52, 692–700. http://dx.doi.org/10.1021/ac50054a024 (41) Antropov, L.I. (1977) Theoretical Electrochemistry, Mir Publishers, Moscow, USSR
Copyright of Journal of AOAC International is the property of AOAC International and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
RESIDUES AND TRACE ELEMENTS
Modified Screen-Printed Ion Selective Electrodes for Potentiometric Determination of Sodium Dodecylsulfate in Different Samples Tamer Awad Ali
Egyptian Petroleum Research Institute (EPRI), 11727, Cairo, Egypt
Gehad G. Mohamed
Cairo University, Chemistry Department, Faculty of Science, 12613, Giza, Egypt
Fabrication and general performance characteristics of novel screen-printed sensors for potentiometric determination of sodium dodecylsulfate (SDS) are described. The sensors are based on the use of ionassociation complexes of SDS with cetylpyridinium chloride (electrode I) and cetyltrimethylammonium bromide (electrode II) as exchange sites in a screen-printed electrode matrix. Electrodes (I) and (II) show fast, stable, and near-Nernstian response for the mono-charge anion of SDS over the concentration range of 1 × 10–2 – 5.8 × 10–7 and 1 × 10–2 – 6.3 × 10–7 mol/L at 25°C and the pH range of 2.0–9.0 and 2.0–8.0 with anionic slope of 57.32 ± 0.81 and 56.58 ± 0.65 mV/decade, respectively. Electrodes (I) and (II) have lower LODs of 5.8 × 10−7 and 6.3 × 10−7 mol/L and response times of about 8 and 13 s, respectively. Shelf life of 5 months for both electrodes is adequate. Selectivity coefficients of SDS related to a number of interfering cations, and some inorganic compounds were investigated. There were negligible interferences caused by most of the investigated species. The direct determination of 0.10–13.50 mg of SDS by electrodes (I) and (II) shows average recoveries of 99.96 and 99.85%, and mean RSDs of 0.83 and 1.04%, respectively. In the present investigation, both electrodes were used successfully as end point indicators for determination of SDS in pure pharmaceutical preparations and real spiked water samples. The results obtained using the proposed sensors to determine SDS in solution compared favorably with those obtained by the standard addition method.
S
odium dodecylsulfate (SDS) is a highly effective surfactant used in a variety of tasks requiring the removal of oily materials and residues. It is a component in industrial products, including engine degreasers, floor cleaners, and car wash soaps (1). It is used in lower concentrations in toothpastes, shampoos, and shaving foams. It is an important component in bubble bath formulations for its thickening effect and its ability to create lather. In medicine, SDS (Figure 1) is used rectally as a laxative in Received October 16, 2012. Accepted by AK March 12, 2013. Corresponding author’s e-mail: [email protected] DOI: 10.5740/jaoacint.12-388
enemas and as an excipient on some dissolvable aspirins and other fiber therapy caplets (2). Wide application of surfactants may result in the pollution of surface waters. Their determination—as intermediates, in final formulations, and in the environment—has been the focus of many analytical approaches, including titrimetry (3, 4), spectrophotometry (5, 6), spectrofluorimetry (7, 8), and chromatography (9, 10), as well as electrochemical methods incorporating ion-selective electrodes (ISEs; 11–13). However, some of these methods, such as spectrofluorimetry and chromatography, need expensive equipment and special pretreatments. Ionic surfactants have usually been determined by a two-phase titration method (14, 15). The main disadvantages of this method are the limited application to strongly colored and turbid samples, the toxicity of organic chlorinated solvent used, the formation of emulsion during titration which can disturb visual end point detection, and the numerous matrix interferences, etc. Most of these limitations can be overcome by using ISEs as indicators in potentiometric surfactant titration. All surfactant titrations are based on antagonist reaction, whereby an ionic surfactant reacts with an oppositely charged ion, usually of another surfactant, to form a water-insoluble salt (ion-pair). The first such electrodes used were made of coated wire and based on ion-association complexes of quaternary ammonium cation and surfactant anion (16–18). The screenprinting technique may be one of the most promising approaches in producing simple, rapid, and inexpensive biosensors (19–21). The biosensors based on screen-printed electrodes have been extensively used for detection of biomolecules, pesticides, antigens, and anions (22, 23). Electrochemical biosensors based on screen-printed electrodes (SPEs) meet the requirements of in situ screening devices in that all the equipment needed for electrochemical analysis is portable. SPEs have all the major performance characteristics of biosensors: sample preparation is minimal; they are quick, cheap, and easy to use; and they are small and can be miniaturized with new technology (24–26). In this study, modified SPEs were fabricated for rapid and sensitive quantification of SDS in both pure pharmaceutical preparations and water samples. The new fabricated SPEs, as potentiometric SDS sensors using home-made printing carbon ink, are developed based on the use of ion-pair compounds of SDS with cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB) as the electroactive substances, and dibutylphthalate (DBP) as plasticizer. This study investigated the general performance
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 117 University, using a PerkinElmer 2400 CHN Vario Elemental Analyzer (Santa Clara, CA). Preparation of SDS-Modified Screen-Printed Electrodes Figure 1. Structure of sodium dodecylsulfate.
