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Volume 47, 1914
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Art. LII.—On the Influence of Pressure on the Solubility of Tricalcic Phosphate in Carbonic-acid Solutions.

[Read before the Canterbury Philosophical Institute, 1st July, 1914.]

Robert Wadington, in 1871 (Journ. Chem. Soc., p. 80), treated samples of bone-ash repeatedly with a solution of carbon dioxide in water, and found a solubility of tricalcic phosphate of 0.147 gram per litre. T. Schloesing (Comp. Rend., 1900, vol. 131, pp. 149–53) determined the solubility of freshly precipitated tricalcic phosphate in pure water, in carbon-dioxide solutions of various strengths, and in similar solutions containing calcium bicarbonate. He found in 1 litre of—

Water saturated with carbon dioxide Water ⅕ saturated Water ½5 saturated Pure water Calcium-bicarbonate solution
91.9 48.5 6.9 1 1

milligrams of phosphoric acid, as P2O5. This result seemed to suggest that the solubility of the tricalcic phosphate depended, to some extent at least, on the amount of carbon dioxide in solution.

As no record dealing with the question could be found in the Journal of the Chemical Society, nor in the Journal of the Society of Chemical Industry, it was determined to ascertain the effect of carbon dioxide at higher pressures than atmospheric upon the solubility of the phosphate.

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After the breaking of the first apparatus, which was composed partly of glass, the apparatus used throughout the work was that shown in fig. 1, half in view and half in section. It consists of a heavy copper tube, fitted with a heavy cap, through which passes a copper capillary tube. The

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Fig. 1.

latter is connected to a bomb of liquid carbon dioxide, and a pressure-gauge registering up to 40 atmospheres. The apparatus is shaken by carrying the bottom round in a horizontal pulley run by an electro-motor. The tube bears at the side a small needle valve, through which the solution can be run off.

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A small quantity of Merck's pure precipitated tricalcic phosphate is placed in the apparatus, and about 330 c.c. of water added. After saturation at the required pressure with carbon dioxide, the whole is shaken for five hours, then allowed to stand for eighteen hours, after which the solution, which is quite clear and bright, is run off through the needle valve. The phosphate is estimated by an adaptation of Woy's molybdate method (Chem. Zeit., vol. 21, p. 442). The phosphate is precipitated as ammonium phosphomolybdate, which, after solution in ammonia and reprecipitation by nitric acid, is heated in a Gooch crucible to dull redness. By this means a dark-blue substance is formed, phosphomolybdic anhydride, 24MoO3. P2O5, which contains 3.946 per cent. of P2O5.


Series I.—The first series of experiments performed in the glass apparatus was unsatisfactory, but showed an increase of solubility with increase of pressure.

Series II.—About 0.75 gram of tricalcic phosphate was taken in 330 c.c. of water. The range of pressures was from 0.5 atmosphere up to 18 atmospheres, at half and 1 atmosphere intervals. The solubility of the phosphate, in terms of grams of P2O5 per litre of solution, varied from 0.0995 at 1 atmosphere to 0.3804 at 18 atmospheres, the curve being shown in fig. 2. The general trend shows a regular increase of solubility with increase of pressure.

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Fig. 2.

During this series of runs the enamel with which the apparatus was lined began to flake off from the interior, until at the end practically none remained. The surface was only very slightly attacked, and on no occasion could any appreciable blue cuprammonium salt be obtained on adding ammonium hydroxide and filtering. The tube was cleaned thoroughly and re-enamelled.

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Series III.—Experiments made to ascertain, if possible, the effect of varying the amount of phosphate taken and the time of stirring showed that the solubility is increased both by increasing the amount taken and the time of shaking. Hereafter, therefore, weighed quantities of the phosphate were taken, the contents of the tube were shaken for five hours, then allowed to stand until run off for estimation.

Series IV.—0.25 gram was taken, giving results as shown in fig. 3. The two higher results were obtained by shaking for ten hours.

Series V.—1 gram taken. Both curves IV and V, though widely separated, show marked regularity, as far as they were taken.

Series VI.—0.5 gram taken. Here (see series VI, fig. 3) a very large drop was observable between 25 and 30 atmospheres, for which there was no accounting.

