Journal of Dentistry, 4, 28-32

The ‘solubility and disintegration’ test for zinc phosphate dental cements: the use of small specimens A. D. Wikon, G. Abel B. G. Lewis

BSc, C Chem, FRIC .

Laboratory of the Government Chemist, Department ABSTRACT

The use of small test specimens for the evaluation of the ‘solubility and disintegration’ of zinc phosphate dental cement combined with the colorimetric determination of phosphate leached is proposed. Comparison with the existing gravimetric procedure has shown an adequate correlation between the two methods. The effect of pH has been investigated.

INTRODUCTION IN the original specification for dental cements which emanated in the 1930s from the classic

work of Paffenbarger et al. (1934, 1938) the amount of cement paste required for each test specimen greatly exceeded the normal amount used clinically by a dentist for a single restoration. In zinc phosphate cements the reaction is markedly exothermic and bulk is important, for it will affect the amount of heat dissipated during and after mixing. For this reason it is desirable that the amount of material used in a test approximates to that used in clinical practice. Little attention was paid to this important point until the introduction of encapsulated silicate cement materials necessitated its reconsideration since these capsules obviously contained only sufficient material for a restoration and not for a standard test specimen. The need

of Industry, London

for smaller specimens was considered both by a national BSI committee* and an international IS0 committee?, and the Laboratory of the Government Chemist developed techniques which utilized small test specimens of similar bulk to a large clinical restoration. In particular, a method was developed for determining the initial solubility of dental silicate cements using small discs, and a sensitive calorimetric technique for estimating eluted phosphate (Wilson and Batchelor, 1971). This followed early work by Wilson and Batchelor (1967) on the chemical nature of the species eluted from dental silicate cements in the ‘solubility and disintegration’ test where less than a milligram of material is eluted. The use of a sensitive instrumental method to determine chemical entities leached, rather than weighing a residue after evaporation, is for the zinc phosphate cement an even more apposite technique than for the dental silicate cement, because the quantities of material eluted are less. Wilson et al. (1970) studied the chemical entities eluted from a zinc phosphate cement over an extended period, and more recently Wilson et al. (1974) have compared the chemical *British Standards Institution, Technical Subcommittee DNMH 3/3-Dental Cements. tInternational Organization for Standardization, Working Group l-Filling Materials, of the IS0 Technical Committee TC/lObDentistry.

Wilson et al. : Zinc Phosphate Dental Cements Table I.-Cements Cement

P

used in the study

: L ratio (s/ml) 2.0

Brand

Manufacturer

Racim Ames S. S. White CAS De Trey Tenet Alston DSD

;:; 2.5 2.5 245 2.4 2.5 P

29

: L ratio = powder

Dental Fillings Ltd, London W. V. B. Ames 8 Co., Fremont, Ohio, USA S. S. White Dental Mfg Co., London Amalgamated Dental Co., London Amalgamated Dental Co., London Vivadent, Liechtenstein Dental Mfg Co. Ltd, London Davis, Schottlander and Davis, London

: liquid ratio.

composition of eluatesfromanumberof brands with figures obtained for the standard ‘solubility and disintegration’ test and found a good correlation with the phosphate leached. They concluded that the latter measurement could well replace the standard procedure with advantage because of a greater sensitivity and accuracy combined with a reduced manipulative time. The procedure can be readily adapted for use with small specimens because of its sensitivity. The present paper describes this adaptation, measures the variation of the method and correlates results with those obtained using the standard specification procedure. This work was carried out as part of the programme of work of a joint FDI/ISO Task Group* which is engaged in the revision of the IS0 Recommendation R 1566 for dental zinc phosphate cement.

MATERIALS

AND METHODS

Altogether 8 brands of dental zinc phosphate cement were employed in this study (Table I). Cements were mixed at 2311 “C and 50% 5 per cent relative humidity, using the technique and consistency recommended in the IS0 Recommendation R 1566. Small cement discs (10 mm diameter x 1-Omm thick) and standard cement discs (20 mm diameter x 1.5 mm thick) were prepared by the method described by Wilson and Batchelor (1971). They were hardened for 1 hour (at 37 “C) and then placed, *Task Group 4 of the Joint Dentaire Internationale)/ISO revising IS0 recommendations.

