252 WSL,I/S: ARAGONITS FROM SBA-WATER 



What follows is therefore a very brief statement of the methods 

 employed, which may serve to indicate the nature of the problem 

 and the methods used. For the most part the ion concentrations 

 have been calculated from freezing-point data, using certain 

 empirical equations for the ionic equilibria involved. 



The calcium concentration of sea-water [Ca] is about 0.0108 

 molal. This calcium doubtless exists in sea- water in the form 

 of various molecules and complex ions, but on account of the 

 great excess of sodium chloride it seems probable that the other 

 calcium compounds must be very largely transposed into calcium 

 chloride. If so, the calcium-ion concentration can probably best 

 be approximated by considering the equilibrium 



CaClo :^ Ca + + -F 2CI- (3) 



Neglecting intermediate ions and assuming that the ionization 

 of calcium chloride is similar to that of barium chloride, as deduced 

 by G. N. Lewis from freezing-point data,^ the writer has found 

 that this equilibrium can be represented by the following em- 

 pirical equation over a moderate range around 0.0 1 molal. 



[Ca^^] [Cl-]^ 



T „,. • = 0.216 (4.) 



In order to compute the calcium-ion concentration [Ca ++] by 

 means of equation (4) it is necessary to assume a value for the 

 chlorine-ion concentration [Cl~] in sea-water. For the reason 

 given above a quantity of chlorine sufficient to combine with the 

 calcium and magnesium may be subtracted from the total 

 chlorine and the remainder considered to be present as sodium 

 chloride, ionized and un-ionized. The result is about 0.432 mol 

 of sodium chloride. From extrapolation of the curve for the 

 ionization of sodium chloride based on the data of Lewis for dilute 

 solutions it is assumed that the ionization at 0.432 molal would 

 be about 55 per cent. This gives 0.237 for [Na + ] and [Cl~], 

 which is the value that will be used in calculating the extent to 

 which the ionization of the minor constituents is repressed by the 

 excess of sodium and chlorine ions in sea- water. 



From equation (4) it is calculated that when [Cl~] equals 

 0.237 and [Ca + + ] -f [CaCls] = 0.0108, [Ca ++] will have the 



■* Journ. Amer. Chem. Soc. 41: 1959. 1919. 