characteristics of the fabricated electrodes using the respective ion-pair compounds, Dodecylsulfate-cetypyridinium (DS-CP, electrode I) and Dodecylsulfate-cetyltrimethyl-ammonium (DS-CTA, electrode II). It was found that both electrodes can be successfully applied for determination of pure SDS in pharmaceutical preparations and spiked water samples. Experimental Reagents All reagents were of analytical grade and bidistilled water was used throughout the experiments. Tricresylphosphate (TCP) from Alfa Aesar (Ward Hill, MA) was used for preparation of the sensors. Other types of plasticizers, namely dioctylphthalate (DOP), DBP, o-nitrophenyloctylether (o-NPOE), and dioctyl sebacate (DOS), were purchased from Sigma-Aldrich (St. Louis, MO), Merck (Whitehouse Station, NJ), Fluka (St. Louis, MO), and Merck, respectively. Relatively high MW polyvinyl chloride (PVC; Sigma-Aldrich) and graphite powder (synthetic 1–2 µm; Sigma-Aldrich) was used for the fabrication of different electrodes. CPC, C21H38NCl, 339.99 g/mol; CTAB, C19H42BrN, 364.5 g/mol; and SDS, C12H25SO4Na, 288.38 g/mol, were purchased from Sigma-Aldrich, Fluka, and BDH (Radnor, PA), respectively. Pharmaceutical formulations containing surfactants, such as Leaders Care solution (Leader’s Pharm Co., for Multipharma; each 120 mL contains 4 g SDS), was used. Anionic surfactants in wastewater sample (Aga area, Dakahlia, Egypt) and seawater sample (Marsa Matrouh, Egypt, seawater 1; and Baltim area, Kafr El Sheikh, Egypt, seawater 2) from the Mediterranean Sea were collected. Formation water sample were also collected (Karama, Al-Wahhat Al-Bahhriyah, Qarun Petroleum Co., Egypt). Apparatus Laboratory potential measurements were performed using a HANNA pH meter, Model 211 (Hanna Instruments, Județul Cluj, Romania). Silver-silver chloride double-junction reference electrode, 6.0726.100 (Metrohm USA, Riverview, FL) in conjugation with different ion selective electrodes, was used. Commercial surfactant electrodes, Cationic Surfactant Electrode, 6.0507.120, (Metrohm USA) were used as a second sensing electrode in comparing the results for determination of cationic surfactants. A digital burette (Cole-Parmer, Vernon Hills, IL) was used for the measurement of surfactants under investigation. Microanalysis for carbon, hydrogen, nitrogen, and sulfur were carried out at the Microanalytical Centers, Cairo
Modified SPEs were printed in arrays of six couples consisting of the working and the reference electrodes (each 5 × 35 mm) following the procedures previously described (24–28). A polyvinyl chloride flexible sheet (0.2 mm) was used as a substrate, not affected by the curing temperature or the ink solvent and easily cut by scissors. A pseudo silver-silver chloride electrode was printed using self-made PVC ink that contains silver-silver chloride (65 + 35%) and is cured 30 min at 60°C. The working electrodes were then fabricated. They were printed using self-made carbon ink prepared by mixing 2.5–19 mg DS-CP or DS-CTA ion pairs, 450 mg DBP, 1.25 g PVC 8%, 0.75 g carbon powder, and were cured 30 min at 50°C. A layer of insulator was then placed onto the printed electrodes, leaving a defined rectangular-shaped (5 × 5 mm) working area and a similar area (for the electrical contact) on the other side. Fabricated electrodes were stored at 4°C and used directly in the potentiometric measurements. Preparation of Ion Exchangers The ion exchangers of SDS with DS-CTA and DS-CP –2 were prepared by slow addition of 15 mL of 10 mol/L SDS –2 to CTAB or CPC solutions (10 mol/L). The mixtures were stirred 10 min, the precipitates were filtered off through No. 42 Whatman filter paper, washed with double-distilled water, dried at room temperature, and finely ground to fine powder (28, 29). Calibration of Modified SPEs The modified SPEs were calibrated by immersion, together with a reference electrode, into a 25 mL beaker containing 2.0 mL of acetate buffer solution at pH 3. Then, a 5 mL aliquot of SDS solution was added with continuous stirring to give final SDS concentration ranging from 1 × 10–2 to 1 × 10–7 mol/L, and the potential was recorded after stabilization to ±0.4 mV. A calibration graph was then fabricated by plotting the recorded potentials as a function of –log [DS–]. The resulting graph was used for subsequent determination of unknown SDS concentration using the same procedure (25, 28). Preparation of Surfactant Solutions Stock solutions (1 × 10–2 mol/L) of SDS, CTAB, and CPC were prepared by dissolving 0.228, 0.3645, and 0.3399 g in doubledistilled water (1 L) with continuous stirring, respectively. Lower concentrations were prepared by accurately diluting from the stock solution. The adsorption of surfactant on the inner surface of vessels was eliminated, as previously reported (25, 27, 30). Determination of Potentiometric Selectivity Coefficient Potentiometric selectivity coefficient (KSDS,jpot) was evaluated according to International Union of Pure and Applied Chemistry (IUPAC) guidelines using the separate solutions method (SSM;
118 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 (a)
Determination of SDS in water samples.––In a 10 mL beaker, a 3 mL aliquot of water sample was transferred, definite concentration of SDS was added, and a 2.0 mL citrate buffer of pH 3.0 solution was added. SDS was quantified in the water samples by using potentiometric titration with CPC and CTAB and SPEs and commercial surfactant electrodes, in addition to the two-phase titration method (14, 15, 25, 27, 34). Potentiometric Titration
(b)
For the potentiometric titration method, an aliquot of the sample solution containing 0.100–13.5 mg SDS was titrated –2 against 10 mol/L CTAB solution. The equivalence points were estimated from the first derivative of the S-shape titration curves (25, 27). Results and Discussion
Figure 2. Effect of IP content on calibration of modified SPEs using (a) electrode (I), (b) electrode (II).
