The action of neutral salts on the osmotic pressure and other qualities of gelatin

Abstract

Hofmeister and his followers had stated that chiefly the anion of a neutral salt influenced the swelling of gelatin, while the cation had little or no effect. Later on it was stated by colloid chemists that the action of neutral salts on the physical qualities of colloids is the algebraic sum of the opposite action of the two oppositely charged ions of the salt used. Neither of the two statements is correct. The former authors were led to erroneous conclusions by the fact that they always investigated the effect of the salt on the protein in the presence of a high concentration of the salt. Now it happens that the presence of the salt suppresses part of those physical changes in the behavior of the colloids which are caused by the action of the salt upon the protein. The writer avoided this error by using either finely pulverized gelatin or dried pig's bladder, and by washing away the excess of salt after the latter had had a chance to act on the gelatin. Contrary to the statement current in colloid chemistry, the writer found by this method that (as long as the salts are not used in excessive concentrations) only one of the two ions of the salts acts upon the protein. Common gelatin, as well as gelatin previously treated by a base, is influenced exclusively by the cation of the neutral salt used, while gelatin which had previously been treated with an acid is only influenced by the anion of the salt used. In the former case, treatment of gelatin with salts with monovalent cation (NaCl, Na2SO4, etc.) causes an increase in swelling, in viscosity, in the quantity of alcohol required for precipitation, and in osmotic pressure of the gelatin, after the excess of salt is washed away; while when the cation of the salt used is bivalent no such increase is observed.