Determination of the net electric charge of a polyacrylamide molecule by Donnan potential measurements

The net electric chargez_pof a polyacrylamide molecule (Praestol, Mol. Wt. 3.4×10⁶) in aqueous solution was determined for two pH values (7.2 and 10.0) by measuring the Donnan e.m.f. as function of the solutions NaCl contentc (2.7 mM<c<74 mM). Taking into account a non-ideal behavior of the counterions (Na⁺) mobility, the numerical values ofz_pturned out to be 3100 and 5500 electric charge units for pH=7.2 and 10.0, respectively. A measurable contribution of the assumed non-ideal model for the Na⁺ ions is found forc<15 mM.