27, 28, 31, 32), in which the potential of the cell comprising the electrodes (I) and (II) modified SPEs and a reference electrode is measured with two separate solutions, one containing the SDS of the activity aSDS (but no j) and the other containing the interfering ion j of the same activity aSDS = aj (but no SDS). Different interfering anions and cations of a concentration of –3 1 × 10 mol/L at pH 3.0 are used, and the results are obtained by using the following equation: pot
log K SDS, j
= ((E1-E2)/S)
+ (1+ (Z1/Z2))log a)
pot
where KSDS,j is the potentiometric selectivity coefficient, E1 is −3 the potential measured in 1 × 10 mol/L SDS, E2 is the potential −3 measured in 1 × 10 mol/L of the interfering compound (j), Z1 and Z2 are the charges of the SDS and interfering species j, respectively, and S is slope of the electrode calibration plot. Sample Analysis Potentiometric titration of SDS in pure pharmaceutical samples.—Bidistilled water was added to a known volume of Leader’s Care solution to make up 25 mol in a calibrated volumetric flask. A 1 mL aliquot of the dilute solution was transferred to a 10 mL beaker containing a 2 mL citrate buffer of pH 3.0. SDS was quantified in the pharmaceutical preparations by using potentiometric titration with CPC and CTAB standard solutions (25, 27) and screen-printed electrodes. The results obtained were compared with those previously reported using the standard addition method (25, 27, 33).
The sparingly soluble ion-pairs of DS-CP and DS-CTA, which formed instantaneously upon the addition of SDS solution to CPC and CTAB solutions, were used for fabrication of modified SDS ion-selective electrodes. Using elemental analysis, the composition of the ion pairs formed is 1:1 ion pair (found (calculated): C = 67.35% (67.76%), H = 12.47% (12.20%), N = 2.56% (2.55%), and S = 5.72% (5.82%) for DS-CTA (C31H67NSO4) ion pair; and C = 69.15% (69.60%), H = 11.53% (11.07%), N = 2.34% (2.46%), and S = 5.64% (5.62%) for DS-CP (C33H63NO4S) ion pair. Optimization of Electrode Performance Under Batch Condition The modified electrodes were prepared and optimized by testing the nature and content of the modifier, effect of soaking time, type of plasticizer, pH, temperature, response time, durability of sensors, and application. Effect of Ion-Pair Content Nine electrodes were prepared. The amounts of carbon powder and DBP were held constant in each electrode. The proportions of modifier in these nine electrodes were 2.5, 5.0, 10.0, 12.5, 14.0, 15.0, 16.0, 17.0, and 19.0 mg of DS-CP or DS-CTA ion pairs while the other constituents were unchanged. The Nernstian slopes for the resulting calibration graphs and correlation coefficients were 49.81 (0.985), 50.70 (0.989), 53.53 (0.990), 55.90 (0.995), 56.40 (0.997), 57.32 (0.999), 56.95 (0.998), 50.45 (0.979), 46.95 (0.970), and 44.38 (0.983), 45.88 (0.985), 49.24 (0.987), 52.23 (0.989), 53.87 (0.990), 54.35 (0.992), 55.00 (0.995), 56.58 (0.998),and 55.90 (0.996) mV/decade for 2.5, 5.0, 10.0, 12.5, 14.0, 15.0, 16.0, 17.0, and 19.0 mg of ion pairs for electrode I and electrode II, respectively. Figure 2 shows that –7 –2 the linear ranges of the electrodes are 5.8 × 10 – 1.0 × 10 and –7 –2 6.3 × 10 – 1.0 × 10 mol/L for electrodes I and II, respectively. These results show that electrodes (I) and (II), which contain 15.0 and 17.0 mg ion pairs, have a higher Nernstian slope and a wide range of linearity. The 15.0 and 17.0 mg ion pair was chosen as the optimum amount for the fabrication of the SDS
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 119 (a)
Table 1. Response characteristics of SDS-modified screen-printed electrode Parameter Slope, mV/decade
Electrode I
Electrode II
57.32 ± 0.81
56.58 ± 0.65
0.999
0.998
Correlation coefficient, r Lower detection limit, mol/L
5.8×10
Response time, s Working pH range Usable range, mol/L Intercept, mV
RSD% a
6.3×10−7
8
13
2-9
2-8
5.8 ×10−7 – 1.0 ×10–2 6.3 ×10–7 – 1.0 ×10–2 243.19 ± 0.27
231.74 ± 0.92
5
4–5
0.83
1.04
Life time, months
(b)
−7
a
Average of four determinations.
