Slow Modification of the Constitution of Solutions of Certain Salts

By Spring, W. Inst. chim. gen., Liege. Rec. trav. chim. (1910), 29, 163-72; SciFinder (Chemical Abstracts Service: Columbus, OH); https://scifinder.cas.org (accessed July 8, 2010). Solns. of a number of salts (KNO3, KBr, Na2SO4, K2Cr2O7, ZnSO4, CuCl2, CuSO4, MnCl2, AlCl3, etc.) prepared in 1905 and kept at ordinary temp. from the action of direct sunlight were examined at intervals during the following four years and were found to become more and more transparent. The degree of transparency was judged by the degree of luminosity in the soln. of a beam of light projected through it by an electric lantern (50 volts and 12 amp.). To ascertain if there had been any molecular change in the solns., during the four years the conductivities of the original and the four year old solns. were determined and compared. The cond. of the original solns. was determined from portions of the 4-year-old solns. which had been evaporated to dryness and redissolved to the original vol. In the majority of cases the 4-year-old solns. showed markedly smaller conds. This may be explained by a hydrolysis of the salt which occurs when the soln. is first made. The "hydrate" [hydroxide?] thus formed is in a colloidal condition and does not affect the electrical resistance; this colloidal soln. by reflection renders more luminous a beam of light passed through the soln. The free acid initially present is the cause of the high conductivity of the solution. In the old solns. the colloidal hydrate and the free acid have greatly decreased, and the conductivity is therefore smaller than before. The state of equilibrium of such a hydrolysis, when once displaced by change of temp., etc., is restored very slowly. The soln. becomes more optically transparent owing to the gradual disappearance of the colloidal hydrate. The K2Cr2O7 soln. alone was anomalous and showed a decrease in resistance of almost one-half on standing four years. This soln. had become yellow, but on evaporating the soln. and redissolving to the same vol. the color was reddish yellow as in the original soln. Reprinted with the permission of the American Chemical Society. Copyright © 2010. American Chemical Society (ACS). All Rights Reserved