THE RELATIONS OF EQUILIBRIUM BETWEEN THE CARBON DIOXIDE OF THE 

 ATMOSPHERE AND THE CALCIUM SULPHATE, CALCIUM CARBONATE, AND 

 CALCRIM BICARBONATE OF WATER SOLUTIONS IN CONTACT WITH IT. 



When two difficultly soluble salts, such as barium sulphate and barium 

 carbonate, are formed in a given medium, for instance by the addition of 

 barium chloride to a mixture of potassium sulphate and carbonate, they are 

 precipitated, as is well known, in the order of their insolubility. In the 

 given case barium sulphate, the less soluble salt, is precipitated first, the 

 more soluble carbonate last. We apply this principle in the familiar case 

 of the volumetric determination of chlorides by titration with silver nitrate, 

 in which potassium chromate is used as indicator; silver chromate is very 

 difficultly soluble, but silver chloride is less so, and is precipitated almost 

 completely and within the limits of exact quantitative analysis, before any 

 solid red silver chromate can permanently be formed, the first persisting 

 appearance of the latter being taken, indeed, as the evidence or indication 

 that the precipitation of the chloride has just been completed. 



The application of the laws of physical chemistry, especially those of 

 chemical and physical equilibrium, to such cases of precipitation shows, how- 

 ever, that there must be a limit to this principle of the order of precipitation; 

 the principle itself may be derived by the application of these laws and is 

 always, of course, subject to them. The existence of such a limit and its rela- 

 tions to the solutions in contact with given salts have been investigated 

 experimentally in a number of cases, the most notable investigation being 

 the classical one of Guldberg and Waage * on the conditions of equilibrium 

 between barium sulphate and carbonate, and potassium sulphate and car- 

 bonate. The limiting values in all such cases are of extreme interest, as 

 from them certain definite conclusions may be drawn as to the nature of 

 precipitates formed in given cases; or, vice versa, from the nature of the pre- 

 cipitate, conclusions as to the composition of the medium may be drawn, 

 and the existence of a limiting value leads to the possibility of a complete 

 reversal of the usual order of precipitation under given conditions. 



As has just been mentioned, and as will be shown in detail presently, 

 the limiting value is for two given, little-soluble salts most intimately asso- 

 ciated with the composition of the liquid medium from which precipitation 

 occurs; the composition of the medium may in turn be dependent for one 

 or more of its essential components, e.g., dissolved carbonic acid, on the 

 nature of the atmosphere above the solution, with the result that the com- 

 position of the atmosphere may become a function in the mathematical 

 expressions deduced by the application of the laws of physical chemistry 

 to the facts of precipitation. 



» Journal fur Praktische Chemie (2), 19, 69 (1879). 



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