solutions (35, 36). The SPEs should be kept dry in an opaque closed vessel and stored in a refrigerator while not in use. Influence of Plasticizer Composition
Figure 3. Effect of plasticizer type on calibration of modified SPEs using (a) electrode (I) and (b) electrode (II).
electrodes. A non-curve response was observed with electrodes containing lower or higher ratios of the modifier. Effect of Soaking Time Freshly prepared SPEs must be soaked in DS-CP and DS-CTA ion pairs suspended solution to activate the surface of the screenprinted layer and form an infinitesimally thin gel layer at which ion exchange occurs. How much time this preconditioning process requires depends on diffusion and equilibration at the electrode-test solution interface; if equilibrium can be established quickly, then a fast response is possible (35). Thus, the performance characteristics of the DS ion-selective SPE was investigated as a function of soaking time. For this purpose, the SPEs [electrode (I) and (II)] were soaked in aqueous suspension of DS-CP and DS-CTA ion pairs, and the titration curves were plotted, from which the total potential changes were recorded after 0, 5, 10, 15, 20, 25, 30, 45, 60, 120, 180, and 360 min. The optimum soaking time was 15 min, when the slope of the calibration curves was 57.32 and 56.58 mV/decade at 25°C, for electrodes (I) and (II), respectively. It is recommended that soaking time be no more than 10 and 15 min for electrodes (I) and (II), respectively, to avoid leaching, although very little of the electroactive species does leach into the bathing
The effect of plasticizer composition was studied on the characteristic performances of electrodes (I) and (II). SPE sensors were prepared using different plasticizers, namely, DBP, DOS, DOP, o-NPOE, and TCP. Sensors incorporated paste plasticized with DBP, TCP, and DOP showed less response [slope 57.32, 55.85, 54.65, and 56.58, 56.12, 51.98 mV/decade for electrodes (I) and (II), respectively] for SDS. This response probably resulted from poor solubility of ion associate in the plasticizer. In the case of DOS [slope 44.95 and 40.24 mV/decade for electrodes (I) and (II), respectively], a negligible or very poor response was obtained. The solubility of the complex was examined by its ability to form transparent solution in the plasticizer. In the case of o-NPOE plasticizer (slope 57.96 and 57.84 mV/decade for electrode (I) and (II), respectively), a clear transparent solution was formed. Whereas, in the case of TCP, DOP, and DOS plasticizers, turbid and suspended solutions were obtained, which indicates that the complex solubility increases in the order of o-NPOE > DBP > TCP > DOP >DOS (Figure 3). o-NPOE may improve the SPE selectivity due to its high dielectric constant (ε = 24). It affects considerable dissolution of ion-association within the SPE; consequently enhances its partition coefficient in the SPE and also provides suitable mechanical property of the SPE compared with less permittivity plasticizers. Performance of Sensors The potentiometric performance characteristics of the anionic surfactant sensors based on use of DS-CP and DS-CTA ion pairs as electroactive materials and DBP as a plasticizer in SPE were evaluated according to IUPAC recommendations (37, 38), and the results are given in Table 1. In citrate buffer of pH 3.0, the sensors –2 –7 displayed linear and stable response for 1 × 10 – 5.8 × 10 and –2 –7 1 × 10 – 6.3 × 10 mol/L SDS at 25 °C with anionic slope of −7 57.32 ± 0.81 and 56.58 ± 0.65 mV/decade, and LODs of 5.8 × 10
120 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 on the number of times used (39). After 6 months, the electrode response deteriorated, which may be attributed to the aging of the SPE matrixes, ion pair, as well as plasticizer (40). Repeatability/Reproducibility
Figure 4. Dynamic response time of (a) electrode (I) and (b) electrode (II).
