1.25 per cent gelatin solutions containing enough NaOH to bring them to pH 7.367 (or KOH to pH 7.203) were made up with various concentrations of NaCl, KCl and MgCl2, alone and in mixtures, up to molar ionic strength. The effects of these salts on the pH were observed.

MgCl2 and NaCl alone lower the pH of the Na gelatinate or the K gelatinate, in all amounts of these salts.

KCl first lowers the pH (up to 0.01 M K+), then raises the pH.

Mixtures of NaCl and KCl (up to 0.09 M of the salt whose concentration is varied) raise the pH; then (up to 0.125 M Na+ or K+) lower the pH; and finally (above 0.125 M) behave like KCl alone.

Mixtures of MgCl2 and NaCl raise the pH up to 0.10 M Na+, and lower it up to 0.15 M Na+ regardless of the amount of MgCl2. Higher concentrations of NaCl have little effect, but the pH in this range of NaCl concentration is lowered with increase of MgCl2.

Mixtures of MgCl2 and KCl behave as above described (for MgCl2 and NaCl) and the addition of NaCl plus KCl to gelatin containing MgCl2 produces essentially the same effect as the addition of either alone, except that the first two breaks in this curve come at 0.07 M and 0.08 M [Na+ + K+] and there is a third break at 0.12 M.

In this pH range the free groups of the dicarboxylic acids and of lysine are essentially all ionized and the prearginine and histidine groups are essentially all non-ionized. The arginine group is about 84 per cent ionized. Hence we are studying a solution with two ionic species in equilibrium, one with the arginine group ionized, and one with it non-ionized.

It is shown that the effect of each salt alone depends upon the effect of the cation on the activity of these two species due to combination. The anomalous effects of cation mixtures may be qualitatively accounted for if one or both of these species fail to combine with the cations in a mixture in proportion to the relative combination in solutions of each cation alone.

Special precautions were taken to ensure accuracy in the pH measurements. The mother solutions gave identical readings to 0.001 pH and the readings with salts were discarded when not reproducible to 0.003 pH. All doubtful data were discarded.

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