738 



SCIENCE 



[Vol. LVI, No. 1401 



When little acid, e. g., HCl, is added to the 

 solution of isoelectric -gelatin, gelatin chloride 

 is formed and some free acid remains, due to 

 hydrolytic dissoeiiation. Hence both a (the 

 concentration of CI ions in combination -with 

 protein) and y (the CI ions of the free HCl 

 existing through hydrolysis) increase, but 3 

 inioreases at fii'st more naipidly than y and hence 

 the excess of concentration of ions inside over 

 that of ions outside increases until the greater 

 part of protein is transformed into protein 

 chloride, when the excess of crystalloidal ions 

 inside over those outside reaches a maximum. 

 From then on z increases comparatively little 

 While y increases considerably with further 

 addition of acid, so that z becomes negligible 

 in comparison with y. Thiis explains why the 

 Donnan coi-reotion becomes zero again when 

 enough acid is added, and why tie observed 

 osmotic pressure becomes as low again as at 

 the isoelectric point. 



In the same way it can be shown why the 

 addition of salt has only a depressing effect on 

 the osmotic pressure. Let us assume that there 

 is inside the bag a gelatin chloride solution of 

 Pjj 3.0 to which NaOl is added, z (the concen- 

 tration of CI ions in comibiMation with 'the 

 gelatin) will not increase wdith the addition of 

 salt, wliile y (the concentration of the CI ions 

 not in comlbination with gelatin) will increase. 

 Hence with the increase in the concenti-ation 

 of the salt the value of 



y42/2 -f- iyz -\- z2 — y42/3 -f lyz 

 will become smaller, finaUy approaching 



pH 16 la 20 Z2 Z4 2£ 7J 30 3: 34 36 33 40 42 44 « 



Fig. 3 



Wihen another salt than a chloride, e. g., 

 NaNOa, is added to a solution of gelatin chlo- 

 ride, we may assume that the gelatin in solu- 

 tion is gelatin nitrate. 



Fig. 3 gives a comparison of the curves for 

 the observed osmotic pressure and for the 

 Donnan correction. Both curves rise in a par- 

 allel way from the isoelectric point reaching a 

 maximum which is 450 mm. H2O pressui-e in 

 the case of the observed osmotic pressure and 

 slightly lower in the ease of the Donnan eor- 

 reetion. The observed osmotic pressure should 

 be higher than the Donnan correction by the 

 osmotic pressure due to the protein solution 

 itself. A difference exists an the values 

 between pu 4.6 and 3.2 but disappears later, 

 and this difference is in all proba/bility the ex- 

 pression of value a, i. e., the osmotic pressure 

 due to the protein itself. The disappearance 

 of this difference at Ph below 3.2 is pix>bably 

 due to the fact that an erix)r of one unit in the 

 second decimal of the pjj causes a considerable 

 error in the calculations of z which increases 

 when the pu is too low. 



Fig. 3 shows that when we correct tihe ob- 

 served osmotic pressure for the Donnan effect 

 it follows that the infiuenee of the pjj of the 

 acid on the osmotic pressure is entirely or prac- 

 tically entirely due to the excess of the con- 

 centa'ation of crystalloidal ions inside the mem- 

 brane over that outside and that this excess is 

 caused by the Donnan equdibriiun. The 

 osmotic pressure of the protein itself is either 

 not altered at all by the addition of acid or if 

 it is altered the effect is too small to be no- 

 ticeable. There is then nothing left for the 

 "dispersion theory" or for any other of the 

 ooUoidal speculations to explain. These re- 

 sults were confinned for crystalline egg albu- 

 min and casein by the writer and for edestin 

 by Hitchcock. We now understand why only 

 the valency and not the nature of tihe ion plays 

 a role in the osmotic pressure of protein solu- 

 tions. The equilibrium equation is one of the 

 second degree when tlie ion with which the pro- 

 tein is in combination is monovalent while it is 

 of the third degree when the ion is divalent. 

 Only the valency of the ion and not its nature 

 enters into tihe Donnan equation. 



We can therefore summarize these results by 