and 6.3 × 10−7 mol/L, and response times of about 8 and 13 s for electrodes (I) and (II), respectively. Response Time The static response time of the electrodes was measured after successive immersion of the electrodes in a series of SDS solutions, in each of which the SDS concentration increased −7 −3 10-fold, from 1.0 × 10 to 1.0 × 10 mol/L. The static response −3 time thus obtained was less than 8 and 13 s for 1.0 × 10 mol/L SDS using electrodes (I) and (II), respectively. At lower concentrations, however, the response time was a little longer and reached 10 and 14 s for electrodes (I) and (II), respectively. The actual potential versus time traces is shown in Figure 4. The potentials remained constant for approximately 3 min, after which a very slow change within the resolution of the meter was recorded. The sensing behavior of the SPE did not depend on whether the potentials were recorded from low to high concentrations, or vice versa. The electrodes were found to have fast response for 45 days without any significant change observed in the working concentration range, slope, or response time (25, 27). Lifetime of Electrodes The electrodes were used for 4–5 months without any significant effect on the paste potential. The lifetime of the electrodes was determined by reading the potential values of the calibration solutions and plotting the calibration curves for the 5 month period. The slopes of the electrodes (57.32 ± 0.81 and 56.58 ± 0.56 mV/decade change) showed a gradual decrease after 105 and 95 days. After 135 and 120 days, the slopes reached approximately 54.85 ± 1.25 and 52.45 ± 2.06 mV/decade change. It can therefore be concluded that the lifetime of the proposed electrodes prepared by using electrodes (I) and (II) is at least 5 months, which is much better than previously reported ISEs (27, 28, 39). After this period, the slight change in the observed slopes could be corrected by conditioning the SPE with 0.01 mol/L SDS solution for 1–3 days. With this treatment, the assembly could be used for 2 months more. The lifetime of SPEs mainly depends on the type of ion pair and plasticizers used, and
The reproducibility of the electrodes were examined by preparing six different electrodes [electrode (I) and (II)] with the same paste composition and determining the slope of the calibration curve obtained for each electrode. The data reveal that the average slopes obtained for four replicates was 57.75 ± 0.88 and 56.21 ± 1.32 mV/decade with RSDs of 1.12 and 2.15% for electrodes (I) and (II), respectively. The repeatability of the method was examined by measuring the potential response of different concentrations of SDS over a wide time interval of 3 weeks. The average slopes found with SD were 57.13 ± 1.25 and 56.05 ± 2.15 mV/decade (RSD = 0.72 and 1.25%) for electrodes (I) and (II), respectively. The LOD, which is evaluated according to IUPAC recommendations (37), was 5.8 × 10−7 and 6.3 × 10−7 mol/L for electrodes (I) and (II), respectively. Influence of pH To investigate the pH effect on the potential response of the electrodes, the potentials were measured for a fixed concentration of SDS solution (1.0 × 10−3 – 1.0 × 10−4 mol/L) having different pH values. The pH varied from 1 to 11 by addition of HNO3 or NaOH. The potential variation as a function of pH is plotted in Figure 5. The composition of the electrodes was kept constant during all experiments. The results showed that the potential of electrodes is constant between pH 2–9 and 2–8 for electrodes (I) and (II), respectively. Thus, the electrodes work satisfactorily in the pH range 2–9, as no interference from H+ or OH− is observed in this range. Fluctuations above pH 9.0 might be justified by the formation of the soluble and insoluble hydroxyl complexes of dodecylsulfate in the solution. Fluctuations below pH 2.0 were attributed to the partial protonation of the SDS used (27). Effect of Test Solution Temperature Thermal stability of the electrodes was tested by fabricating calibration graphs for the electrode potential, Eelect, vs p[SDS] at different test solution temperatures (20, 25, 30, 35, 40, 45, 50, 55, and 60°C). The electrode potentials from these graphs at p[SDS] = 0 were obtained and plotted versus (t-25), where t is the temperature of the solution in °C (Figure 6). A straight-line plot is obtained by using Antropov’s equation (41): E° = E°(25) + (dE°/dt)(t-25) where E°(25) is the standard electrode potential at 25°C. The slope of the straight-line obtained represents the isothermal coefficient of the electrodes [0.00157 and 0.00113 V/°C for electrodes (I) and (II), respectively]. The value of the obtained isothermal coefficient of the electrodes indicates that they have fairly high thermal stability within the investigated temperature
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 121
Figure 6. Variation of the cell electromotive force with the temperature for (a) electrode (I) and (b) electrode (II).