FDI (FBdkration Working Group

Table //.-Effect test medium on Volume of test medium* 10 ml 20 ml 50 ml

of volume of results obtained Experimental value? 1.02 1.34 1.58

*Cement A2; powder : liquid ratio 2 g/ml. tResults are given as mg PO4 eluted/g cement and are each the mean of 5 determinations.

singly, in an exact volume of water (1650 ml according to the experiment) contained in a polythene vessel for 23 hours. After removal of the specimens, solutions were transferred to volumetric flasks (50 ml for small specimen work and 100 ml for standard specimens), and diluted to the calibration mark. Aliquots of 10 ml were taken and the phosphate content determined by the ‘molybdenum-blue’ calorimetric method, as described by Wilson and Batchelor (197 1). In studying the effect of pH standard buffer solutions were prepared covering the pH ranges 4.5-6.0 (0.01 M acetic acid/acetate buffers) and 6.0-7.5 (0.01 M barbitone buffers) in 0.5 steps.

RESULTS AND DISCUSSION A number of factors affect the value of the experimental results. The amount of phosphate eluted from the test specimens was found to increase with the volume of the test solution (Table II). The results suggest that a proportion of the soluble sodium dihydrogen phosohate ions are adsorbed on to the surface of the

30

Journal of Dentistry, Vol. ~/NO. 1

cement, thus reducing the amount found in solution. Clearly, in this situation where there is a competition between the gel surface and the solution for the dihydrogen phosphate ions, increase in volume will reduce the amount adsorbed on to the gel surface. In subsequent experiments the volume of test solution employed was standardized at 20 ml, and results using single small discs in this volume of water were compared with those obtained from standard discs immersed in 50 ml water (Table ZZZ). The weight fraction of phosphate eluted from the small specimens is in every case greater than that eluted from standard size specimens-the expected result in view of the previous discussion. Table ///.-Comparison of results using both small and standard size test specimens Cement Al A2 61 :: c2 Dl 02 El E2 Fl F2 Gl G2 Hl H2

Small discs in 20 ml 1.94 1.34 16.2 11.1 196 1.13 2.40 0.54 1.88 0.73 1.31 1.43 2.01 2.19 0.74 0.66

Standard discs in 50 ml 0.69 0.70 11.29 6.69 0.81 0.70 I.10 0.17 0.95 0.29 0.88 0.83 0.98 1.07 0.36 0.29

Results are expressed as mg PO4 eluted/g cement.

Table /K-Variation

The variation in each method was estimated from ten results obtained on cement A (Table IV). The coefficients of variation were almost identical, showing that the use of small specimens does not introduce any additional scatter. The standard deviation (0.21) is similar to that (0.26) found by Wilson and Batchelor (1971) for dental silicate cements. However, the coefficient of variation is larger as the amounts of phosphate eluted are less. The effect of the pH of the test solution on the amount of phosphate eluted is presented graphically in Fig. I, which shows that results are initially affected by this parameter. Phosphate extraction is minimal at pH 50 and increases very rapidly as the pH is reduced below this value and increases somewhat more moderately as the pH increases above this value. The value for the result in distilled water is several times the minimum value at pH 5.0. In view of these results some importance must be attached to the purity of the distilled water as regards pH, which should correspond to that of pure water in equilibrium with the carbon dioxide present in the atmosphere (pH 575.8). Probably, a departure from the use of distilled water is desirable and a buffered test solution would make the test a more robust one. The reason for the observed effect of pH on phosphate extracted lies in the amphoteric nature of the associated metal ion zinc, which goes into solution under acid conditions as the Zn2+ cation, and in alkali media as the zincate anion [Zn0,12-. Zinc compounds, then, tend

in each method using small and standard size test specimens*

Results obtained Range Mid-range Mean value Median value Estimated standard deviation Coefficient of variation Standard error of mean

Small discs in 20 ml

Standard discs in 50 ml

2.42, 2.20, 1.96, 1.93, 1.84, 1.84, 1.84, 1.82, 1.81, 1.70, 1.70-2.42 2.06 1.94 1.84 0.215 11% 0.068

1.72, 1.59, 1.50, 1.48, 1.46, 1.39, 1.38, 1.29, 1.25, 1.24, 1.24-l .72 1.48 1.43 1 .42 0.15 10% 0.0474

*Cement Al ; powder : liquid ratio 2 g/ml. Results are expressed as mg PO4 eluted/g cement.

