OCCLUSION OF DISSOLVED SUBSTANCES. 3 1 



THE OCCLUSION OF DISSOLVED SUBSTANCES BY ARGENTIC 



CHLORIDE. 



It is well known that all precipitates have a tendency to carry 

 down with them other substances from the solution in which they are 

 formed.* According to the nature of the precipitate, the mechanism 

 of this contamination varies. A basic precipitate such as ferric hydrox- 

 ide is likely to carry down traces of acid in the form of basic salt ; an 

 acid precipitate such as silica is likely to carry down traces of base ; 

 many substances are contaminated in the act of precipitation by the 

 admixture of isomorphous substances in solid solution ; all or nearly 

 all crystals contain within themselves minute cells inclosing mother 

 liquor ; and very finely divided precipitates, because of their large sur- 

 face, pertinaciously adsorb in varying amount impurities from the super- 

 natant solution. Obviously contamination from any of these sources 

 must be scrupulously avoided in accurate work, and the peculiar qual- 

 ities of each precipitate under investigation must be studied. 



As has been said, argentic chloride was known by Stas to occlude 

 argentic nitrate, which is easily detected by its blackening effect upon 

 fusing the contaminated chloride. f This test is a delicate one. Stas 

 found that upon working with solutions of argentic nitrate weaker 

 than decinormal, the occlusion was so slight as to be negligible ; there- 

 fore in his early experiments he took great pains never to use solutions 

 stronger than this. Later he seems to have overlooked this precaution, 

 since he used much more concentrated solutions. Although realizing 

 that argentic nitrate is occluded by the precipitate, Stas seemed never 

 to have guessed that other salts might also be taken up in this way. 

 Evidently the question was one which needed further investigation, 

 and the outcome of the investigation is recorded below. 



It was convenient at first to determine whether or not the occlusion 

 of sodic nitrate or sodic chloride would also affect the appearance of 

 the silver salt on fusion, in order to discover an easy qualitative test 

 for the presence of either. As a matter of fact, the former salt was 



*Jannasch & Richards, J. Prkt. Chem., 39, 321 (1889) Schneider, Z. phys. 

 Chem., 10, 425 (1892) ; Richards, Proc. Am. Acad., 35, 2>77 (1900) ; Z. phys. 

 Chem., 46, 189 (1903). 



tRichards, Proc. Am. Acad., 29, 76 (1893). The blackening occurs in full 

 measure only when the argentic nitrate is occluded from aqueous solution. Per- 

 fectly dry argentic nitrate fused with dry chloride is but slightly decomposed, and 

 the fused mass is merely gray, not black. The probable explanation is obvious. 



