The Several Forms of Calcium Carbonate. 497 



The Solubility of the Several Forms. 



The solubility of any carbonate has a definite meaning only 

 if the partial pressure of C0 2 above the solution at equilibrium 

 is specified ; in other words, we are dealing, not with a binary 

 system, but with a ternary system R 2 — C0 2 — H 2 0, and 

 accordingly the concentration of all three components must 

 be definite at equilibrium.* It follows, therefore, that the 

 partial pressure of C0 2 must be carefully controlled and meas- 

 ured if the results are to be significant ; thus, as an instance of 

 the influence of C0 2 , a change in the proportion from 3 to 5 

 parts per 10000 — an increase which may readily occur in the 

 air of a laboratory within a very short time — alters the amount 

 of calcium in a solution in equilibrium with calcite in the pro- 

 portion of 63 to 75. In other words, the solubility of a car- 

 bonate in distilled water is indefinite unless the concentration 

 of free CO„ in the solution — or the partial pressure of C0 2 in 

 the atmosphere in contact with the solution — is stated ; and it 

 is precisely the first increments in the amount of free CO, 

 which exert the greatest influence upon the solubility of a car- 

 bonate. 



The existing data on the solubility of calcite have been col- 

 lated and discussed in two previous papers,f to which the 

 reader desirous of fuller information is referred. We now 

 have concordant data extending over the whole range within 

 which calcite is the stable phase in contact with water at ordi- 

 nary temperatures ; at 25° this range of partial pressures 

 extends from an extremely small value (about 10" ' 4 atm.), the 

 point at which calcium hydroxide becomes the stable phase, to 

 about 15 atm., at which point calcium bicarbonate is the stable 

 phase. In the comparison of these solubilities there is a great 

 advantage in considering the solubility-product constant;}: 

 instead of the amount of carbonate dissolved ; for, whereas the 

 latter varies with the C0 2 pressure and with the concentration 

 of other salts, the former is practically an invariable quantity 

 for each solid phase at a given temperatnre§ and constitutes, 

 therefore, abetter basis of comparison. The solubility-product 

 constant of calcite at the temperature t, for values of £ between 

 0° and 30°,_is given with sufficient accuracy by the expression 

 log K' c = 8-087 — 0-006Z ; whence the solubility of calcite 



* The sanie is true of sulphides with water. 



f Johnston, J. Am. Chem. Soc.,xxxvii, 2001, 1915 ; Johnston and William- 

 son, ibid., xxxviii, May, 1916. 



% The solubility product is [Ca++] [COT], where [Ca++] and [CO!] repre- 

 sent the actual concentration of these ions in the solution ; whenever the 

 solution is in equilibrium with a definite solid phase (e. g. calcite) this pro- 

 duct attains a constant value characteristic of the particular solid phase. 



§ The influence of change of hydrostatic pressure is so small that it is neg- 

 ligible. 