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Fig. 3.

An attempt to confirm this peculiarity was made in Series VII, but the enamel began to flake in the second run. Later it flaked during the first run after re-enamelling, and on evaporating the solution the solid residue had a pale blue-green tint in the mass, and gave a slight blue coloration with ammonia. The slight surface action on the copper probably had some effect upon the solubility, which is here very irregular. After trying varnish in the tube without success, it was plated, as continuously as its structure would permit, with standard gold.

Series VIII was undertaken to verify, if possible, the peculiar results of series VI. The results, at pressures from 42 down to 4.4 atmospheres,

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while consistent with one another, were not so with those of series VI. They show a regular increase of solubility with pressure. Here again after a few runs the presence of copper was found on evaporating the solution, rather more being present at high than at low pressures. The gold appeared to be quite intact, except for a ring about in. wide round the top, above the part wetted by the solution during shaking.

Series IX (1 gram) and Series X (2 grams) also gave very regular results.

In order to show more clearly the relations between amount of phosphate taken, pressure, and solubility, all the curves are placed together in fig. 3.

Analyses were made of the following materials:—

A. The original material from the bottle.

B. The undissolved phosphate left in the bottom of the copper tube was collected, and dried at 100° C. It contained a very slight trace of copper, which was not estimated, and gave a very slight effervescence with nitric acid.

C. The phosphate was obtained by evaporating on the water bath the solution not required for estimation. The sample which was analysed for copper was from several high-pressure solutions, and contained more copper than usual. This material also gave an effervescence on treatment with nitric acid, showing the presence of a carbonate, derived probably from calcium bicarbonate in the solution. This carbon dioxide, however, was in too small a quantity to be estimated directly.

The results of average analyses were:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

A. B. C.
CaO 48.68 48.90 50.45
P2O5 38.77 38.67 37.29
Loss (at red heat) 11.07 11.15 10.86
CuO 1.19
98.52 98.72 99.79

The loss at red heat in A must be due to water only, and the analysis corresponds to 3.18CaO. P2O5.2.43H2O. If in B the loss is assumed to be due wholly to water, which is nearly true, the analysis shows the material to be 3.2CaO. P2O3. 2.3H2O. In C, the ratio of CaO to P2O5 is 3.44. But the maximum of CaO which can combine with P2O5 is 3CaO. Hence the remaining 0.44 CaO must be combined in some other way, which under the circumstances may be assumed to be with CO2, as CaCO3. Similarly, if the CuO is in the form of carbonate, the amount of CO2 present will be 5.72 per cent., leaving 5.04 per cent. of loss at red heat as 1.07 molecule of water. Thus the analysis gives 3CaO. P3O5. 0.44CaCO3. 0.02CuCO3. 1.07H2O. It is also possible that some of the CuO may be combined with P2O5 instead of with CO2.

The soluble compound which is formed is unstable, since it is deposited as a precipitate when the carbon dioxide is removed, either on standing, or more completely on boiling. The presence of calcium carbonate, or in solution calcium bicarbonate, indicates that an interaction has taken place, the carbonic acid having attacked the tricalcic phosphate with the formation of some calcium bicarbonate.

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The solubility of tricalcic phosphate in carbonic-acid solution increases directly with increasing pressure of carbon dioxide.


The solubility also increases with increasing time of contact between solvent and solute.


Differences of temperature appear to have very little effect.


The solubility is irregular when the surface of the solution-vessel is not uniform throughout a series of runs.


The presence of small quantities of copper in solution causes a small decrease in the solubility


An interaction takes place between the tricalcic phosphate and the carbonic acid.


In (2) practically no difference is observable between a solution run off after standing in the solution-vessel for eighteen hours and the same solution after standing forty hours. This is due to the sinking of the undissolved portion of the phosphate to the bottom of the vessel, and the consequent removal from the sphere of action.

In (3) any increase in solubility with rising temperature is probably counteracted by the concomitant lessened solubility of the carbon dioxide.

It is suggested that further work might be carried out to determine the solubility with varying times of contact between solid and solvent, and also to ascertain the effect of metallic salts, such as those of ammonium, calcium, and copper. More accurate work also could be done if the vessel were lined with a quite continuous coat of platinum.