THE ACTION OF NEUTRAL SALTS 99 



cases that ion which has the opposite sign of charge to that of 

 the protein ion depresses the osmotic pressure, swelling, and 

 viscosity of proteins. 



2. ION SERIES AND THE ACTION OF SALTS ON PROTEINS 



From what has been said, it is clear that only one of the ions of 

 a neutral salt influences the physical properties of a protein, 

 namely that ion which has the opposite sign of charge to the 

 protein ion; and this influence is of a depressing character. We 

 will now show that this effect depends only upon the valency of 

 the depressing ion and that different ions of the same valency 

 have the same depressing effect. It is necessary to compare the 

 relative depressing action of low but equal concentrations of 

 different salts upon the physical properties of a gelatin salt, for 

 example, gelatin chloride of a definite pH; e.g., 3.0. As can be 

 easily surmised, the addition of a salt will in many cases alter the 

 pH of the solution and this alteration will be larger in the case of 

 certain salts, e.g., Na acetate, than in the case of others, e.g., 

 NaCl. Unless we take into consideration these variations in the 

 pH caused by the addition of salts there will be danger of 

 erroneously ascribing the influence of a variation in the hydrogen 

 ion concentration to an influence of the nature of the anion. 

 The Hofmeister ion series, as far as they refer to proteins, are 

 largely due to this error. 



The method of our experiments was as follows: 50 c.c. of a 1.6 

 per cent solution of originally isoelectric gelatin contained enough 

 HC1 to make the pH = 3.0. To this were added 50 c.c. of H 2 O 

 or of a salt solution of different molecular concentration, and the 

 viscosity of this mixture was measured using those precautions 

 which were described in the preceding chapter. 



Figure 33 gives the curves representing the depression of the 

 relative viscosity of a gelatin chloride solution of pH 3.0 by dif- 

 ferent concentrations of salts with monovalent anion; namely, 

 NaCl, NaH 2 PO 4 , NaCNS, NaH tartrate, NaH 2 citrate, and Na 

 acetate. The curve for Na 2 SO4 is added for comparison. The 

 monosodium salts of weak dibasic and tribasic acids dissociate 

 electrolytically into a Na ion and a monovalent anion, H 2 PO4, H 

 tartrate, H 2 citrate, etc. All the salts mentioned in Fig. 33 are 

 therefore salts with monovalent anion with the exception of 



