Two samples of a standard gelatin were studied, both prepared according to published specifications and washed free from diffusible electrolytes. The isoelectric point of this material was determined in four ways.
1. The pH values of solutions of gelatin in water approached the limit 4.86 ± 0.01 as the concentration of gelatin was increased.
2. The pH values of acetate buffers were unchanged by the addition of gelatin only at pH 4.85 ± 0.01. This gives the isoionic point of Sørensen, which is the isoelectric point with respect only to hydrogen and hydroxyl ions.
3. Gels of this gelatin made up in dilute HCl or NaOH, or in dilute acetate buffers, exhibited maximum turbidity at pH 4.85 ± 0.03.
4. Very dilute suspensions of collodion particles in 0.1 per cent gelatin solutions made up in acetate buffers showed zero velocity in cataphoresis experiments only at pH 4.80 ± 0.01.
No evidence was found for the assumption that gelatin has two isoelectric points at widely separated pH values.
It is concluded that the isoelectric point of this standard gelatin is not far from pH 4.85.