August 26, 1922] 



NA TURE 



287 



Naturally, the earliest physical properties to be in- 

 vestigated were the viscosity of the sol and the swelling 

 of the gel, and it was soon found that the relations 

 were very complicated, depending on previous history, 

 even in systems made up from gelatin and pure water 

 alone. For example, shaking, or repeated passage 

 through a viscometer, will decrease the viscosity of a 

 gelatin sol ; at ordinary temperatures the viscosity of 

 a freshly made sol gradually increases, whilst that of 

 a freshly diluted sol gradually decreases ; in a freshly 

 made gel the intensity of the Tyndall effect gradually 

 increases ; and so on, all indicative of the formation 

 of a structure and of the attainment of an equilibrium 

 of some kind. 



If the results obtained with gelatin in pure water 

 are so complicated it is no wonder that they are still 

 more so in the presence of acids, bases, and salts. Von 

 Schroeder showed that in the presence of either hydro- 

 chloric acid or sodium hydroxide a maximum viscosity 

 of the sol is attained at a low concentration of either 

 of these substances. Again, according to other in- 

 vestigators, the effect of equivalent (tenth normal) 

 solutions of various acids on the swelling is indicated 

 by the following series, which is known as a Hofmeister 

 series, after the investigator who was the first to 

 examine the effects of different salts on the physical 

 properties of the proteins : 



HC1=>HN0 3 > acetic acid > HjSOj ^ boric acid. 

 With the sodium salts of various acids the swelling 

 decreases in the order : 



Thiocyanates > iodides > bromides > nitrates > chlorates > 

 chlorides > acetates > tartrates > citrates > sulphates. 



Moreover, the order in the series may be affected by 

 the concentrations of the substances used. 



Such series are very difficult to understand, since 

 the order of the compounds does not bear much relation 

 to their ordinary chemical properties ; for example, 

 it is difficult to understand why acetic acid comes 

 between nitric and sulphuric acids. 



A way out of such difficulties has been found in 

 recent years by the realisation that gelatin, like other 

 proteins, behaves as an amphoteric substance and that 

 its properties in solution depend on the hydrion con- 

 centration. For progress in this direction we are 

 chiefly indebted to the work of Procter in England, 

 Pauli in Austria, and Loeb in America, the basic ideas 

 being due to Michaelis and Sorensen. 



Gelatin is a stronger acid than base, so that hydrion, 

 in the form of an external acid, has to be added to 

 the solution in order to bring the gelatin to the iso- 

 electric condition. At the isoelectric point the hydrion 

 concentration, C H , is approximately 2-5 x io~ 5 , that is, 

 the pH (= -log C H ) is 4-7, which is on the acid side 

 of the neutral point of water (pH = yo). The theory 

 of amphoteric electrolytes shows that at the isoelectric 

 point their solutions should contain a maximum number 

 of neutral particles and should therefore possess peculiar 

 properties ; in accordance with this it is found that 

 the properties of swelling, viscosity, osmotic pressure, 

 etc., show a minimum at that point. 



On the acid side of the isoelectric point, i.e. at 

 pH<4'7, gelatin should behave as a base and form 

 gelatin-acid salts, whilst on the alkaline side, pH>4"7, 

 it should act as an acid and form metal gelatinates. 

 Loeb has endeavoured to show that this is true in 



NO, 2756, VOL. I IO] 



several ways, of which the following may be quoted, 

 where use is made of silver nitrate and gelatin which 

 is brought to different pH's, all less than pH = 7-o, by- 

 treatment with varying concentrations of nitric acid. 

 It can be predicted that on the alkaline side of the 

 isoelectric point the gelatin, when treated with silver 

 nitrate, will combine with the silver forming a silver 

 gelatinate, and that the amount formed will be greater 

 the higher the pH. If such a silver gelatinate is 

 formed the silver should not be readily washed out 

 by water and should remain in the gelatin after washing. 

 On the acid side of the isoelectric point the gelatin 

 should form gelatin nitrate, and it should be easy to 

 remove the silver by washing. The following analytical 

 figures show the agreement between theory and ex- 

 periment. 



c.c. o-oiN-Ag in combination with 0-25 gm. Gelatin at different pH's. 



pH 3-6 3-7 3-9 41 4"3 4'6 47 5'° 5*3 57 6-i 6-4 

 c.c. 0-5 0-3 0-3 o-2'o-2 0-2 0-55 1-25 3-2 4-0 4-85 4-9 



The retention of the silver by gelatin at a pH>4"7 is 

 well shown by the fact that if test tubes containing 

 samples of the various gelatins are exposed to light, 

 those which are on the alkaline side of the isoelectric 

 point blacken, whereas those on the acid side do not, 

 but remain transparent even when exposed to light for 

 months. 



Results similar to those with silver nitrate are 

 obtained when a nickel or copper salt is used. With 

 potassium ferrocyanide the gelatin should retain the 

 ferrocyanide, as gelatin ferrocyanide, on the acid side 

 of the isoelectric point, and this is found to be the case. 



Results such as the above indicate the necessity of 

 knowing the pH when any investigations are carried 

 out, and also of making comparisons of any particular 

 property at the same pH. When such comparisons 

 are made, Loeb has shown that the Hofmeister series, 

 with their anomalies, disappear ; for example, the 

 various monobasic acids, and acids such as phosphoric, 

 oxalic, and citric acids, which dissociate into two ions 

 at ordinary dilutions, have the same effect on swelling, 

 viscosity, etc., at the same pH. Dibasic acids, such 

 as sulphuric acid, which dissociate into three ions at 

 ordinary dilutions, should, and do, give different effects 

 from the monobasic acids. Similar results were found 

 with alkalis, and abnormal effects produced by such 

 salts as sodium acetate were shown to be due to the 

 alteration of the pH of the gelatin solutions when the 

 salt was added. 



The increased swelling, viscosity, etc., which take 

 place on either side of the isoelectric point and reach 

 a maximum at pH's of about 3-5 and 8-5 respectively, 

 are attributed by Pauli to the greater hydration of the 

 gelatin ions formed, as compared with that of the 

 neutral molecule, but Loeb is not in agreement with 

 this. The latter postulates the existence in any protein 

 solution of molecularly dispersed particles, floating side 

 by side with submicroscopic particles occluding water, 

 the amount of which is regulated by the Donnan equili- 

 brium (Procter was the first to apply the Donnan 

 equilibrium to the study of gelatin solutions). The 

 osmotic effects are determined by the molecular par- 

 ticles, the viscosity effects by the submicroscopic 

 particles. Any influence in the solution (change in 

 H-ion concentration) by which the molecular dispersion 



