AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 249 



mination based on the direct solubility of calcium carbonate in water. 

 The latter method gives excellent results with salts like silver chloride, 

 calcium or barium sulphate, but calcium carbonate is largely hydrolyzed 

 by water into the hydroxide and carbonic acid and bicarbonate, and this 

 hydrolysis, together with the possibility of absorption of carbon dioxide 

 and consequent change in the equilibrium conditions, affects these direct 

 determinations. Bodlaender has estimated that about 80 per cent of the 

 calcium carbonate is decomposed by water in its saturated solution in pure 

 water. In Schloesing's experiments the hydrolysis is practically completely 

 overcome by the measurements being made in the presence of an excess of 

 carbon dioxide. No calculations were made by Bodlaender as to the extent 

 of any hydrolysis in these experiments of Schloesing, but we can readily 

 determine it as follows: In experiment 1, in which we have the smallest 

 pressure of carbon dioxide and therefore the most favorable conditions for 

 hydrolysis, we have as the pressure of carbon dioxide (P) 0.000504 atmos- 

 phere. Consequently, according to equation (29) 



CH2CO3 = 0.04415 X 0.000504 = 2.225 X lO"' 



According to the table we have 



Chc03 = 0.001356 



Now, for carbonic acid we have (equation III, p. 242), 



ChXChcOs = 3.04x10-^XCh,co3 

 and inserting the given values for ChcOs ^^^ ChjCOs* we find : 



3.04X10-^X2.225X10- :^^ 



^ 0.001356 ^.yyxiu 



Npw, for the ionization of water at 16°, we have: 



ChXCoh = 0.55X10-" 

 and consequently 



0.55 X 10-" , , ,^_. 

 ^^^- 4.99XlO-> ^^-^X^Q"' 



Calcium bicarbonate, the chief salt in solution, is hydrolyzed according 

 to 



^Ca(HC03)2 + HOH1:; iCa(0H)2 + H^COg 



and since the equivalent concentration of the bicarbonate ions in experi- 

 ment 1 is 0.001356, the part hydrolyzed is 1.1 X 10-*/0.001356, or about 0.08 

 per cent. So, even in this first, least favorable experiment, the hydrolysis 

 is negligible. In experiment 5, we find in a similar way 



Ch = 7.43X10-8 CoH = 7.40X10-8 



and the part hydrolyzed is 7.4x10-^0.0051, or 0.0014 per cent. 



Hydrolysis is reduced therefore to almost nothing, and the value found 

 for KcaC03> 1.26X10~8, needs no correction from this source. 



