86 



DONNAN EQUILIBRIUM AND VISCOSITY 



point increasing with increasing hydro>';tn ion concentration of the 

 solution and finally decreasing again. But the curves for swelling 

 and for volume of sediment were only parallel at the beginning, 

 since the swelling (which occurred at once) w * >^ followed by some 

 of the casein going into solution, or into suspens- . i in the supernatant 

 milky liquid. The longer the experiment lasted the smaller the volume 

 of the sediment became and the larger the mass which went into the 

 supernatant solution. This is expressed in Fig. 7. The upper 

 curve represents the volume of the sediment after 1 hour. The 



xn 



CO 



^ 



10 



pH 1.4 1.6 Ifi 2.0 22 24 26 Zd 3.0 3.2 3.4 3.6 3.8 40 4.2 

 Fig. 7. Swelling and solution of casein chloride in 1 and 22 hours at 20°C. 



suspension of 1 gm. of casein in 100 cc. HCl of different concentration 

 had been kept for 1 hour at 20°, had been shaken repeatedly but not 

 frequently and the suspension was then passed into 100 cc. graduates 

 and allowed to settle at 20° C. After 2 hours the volume of the sediment 

 was measured and the volumes are the ordinates of the curve marked 

 "after 1 hour" in Fig. 7. A similar experiment was made in which 

 the suspension of casein was kept for 22 hours at 20°C. and was 

 allowed to settle during 6 hours also at 20°. The volumes are the 

 ordinates of the second curve in Fig. 7 marked "after 22 hours." 

 The abscissae are the pH of the total solution and suspension. 



