60 COLLOIDS IN BIOLOGY AND MEDICINE 



i.e., there is formed by hydrolysis some colloid acid which cannot 

 diffuse through, although the NaOH may do so. If the colloid is a 

 strong acid, the process terminates rapidly providing the NaOH re- 

 mains in the outer water, and an osmotic pressure may develop within 

 the cell which is chiefly produced by the R and Na components of the 

 colloid electrolytes as occurs, for instance, in the case of congo red. 



If the colloid electrolyte is a weak acid, then proportionally more 

 NaOH diffuses outward, and when the equilibrium is established, it is 

 chiefly by the hydrolytically split colloid acid and the NaOH. Ex- 

 ample: soap solution. 



If the NaOH is constantly removed (by continually renewing the 

 outer water, or by means of a bond which is not at all or slightly 

 dissociated, e.g., carbonic acid) there will finally remain only colloid 

 acid, in fact the weaker the colloid acid the more rapidly the process 

 will terminate. What has been stated for an acid colloid applies 

 of course to a basic one also. 



It follows from these premises that salts even of strong acids and 

 bases may be completely broken up hydrolytically, provided one ion 

 is a colloid which can be held back by a membrane. By membrane 

 hydrolysis it is possible to separate from a neutral salt, either an 

 alkali (intestinal or pancreatic juice) or an acid (hydrochloric acid, 

 in the stomach, or acid urine). It requires no special exposition to 

 show that the same process may be brought about by ultrafiltration. 



The reverse process may occur, however. If there is a colloid acid 

 or base in a cell surrounded by a membrane, e.g., an amphoteric 

 colloid albumin or fibrin, a minimal concentration of H or OH ions 

 in the outer fluids suffice to form a salt with the colloid in the cell 

 which by swelling develops a higher osmotic pressure. 



(6) We shall now consider what occurs when the colloid electrolyte 

 within the membrane has an ion in common with the electrolyte 

 outside, e.g., the Na salt of congo red (RNa) and common salt (NaCl). 

 We then have the following formula: 



Initial condition Equilibrium 



R 



Na 

 (1) 



Cl R 



Na Na 



(1) Cl 



(1) 



Cl 



Na 



(2) 



Na ions cannot pass from space (1) to space (2) since by reason of 

 its colloidal character the anion R cannot follow. 1 However, Cl and 

 with it the same amount of Na will diffuse into (1). The amount of 



1 There are always the same number of anions and cations in a solution. It 

 is impossible to separate them by diffusion for then a free electric charge would 

 be liberated. 



