REACTIONS IN HETEROGENEOUS SYSTEMS 53 



water, therefore the total volume of the disperse phase is 

 increased.* 



The protein solution probably consists of two phases, one a 

 dilute solution of protein in water and the other a concentrated 

 solution of protein in a little water. It is an increased osmotic 

 attraction by the protein for water that causes the con- 

 centrated protein phase to increase in volume, f 



PRECIPITATION OF COLLOIDS 



The precipitation of colloids depends on at least two factors. 

 (i) The surface tension which tends to cause them to run 

 together because larger particles have relatively less surface 

 than smaller particles. (2) The electrical charge on the 

 particles which tends to keep them separated. The less the 

 electrical charge the more easily the particles run together. J 



The cause of the electrical charge on suspended particles 

 cannot be explained in all cases. In some cases it appears to 

 depend on the chemical nature of the colloid; thus ferric 

 hydroxide (basic) has a positive charge and silicic acid (acidic) 

 has a negative charge. By decreasing the electrical charge the 

 particles run together more easily, therefore oppositely charged 

 colloids precipitate each other. 



The influence of electrical charges in precipitating colloids 

 is well shown by the action of monovalent, divalent and 

 trivalent ions. A negatively-charged colloid is precipitated 

 by positive ions in the ratio of the molecular concentrations of 

 x \/x 3 T/X of mono- di- and tri- valent ions respectively. 

 This ratio is explicable by the kinetic theory in this way. 

 To furnish two positive charges it requires two monovalent 

 ions and only one divalent ion. The probability that one 

 divalent ion will come into contact with a colloidal particle is 

 proportional to the reciprocal of the molecular concentration, 



that is -. whilst for two monovalent ions the probability is 



OC 



1 x X = T 2 . Similarly the probability that three positive charges 



X X X 



* W. Pauli and H. Handovsky, Biochem. Zeit., 1909, vol. 18, 

 p. 340, and 1910, vol. 24, p. 239. 



f H. Chick, Biochem. Journ., 1914, vol. 8, p. 261. 



I For the view that a negative surface tension on one side of a surface 

 determines the dispersion of a colloid, see W. Ramsden, Trans. L'pool. 

 Biol. Soc., 1919, vol. 33. 



The total surface tension of both sides of the boundary must be posi- 

 tive, otherwise the phases will mix. 