Figure 5. Effect of pH of the test solution on the potential readings of (a) electrode (I) and (b) electrode (II).
range and show no deviation from the theoretical Nernstian behavior. Selectivity and Interference The most important characteristic of any ion-sensitive sensor is its response to the primary ion in the presence of other ions in solution, which is expressed in terms of the potentiometric selectivity coefficient. Potentiometric selectivity coefficient pot (KSDS,j ) describes the preference of the SPE for an interfering − ion j relative to SDS determined by SSM. The selectivity data so obtained are summarized in Table 2. The selectivity data show that most of the secondary anions are carbonate, nitrate, chloride, acetate, sulfate, thiocynate, oxalate, benzoate, phthalate, dodecylbenzenesulfonate, and dioctylsulfosuccinate. There was no significant interference from most of the tested substances, with the exception of dodecylbenzenesulfonate anion. The selectivity studies showed that the DBP plasticized electrode (I) sensor gave better results than the electrode (II) sensor under the same experimental conditions (Table 2). Analytical Applications The prepared electrodes were successfully used for determination of SDS in aqueous solutions and in pharmaceutical preparations (i.e., Leaders Care) using the standard addition and potentiometric titration methods. The results are summarized in Table 3. Using the potentiometric titration method, different –3 –3 volumes of 1.0 × 10 mol/L of pure SDS and 1.0 × 10 of pharmaceutical preparations were taken, completed to 20 mL with double-distilled water, and the pH was adjusted to 3 using
citrate buffer. The SD-selective sensors in conjunction with a silver-silver chloride reference electrode were immersed in the solution and titrations were carried out with standard 1.0 × 10–3 mol/L CTAB and CPC. The electromotive force was recorded after potential stabilization. The end point was determined by plotting potential versus added titrant volume. The standard addition method was also applied, and the change in potential readings (at a constant temperature of 25°C) was recorded after each addition and used to calculate the concentration of SDS sample solutions (29) by using the following equation: Cs X Cx
Vs
=
E/S - Vx
(Vx + Vs ) 10
where Cx and Vx are the concentration and volume of the unknown sample, respectively. Cs and Vs are the concentration and the volume of the standard, respectively. S is the slope of the calibration graph and ∆E is the change in mV due to the addition of the standard. Determination of the concentration depends mainly on ∆E; therefore, noticeable ∆E was obtained Table 2. Potentiometric selectivity coefficients of various interfering anions using SDS sensors K Interferent, J
(Electrode I)
pot SDS, J (Electrode II)
5.6 × 10
–3
4.5 × 10–3
NO2–
5.3 × 10
–3
3.2 × 10–3
Cl–
6.9 × 10–4
5.7 × 10–4
Acetate
–3
4.7 × 10
3.0 × 10–3
SO42–
6.8 × 10–4
4.5 × 10–4
Oxalate
–3
5.5 × 10
4.4 × 10–3
SCN–
3.5 × 10–3
1.9 × 10–3
Benzoate
–3
3.7 × 10
2.8 × 10–3
Phthalate
2.8 × 10–4
1.7 × 10–4
NO3
6.0 × 10
–3
5.2 × 10–3
CO32–
9.0 × 10–4
7.2 × 10–4
0.55
0.70
HCO3–
–
Dodecylbenzenesulfonate Dioctylsulfosuccinate
6.5 × 10
–3
4.9 × 10–3
122 Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 Table 3. Determination of SDS in pharmaceutical preparations (Leaders Care) using potentiometric titration and standard addition methods Pure solution
Leaders Care
Electrode I Taken, mg
Electrode II a
Found, Recovery, RSD , mg % %
Found, mg
Recovery, %
0.490
98.00
Electrode I a
RSD , %
Taken, mg
Found, Recovery, mg %
Electrode II a
RSD , %
Found, Recovery, RSDa, mg % %
Standard addition method 0.500
0.495
99.00
4.35
2.68
5.00
4.88
97.60
1.56
4.90
98.00
2.68
1.000
0.998
99.80
3.55
0.992
99.20
3.15
10.00
10.05
100.0
2.78
9.93
99.30
3.02
1.500
1.521
101.4
2.75
1.489
99.26
1.59
20.00
19.92
99.60
3.23
19.94
99.70
4.36
0.500
0.498
99.60
2.57
0.496
99.20
4.92
98.40
3.26
5.04
100.8
4.02
1.000
0.999
99.90
1.23
1.03
1.500
1.532
102.1
0.99
1.512
Potentiometric titration method
a
4.06
5.00
103.0
2.98
10.00
9.89
98.90
2.98
9.96
99.60
3.28
100.8
3.76
20.00
20.03
100.2
1.95
19.93
99.65
2.66
Average of five determinations.
