Electrolytes behavior toward colloids

Contrary to their usual behavior, which would place them in the class with the electrolytes, solutions of dyestuffs frequently exhibit a sensitiveness toward electrolytes very similar to that of metal colloids. The precipitates caused by the salts of the alkali metals can be easily returned to the original form just as in the case of suspensions of clay. 

Lea produced his allotropic silver (4) by treating in the cold 200 cc. of a 10 per cent silver nitrate solution with a mixture consisting of 200 cc. of 30 per cent ferrous sulfate, 250 cc. of a 40 per cent sodium citrate solution, and 50 cc. of a 10 per cent sodium carbonate solution. The violet precipitate was filtered and washed in water. In order to remove the impurities the precipitation was repeated a number of times with ammonium nitrate. Finally the solution was evaporated and a mass with a metallic luster obtained that consisted mostly of silver. Lea found that the metal did not diffuse through membranes and that it could be freed by this means from electrolytes. Because of these properties and also because inorganic colloids were considered to be allotropic modifications of the metals in question, Lea decided that his silver must also be an allotropic form. Even today we are not in a position to deny this assertion. Barus and Schneider f have shown that it is not at all necessary to assume allotropic modifications in order to explain the subdivision in water or the behavior of the colloid toward electrolytes. We have therefore no direct evidence that the metal in the colloidal state is not an allotropic modification but the assumption is quite unnecessary and perhaps improbable. On the other hand recent work has shown that allotropy is not so uncommon as it was previously supposed, and if the rule proposed by W. Oswald holds, that the more unstable form appears first, it may very well be that colloidal metals contain, or are, allotropic modifications. 

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