182 THEORY OF COLLOIDAL BEHAVIOR 



It is obvious that if z is small and constant, while y increases 

 more and more (through the addition of NaCl), z becomes a 

 negligible quantity and the term 



a 2y 



approaches / = 1 



7-7 

 2Vy(y 



We can measure the term \/y(y + z) directly by titrating the 

 outside solution for Cl. We cannot determine 2y -f z directly 

 but we can determine y + z by titrating the inside solution for 

 Cl. If both tit rations are made after equilibrium is established 

 we get the value of 



2) 



and the variations of this value with increasing concentration of 

 NaCl are contained in Table XXXV. It is seen that this value 

 is almost 1 when the NaCl solution is M/32. 



Now the value of / ' = does not differ much from the 



Vy(y+*) 



value - / ; as long as y is large in comparison with 2, and 



we can say that with z small and constant and y large and increas- 

 ing rapidly, the two values 



y + z 2y + z 



approach the value 1 almost (but not quite) at an equal rate. 

 Hence, it follows from Table XXXV that if the concentration of 

 NaCl becomes M/32 the value 2y + z 2VtKi + 2 ) must be 

 nearly zero. In this case the osmotic pressure of the 1 per cent 

 gelatin chloride solution must be almost but not quite down to 

 that of the pure gelatin solution as it is at the isoelectric point. 

 The actual observations plotted in Fig. 46 show that for M/32 

 NaCl or M/32 NaNO 3 a 1 per cent solution of gelatin chloride 

 of pH about 3.5 has an osmotic pressure not far from that of 



y -\- z 



isoelectric gelatin. If the values of / == are plotted as 



V y(y + z) 



