By the use of a crosslinked sodium polyacrylate, with a mean polymerization degree of 4850 between two adjacent crosslinks, as a sample, in the system containing various inorganic and organic salts, hydrochloric acid, and sodium hydroxide, examination was made on the effect of the concentration of the salt added, C
s, on the degree of equilibrium swelling, Q
v, of these salts. In the system of univalent-univalent inorganic salts, there was no difference in Q
v according to the kind of cations present but a slight difference in Q
v was seen according to the kind of anions present at around 0.1 N of C
s. This difference appeared markedly in the organic salt system, and the values of Q
v decreased in the order of sodium carbazol-chrome sulfonate>sodium acetate≒sodium benzoate>dl-methylephedrine hydrochloride>ephedrine hydrochloride>pyridoxine hydrochloride. In the system of bivalent-univalent salts, decrease of Q
v was marked and its values were not affected by C
s. In the univalent-univalent salt system, relationship between Q
v5/
3 and C
s2/
3 was linear, which was found not to agree with the swelling theory of Vermaas and Hermans.
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