732 



JOHN JOHNSTON AND E. D. WILLIAMSON 



[H 2 C0 3 ]+[C07]+[HC07]) determine [OH"], the degree of alkalinity 

 (or acidity) of the solution; and that no change can be made in any 

 one of these quantities without affecting each of the others. 



Accordingly the solubility of calcite is significant only if the con- 

 centration of free C0 2 is controlled and measured, for changes in the 

 latter, such as may easily occur, exert a large influence on the amount 

 dissolved. 1 This is evident from Table I, which gives the solubility 



TABLE I 



Solubility of Calcite at i6° for Various Values of P 



at 16 for various values of P not far removed from the proportion 

 normally present in the atmosphere (about 3 parts per 10,000). 

 Calculation shows that except for very small partial pressures of 

 C0 2 the calcium in solution is associated almost entirely with 

 bicarbonate — thus even when P is only 0.0005, the proportion as 

 carbonate is only about 2 per cent, whereas when P is 1 . o, the pro- 

 portion is less than 1 part in 30,000; nevertheless, carbonate is still 

 the solid phase which separates out, an excellent example of the fact 

 that it is the solubility relations and not the "affinity" relations in 

 solution that determine which of the possible stable solid phases 

 shall appear. 



1 Neglect of this factor or, in general, a failure to secure equilibrium conditions 

 is responsible for erroneous statements in the literature. For instance, the solubility 

 as given by Treadwell and Reuter (Z. anorg. Chan., XVII [1898], 170) is not a real 

 solubility at all; acceptance of their figure (238 parts per million) has led several 

 writers astray. Cf. op. cit., p. 2009. Thus on this basis J. C. Jones (Science, XX 

 [1914], 829) concluded that the waters of the Lake Lahontan basin are only about 

 one-twentieth saturated with CaC0 3 . 



