﻿Calcium Carbonate in Water. 967 



carbonate here considered, a very small proportion of carbon 

 dioxide corresponds to an enormous increase in the solubility. 

 For this reason, it has been possible to investigate, by the 

 ordinary methods, the equilibrium of the system even for 

 very small partial pressures of carbon dioxide (Schloesing, 

 Compt. rend, lxxiv. p. 1552 (1872); Treadwell and Reuter 

 (Zeit. anorg. Chem. xvii. p, 170 (1898)). Beyond a certain 

 limit, however, the results become too inaccurate, owing to 

 the diminished solubility. The equilibrium under ordinary 

 atmospheric conditions, for example, which is of importance 

 in water analysis, cannot be determined by the usual volu- 

 metric methods. 



If a current of air free from carbon dioxide be drawn 

 through a bicarbonate solution, equilibrium is re-established, 

 by precipitation of carbonate, and this precipitation will 

 continue until all bicarbonate disappears from the solution, 

 i. <?., until all free carbon dioxide is removed. There will 

 therefore remain a saturated solution of calcium carbonate. 



Much confusion has always existed in regard to the state 

 in which calcium carbonate falls out of solution. The 

 precipitate obtained differs according to the concentration of 

 the solution, the rate at which precipitation occurs, the 

 temperature of experiment, the partial pressure of carbon 

 dioxide above the solution, and the presence of other salts. 

 The most complete experiments are those of Rose (Jour. 

 prak. Chem. lxxxi. p. 383 (1860) ; lxxxii. p. 351 (1861)). 

 From bicarbonate solutions above 90° C, arragonite is 

 deposited ; between 90° C. and 30° C, a mixture of calcite 

 and arragonite ; under 30° C. only calcite. The stability 

 of the arragonite varies according to the conditions of 

 formation. 



By double decomposition between concentrated solutions 

 of a calcium salt and a carbonate in the cold, amorphous 

 calcium carbonate is said to be precipitated, transformation 

 to calcite following. The formation of a third crystalline 

 form, also unstable, has lately been described by Meigen 

 (Chemiker-Zeitung, xxxiv. p. 1015 (1910)). 



In the experiments described below the primary object 

 was to determine the degree of solubility of the carbonate, 

 by the complete withdrawal of the carbon dioxide from 

 bicarbonate solutions, at the temperatures of 25° C, 50° C, 

 and 100° C. The values obtained would, it was expected, 

 give some indication of the form in which precipitation had 

 occurred, and also check the hVures for the solubility o£ 

 calcite and arragonite previously obtained. 



The direct preparation of the bicarbonate solutions, by 