Table 4. Determination of SDS in spiked seawater and wastewater samples by potentiometric titration with 10–3 mol/L CPC and CTAB using proposed SPEs SPEs Electrode I Samples
Found, mg
RSD, %
Seawater 1
20.23
Seawater 2
19.55
Electrode II a
Found, mg
RSD, %
0.88
20.10
1.20
19.35
Two-phase method a
Commercial electrode Found, mg
RSD, %a
1.96
20.00
1.40
1.58
19.20
2.10
Found, mg
RSD, %
1.22
19.85
0.98
18.99
a
Wastewater
14.60
0.95
14.51
1.12
14.00
2.00
14.33
1.75
Formation waterb
10.46
1.24
10.32
1.52
10.15
2.13
10.22
1.88
a
Average of five determinations.
b
Formation water sample was collected from Qarun Petroleum Co., Egypt (Karama, al-Wahhat,-al-Bahhriyah).
upon using high concentration of the standard. Electrodes (I) and (II) were also applied for determination of SDS in different real spiked water samples (Table 4). The results obtained using the two modified SPEs were compared with those obtained by the commercial surfactant electrode and the two-phase titration method (Table 4). The data clearly indicate satisfactory agreement between the surfactant contents in different samples determined by the proposed sensors and the standard addition method. Conclusions The modified SPEs developed in the present work exhibited good reproducibility for about 5 months in the concentration range of 5.8 × 10−7 – 1.0 × 10–2 and 6.3 × 10–7 – 1 × 10–2 mol/L, and the pH range of the proposed sensors was 2.0–9.0 and 2.0–8.0. Dodecylsulfate-screen-printed electrodes based on the ion-pair compound of DS-CTA, DS-CP, and DBP as plasticizer were developed. The proposed electrodes were successfully applied to the determination of SDS in pharmaceutical preparations and in real spiked water samples. The analytical method proposed proved to be simple, rapid, and accurate.
References (1) Duarte, M., Giordani, R.B., De Carli, G.A., Zuanazzi, J.A., Macedo, A.J., & Tasca, T. (2009) Exp. Parasitol. 123, 195–198. http://dx.doi.org/10.1016/j.exppara.2009.07.002 (2) Abe, M., & Scamehorn, J.F. (2004) Mixed Surfactant Systems, 2nd Ed., CRC Press, Boca Raton, FL. http://dx.doi. org/10.1201/9781420031010 (3) Kelly, M.A., Altria, K.D., & Clark, B.J. (1997) J. Chromatogr. A 78l, 67–71. http://dx.doi.org/10.1016/S0021-9673(97)00501-3 (4) Erdem, B., Sully, Y., Sudol, E.D., Dimonie, V.L., & El-Aasser, M.S. (2000) Langmuir 16, 4890–4895. http://dx.doi. org/10.1021/la991547y (5) Ghoreishi, S.M., Behpour, M., & Ghafari Farsani, A. (2007) Dyes Pigments 74, 585–589. http://dx.doi.org/10.1016/j. dyepig.2006.03.023 (6) Santalad, A., Srijaranai, S., Burakham, R., Sakai, T., & Deming, R.L. (2008) Microchem. J. 90, 50–55. http://dx.doi. org/10.1016/j.microc.2008.03.007 (7) Culzoni, M.J. Aucelio, R.Q., & Escandar, G.M. (2010) Talanta 82, 325–332. http://dx.doi.org/10.1016/j.talanta.2010.04.043 (8) Zhu, X., Bao, L., Guo, R., & Wu, J. (2004) Anal. Chim. Acta 523, 43–48. http://dx.doi.org/10.1016/j.aca.2004.06.067 (9) Vasanth Kumar, K., & Rocha, F. (2009) J. Chromatogr. A 1216, 8528–8534. http://dx.doi.org/10.1016/j.chroma.2009.09.077 (10) Moliner-Martínez, Y., Cárdenas, S., & Valcárcel, M. (2007) J. Chromatogr. A 1167, 210–216. http://dx.doi.org/10.1016/j. chroma.2007.07.076
Ali & Mohamed: Journal of AOAC International Vol. 98, No. 1, 2015 123 (11) Ghoreishi, S.M., Behpour, M., & Farsani, A.G. (2007) Dyes Pigments, 74, 585–589. http://dx.doi.org/10.1016/j. dyepig.2006.03.023 (12) Rastegari, A.A., Bordbar, A.