I 



68 COLLOID CHEMISTRY OF THE PROTEINS 



and the consequent ionisation proceeds, as is shown by the in- 

 crease in viscosity, which, however, decreases once again when 

 excess of acid has reduced the ionisation, and thus increased the 

 number of neutral particles. The precipitation by alcohol also 

 is reduced to practically nil as the ionisation of the acid protein 

 increases, but reappears again when the ionisation is reduced. 



The difference in properties between ionised and neutral 

 albumin, which has already been sketched in the previous 

 chapter, and to which we shall return later, is reflected in the 

 behaviour of albumin chloride. But, whereas the behaviour 

 of albumin in the iso-electric region (in the presence of buffer 

 solutions) is a question of the existence of ionic and of neutral 

 particles of natural albumin (or of the polyamino-acids of 

 E. Fischer), we have here to deal with the ionic and neutral 

 particles of the albumin salts formed when the protein combines 

 with acids. The structure of these salts must now be discussed, 

 and first of all the salts formed with different acids must be 

 considered. 



The ideas so far presented have been based on the salts of 

 proteins with hydrochloric acid, on account of the ease with 

 which the behaviour on dissociation can be investigated by 

 measurements of the H- and Cl' ion concentrations. Corre- 

 sponding measurements with bromide and iodide have also 

 been possible, but no remarkable differences between these 

 salts and the chloride have appeared. On the other hand, 

 electrometric measurements with polyvalent anions such as 

 S0 4 " and P0 4 '" fail because the accuracy of such measurements 

 'decreases considerably with increasing valency of the anion. 

 It is insufficient even for S0 4 , according to experiments made 

 by Pauli and Hirschfeld.* Hence, it is necessary to rely 

 entirely on measurements of H* ion concentrations when in- 

 vestigating the combination of proteins with acids other than 

 the halogen hydracids. The fraction of the acid which com- 

 bines with the protein can therefore only be determined in 

 this way in the case of strong acids or of weak acids of known 

 dissociation constant. These methods depend on the assump- 

 tion which applies to all salts, that the protein salts are con- 

 * Biochem. Zeitsch., 1914, 62, 245. 



