52 THE PROPERTIES OF PROTEINS 



albumin, purified with special care by repeated recrystallisation 

 and dialysis. 



Consider a series of mixtures with the same content of 

 ammonium sulphate, but containing increasing quantities of 

 sulphuric acid. We get from these a curve in which the free 

 H- ion content (h, the abscissae) increases with the excess of 

 sulphuric acid present (t, the ordinates). Suppose we now 

 choose such a mixture that the curve (e = O, in Fig. 8) contains 

 the iso-electric concentration of H* ions for egg albumin. At 



60 



$ 



60 



80 



- 



10 



e-ffti 



Z 







16 



fS 



2-fO 



-6 



FIG. 8. Ammonium sulphate concentration = 0-36 N 



in all cases. 



e = milligram equivalents of protein nitrogen 

 in 100 c.c. 



this point, the hydrion concentration produced by the same 

 concentration of ammonium sulphate and the same excess of 

 acid should remain unaltered by the presence of egg albumin 

 of any concentration (expressed as e milligram equivalents of 

 protein nitrogen). We find insiJ&aci that the curves obtained 

 for various concentrations of protein intersect the pure ammo- 

 nium sulphate sulphuric acid curve, within the limits of 



