78 



DONNAN EQUILIBRroM AND VISCOSITY 



touch each other, forming a continous jelly. Hence the longer a 

 solution of isoelectric gelatin stands at 20°C. the greater the number 

 of submicroscopic solid pieces of jelly formed in the solution. The 

 submicroscopic pieces of jelly surrounded by a true solution of isolated 

 molecules of gelatin in water are compelled to regulate the amount 

 of water they occlude by the Donnan equilibrium. Hence when 

 we add some HCl to a 1 per cent solution of isoelectric gelatin after 



Z4 

 2.3 

 Z2 

 2.1 

 2.0 

 1.9 

 1.8 

 1.7 

 1.6 

 15 

 1.4 

 1.3 

 1.2 



.2 



8 



I 



1.1 



pHlJ6 IB 2jO 22 24 26 2fl 3.0 3.2 34 36 3.6 40 42 44 



Fig. 3. Increase in viscosity when acid is added to solutions of isoelectric 

 gelatin after they had been standing for 3 and 17 hours respectively. 



the solution had been standing for some hours at 20° we should expect 

 to find a higher viscosity than when we add the same amount of 

 acid to the gelatin solution immediately after it had been rapidly 

 heated to 45°C. and rapidly cooled to 20°C. 



This experiment turns out as expected, as is shown in Fig. 3. When 

 we bring newly prepared 0.5 per cent solutions of isoelectric gelatin 

 to different pH by the addition of HCl we obtain a viscosity curve 

 like the lowest in Fig. 3. When, however, we let the 0.5 per cent iso- 



