118 A TREATISE ON METAMORPHISM. 



If the electrolyte consists of polyvalent ions in the proportion mA:nB, the 

 solubility product takes the form: a m b n = constant." 



From the foregoing follows Ostwald's statement of the first law of pre- 

 cipitation, already given: "Whenever in any liquid the solubility product 

 of a solid is exceeded, the liquid is supersaturated with respect to that solid," 6 

 and therefore precipitation of the salt follows. Of the various salts which 

 may be precipitated from a solution, that one will be precipitated first 

 whose solubility product exceeds its constant of solubility. 



Ostwald illustrates this by the cases already cited: If a solution of 

 BaCL be added to Na 2 S0 4 , BaS0 4 will be precipitated. According to 

 Ostwald's view, this happens because the solubility product of the ions in 

 BaS0 4 is very small. 



The second law of precipitation follows from the faet that "the solu- 

 bility of one salt is depressed in the presence of another having a common 

 ion." c This is equivalent to saying - that "the solubility of each molecular 

 species in a mixture is always smaller than for the particular species when 

 alone.'"* Hence, when to a solution containing certain ions a solution is 

 added which has an ion in common with one of those already in the solu- 

 tion, supersaturation and precipitation are promoted. An example of this 

 is the addition of HC1 to a solution of BaCl 2 . The chlorine ion is common, 

 and if the solution is near saturation before the HC1 is added, BaCL will be 

 precipitated. Again, if one adds a saturated solution of NaC10 3 to a satu- 

 rated solution of KCIO3, an abundant precipitate of the latter salt will form. 



The above law is a general statement which includes the rule that "The 

 addition to a solution of a liquid which is able to form a homogeneous whole 

 with the solution causes precipitation of more or less of the substance in 

 solution if that substance is insoluble in the liquid which is added."" This 

 rule is illustrated by the same examples. It follows from this that "in 

 order to precipitate a substance completely from its solution, an addition of 

 an excess of the precipitant is an advantage."-^ 



The converse of the second law of precipitation is : The solubility of a 

 salt increases on the addition of a second salt containing no ion in common. 



"Ostwald, W., Foundations of analytical chemistry, translated by George McGowan, Macmillan 

 & Co., London, 1895, p. 76. 



* Ostwald, Foundations, cit., pp. 76-77. 



c Nernst, W. , Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 

 1895, p. 446. 



^Nernst, cit., p. 453. 



<• Ostwald, Solutions, p. 90. 



/Nernst, cit., p. 449. 