-K., & Taheri-Kafrani, A. (2009) Colloids. Surfaces. B 73, 132–139. http://dx.doi.org/10.1016/j. colsurfb.2009.05.010 (13) Gerlache, M., Sentürk, Z., Viré, J.C., & Kauffmann, J.M. (1997) Anal. Chim. Acta 349, 59–65. http://dx.doi.org/10.1016/S00032670(97)00277-8 (14) Tsubouchi, M., & Tanaka, Y. (1984) Talanta 31, 633–634. http:// dx.doi.org/10.1016/0039-9140(84)80186-1 (15) Fischer, I., & Franzreb, M. (2011) Colloids. Surfaces A 377, 97–102. http://dx.doi.org/10.1016/j.colsurfa.2010.12.030 (16) Matesic-Puac, R., Sak-Bosnar, M., Bilic, M., & Grabaric, B.S. (2005) Sens. Actuators B 106, 221–228. http://dx.doi. org/10.1016/j.snb.2004.08.001 (17) Kovács, B., Csóka, B., Nagy, G., & Ivaska, A. (2001) Anal. Chim. Acta 437, 67–76. http://dx.doi.org/10.1016/S00032670(01)00987-4 (18) Shafiee-Dastjerdi, L., & Alizadeh, N. (2004) Anal. Chim. Acta 505, 195–200. http://dx.doi.org/10.1016/j.aca.2003.10.062 (19) Albareda-Sirvent, M., Merkoçi, A., & Alegret, A. (2001) Sens. Actuators, B 79, 48–57. http://dx.doi.org/10.1016/S09254005(01)00847-4 (20) Waibel, M., Schulze, H., Huber, N., & Bachmann, T.T. (2006) Biosens. Bioelectron. 21, 1132–1140. http://dx.doi. org/10.1016/j.bios.2005.04.010 (21) Martinez, N.A., Messina, G.A., Bertolino, F.A., Salinas, E., & Raba, J. (2008) Sens. Actuators, B 133, 256–262. http://dx.doi. org/10.1016/j.snb.2008.02.025 (22) Albareda-Sirvent, M., Merkoci, A., & Alegret, S. (2000) Sens. Actuators, B 69, 153–163. http://dx.doi.org/10.1016/S09254005(00)00536-0 (23) Kandimalla, V.B., Tripathi, V.S., & Ju, H. (2008) Electrochemical Sensors, Biosensors and their Biomedical Applications, Elsevier, New York, NY, Chapter 16, pp 503–529 (24) Khaled, E., Mohamed, G.G., & Awad, T. (2008) Sens. Actuators, B 135, 74–80. http://dx.doi.org/10.1016/j. snb.2008.07.027 (25) Mohamed, G.G., Ali, T.A., El-Shahat, M.F., Al-Sabagh, A.M., Migahed, M.A., & Khaled, E. (2010) Anal. Chim. Acta 673, 79–87. http://dx.doi.org/10.1016/j.aca.2010.05.016
(26) Frag, E.Y.Z., Ali, T.A., Mohamed, G.G., & Awad, Y.H.H. (2012) Int. J. Electrochem. Sci. 7, 4443–4464 (27) Mohamed, G.G., Ali, T.A., El-Shahat, M.F., Al-Sabagh, A.M., & Migahed, M.A. (2010) Electroanalysis 22, 2587–2599. http:// dx.doi.org/10.1002/elan.201000096 (28) Mohamed, G.G., El-Shahat, M.F., Al-Sabagh, A.M., Migahed, M.A., & Ali, T.A. (2011) Analyst 136, 1488–1495. http://dx.doi. org/10.1039/c0an00662a (29) Arvand-Barmchia, M., Mousavia, M.F., Zanjanchib, M.A., & Shamsipurc, M. (2003) Microchem. J. 74, 149–156. http:// dx.doi.org/10.1016/S0026-265X(02)00182-0 (30) Raulf, M., Buschmann, N., & Sommer, D. (1995) Fresenius J. Anal. Chem. 351, 526–529. http://dx.doi.org/10.1007/ BF00322727 (31) Ma, T.S., & Hassan, S.S.M. (1982) Organic Analysis Using Ion Selective Electrodes, Academic Press, London, UK (32) Hassan, S.S.M., Mahmoud, W.H., & Othman, A.M. (1997) Talanta 44, 1087–1094. http://dx.doi.org/10.1016/S00399140(96)02199-6 (33) British Pharmacopoeia (2010) The British Pharmacopoeia Secretariat, London, UK (34) Reid, V.W., Longman, G.F., & Heinerth, E. (1967) Tenside Surfact. Deterg. 4, 292–304 (35) Ghoreishi, S.M., Behpour, M., & Nabi, M. (2006) Sens. Actuators, B 113, 963–969. http://dx.doi.org/10.1016/j. snb.2005.04.003 (36) Alizadeh, N., & Mahmodian, M. (2000) Electroanalysis 12, 509–512. http://dx.doi.org/10.1002/(SICI)15214109(200005)12:73.0.CO;2-U (37) Buck, R.P., & Lindner, E. (1994) Pure Appl. Chem. 66, 2527–2536. http://dx.doi.org/10.1351/pac199466122527 (38) Gerlache, M., Sentiirk, Z., Vire, J.C., & Kauffmann, J.M. (1997) Anal. Chim. Acta 349, 59–65. http://dx.doi.org/10.1016/S00032670(97)00277-8 (39) Sadeghi, S., Dashti, G.R., & Shamsipur, M. (2002) Sens. Actuators, B Chem. 81, 223–228. http://dx.doi.org/10.1016/ S0925-4005(01)00956-X (40) Oesch, U., & Simon, W. (1980) Anal. Chem. 52, 692–700. http://dx.doi.org/10.1021/ac50054a024 (41) Antropov, L.I. (1977) Theoretical Electrochemistry, Mir Publishers, Moscow, USSR
Copyright of Journal of AOAC International is the property of AOAC International and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
