22 H. C. COOKE 
tions in which more sodium chloride is present than in the “mini- 
mum solution’? will precipitate silver chloride om dilution. 
Can the addition of sodium chloride to silver chloride solutions ever 
so reduce the amount of silver present that the precipitation of silver 
as sulphide becomes imposszble? 
This relation is determinable mathematically. The solubility 
of silver chloride in water at 20° C. is 0.0016 gm., or 1.1X1075 
mols per liter. Assuming complete dissociation, as is permissible 
at such dilutions for a salt of a strong acid and a strong base, we 
should have present in solution 1.11075 mols of Ag ion, and the 
same number of mols of Cl] ion. The solubility product for the 
salt is therefore (1.1 X10~5)?=1.24X10_™. 
The concentration of silver sulphide aqueous solution is 2.2X 
t0~?7 mols per liter,’ and at such extreme dilution the salt may be 
assumed to be completely ionized. The amount of Ag ion present 
in a saturated solution is therefore 4.4X10~™ mols per liter, and 
the amount of sulphur ion, one-half of this. 
The addition of sodium chloride to silver chloride solution, as 
before shown, decreases the amount of silver ion present in solu- 
tion. To prevent the precipitation of the silver as sulphide, the 
amount of silver ion would have to be decreased till the product of 
the silver ion and the sulphur ion present in solution was equal to 
the solubility product for silver sulphide, ie., 3.9X1075°. If 
we assume the concentration of sulphur ion never greater than the 
amount given above, 2.2X10~'7, then the concentration of the 
silver ion must decrease to 4.4X107~* if precipitation is not to 
occur. Substituting this value into the equation Ca,XCcqj=1.24X 
10~7°, we obtain 2.8X10° as the value of the Cl ion. Disregard- 
ing the infinitesimal amount of Cl ion in combination with Ag ion, 
we should therefore require to have present 2.8 million mols, or 
about 165 million grams, of ionized sodium chloride per liter, to 
prevent the formation of silver sulphide under the given conditions. 
With higher concentrations of sulphur ion, even more sodium 
chloride would be required to prevent the formation of silver sul- 
phide. As such concentrations of sodium chloride are impossible, 
it is evident that the amount of this salt in meteoric waters can 
™ Knox, loc, cit. 