Wilson et al.

: Zinc

Phosphate

31

Dental Cements

-

3’o-

to have a minimum solubility in regions of intermediate pH, and so these present observations can be explained broadly in terms of the known chemical behaviour of zinc. However, the pH for minimum phosphate erosion is lower that the expected one. Previously, Wilson et al. (1970) had found that fully hardened cements, from which soluble reaction intermediates had been removed, showed least erosion under neutral conditions (pH 7); a result in accord with observations made on variations of the solubility of zinc phosphate with pH (Jurinak and Inouye, 1962). Experimental conditions were different in the present work in that the cements were still hardening and contained reaction intermediates. The shift of the minimum to a lower value must, therefore, relate to the effect that the pH of the medium has on the rate of reaction in the cement. It would appear that an acidic medium tends to accelerate the setting reaction, thus reducing solubility attributable to reaction intermediates. Below pH 5.0 the rapidly increasing solubility of the actual zinc phosphate matrix in acids becomes the predominating factor.

4:o

Fig. I.-Effect eluted.

510

6.0

Calorimetric,

4

8.0

of pH on t!!e amount of phosphate

Specification limit

Fig. 2.-Correlation

7.0

A

0

0

A Ten times scale values

Scale values

mg P201/g cement

chart between the existing gravimetric test procedure and the proposed calorimetric method. Batches designated 1 and 2 were tested by two different workers.

32

The standard method described here, which is to be proposed for incorporation in the revised IS0 standard for zinc phosphate cement, was compared with the existing specification method in order to set corresponding pass/fail limits. Both methods were applied to 8 cements by two workers each testing a separate batch. The correlation between the two methods is presented graphically in Fig. 2. Agreement is sufficient for the purpose of the test and the limit proposed for acceptance is that the amount of phosphate extracted from a zinc phosphate cement, under the conditions of the test, should be 2 mg P,O,/g cement. This maximum limit is comparable to the limit of 0.2 per cent in the existing specification test.

CONCLUSIONS The pair of standard discs (20 mm diameter x 1.5 mm thick) used in the specification test for ‘solubility and disintegration’ may be replaced by a single small specimen (10 mm diameter x 1.0 mm thick) provided that the gravimetric procedure is replaced by a calorimetric phosphate determination. The maximum limit proposed for the new test is 2 mg P,O,/g cement compared with 0.2 per cent in the present specification test. Results suggest that the test is sensitive to pH.

Journal of Dentistry, Vol. ~/NO.

1

Acknowledgements The authors thank the Government Chemist, Dr H. Egan, for permission to contribute this paper. Crown Copyright. Reproduced by permission of the Controller of Her Majesty’s Stationery Office. REFERENCES JURINAKJ. J. and INOUYET. S. (1962) Some aspects of zinc and copper phosphate formation in aqueous systems. Soil Sci. Sot. 26,144-147. PAFFENBARGERG. C., SCHOONOVERI. C. and S~UDER W. (1938) Dental silicate cementsphysical and chemical properties and a specification. J. Dent. Res. 25, 32-87. PAFFENBARGER G. C., SWEENEYW. T. and ISAACS A. (1934) Zinc phosphate cements-physical properties and a specification. J. Dent. Res. 21, 1907-1924. WILSON A. D., ABEL G. and LEWIS B. G. (1974) The solubility and disintegration test for zinc phosphate cements-a chemical study. Br. Dent. J. 137, 313-317. WILSONA. D. and BATCHELORR. F. (1967) Dental silicate cements: I. The chemistry of erosion. J. Dent. Res. 46, 1075-1085. WILSON A. D. and BATCHELOR R. F. (1971) Initial solubility and disintegration of dental silicate cements-a test with miniature specimens. Br. Dent. J. 130, 143-146. WILSON A. D., KENT B. E. and LEWIS B. G. (1970) Zinc phosphate cements: chemical study ofin vitro durability. J. Dent. Res. 49, 1049-1054.

The 'solubility and disintegration' test for zinc phosphate dental cements: the use of small specimens.

Journal of Dentistry, 4, 28-32 The ‘solubility and disintegration’ test for zinc phosphate dental cements: the use of small specimens A. D. Wikon, G...
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