John Johnston

THE SOLUBILITY-PRODUCT CONSTANT OF CALCIUM AND MAGNESIUM CARBONATES.

T H E SOLUBILITY-PRODUCT CONSTANT OF CALCIUM AND MAGNESIUM CARBONATES. BY JOHN JOHNSTON. Received June 17, 1915. In connection with certain calculations undertaken primarily to ascertain what degree of purity of magnesium carbonate might be expected if i t were deposited by evaporation at constant temperature of a solution containing both magnesium and calcium carbonates, i t became necessary to determine the appropriate solubility-product constants. Values of these quantities were calculated some years ago by Bodlander,* but these must now be revised because some of the constants employed by him are now known to be erroneous. Bodlander’s calculations for calcium carbonate were revised later by S t ieg l i t~ ,~ in connection with calculations relative to the effect of a change in the amount of carbon dioxide in the atmosphere upon the proportion of calcium carbonate in gypsum deposited by the evaporation a t constant temperature of solutions (e. g., sea water) containing both. The available data have now all been recomputed so as to obtain really comparable values of these solubilityproduct constants which should be consonant with present-day knowledge of the various quantities involved in the calculations. With a knowledge of these constants one can make certain deductions which are important in relation to the question of the nature and character of the solid deposited when solutions containing both carbonates are evaporated down. The constant cannot be obtained directly from observations on the solubility of the carbonate Me03 in pure water for two reasons: ( I ) because of the hydrolysis, the amount of which is in the first instance unknown; (2) because, as will be shown later, there cannot be true equilibrium except the solution be in contact, and have came to equilibrium, with a definite partial pressure of COz which must be measured. But if the partial pressure of COz is known, and the conqentration of the saturated solution in contact with i t (and with a definite solid phase) is deter4 mined, the solubility-product constant can readily be calculated. The equilibrium between the solid carbonate MC03, the solution, and an atmos-