14 IMMUNO-CATALYSIS 



In the solvolytic processes, the basicity of the solvent is closely 

 related to the acidity displayed by the acid. A system of conjugate 

 acid-base pairs is put in equilibrium, A~ being the conjugate base of 

 the acid HA, H+.S the conjugate acid of the solvent S. 



A striking example of the efficacy of this theory is found in the 

 consideration of aqueous hydrochloric acid solutions. The substance 

 HCl is not itself an electrolyte. In the pure state its bond is strongly 

 covalent and it is a poor conductor of electricity. The theory states that 

 its acidity and electrolytic behavior in w^ater solution is due to a 

 frotolytic reaction with the solvent: 



HCl -{- H2O ^ H3O+ + Cl- 

 the hydro- 

 nium ion 

 CH+.H2O) 



The hydrochloric acid, even up to rather high concentrations, ap- 

 pears to be completely ionized because of this process. This is simply 

 due to the fact that the chloride ion is a weak base compared to the 

 H2O molecule, so that the equilibrium is shifted rather completely to 

 the right. 



In the case of a typical weak acid, such as acetic acid, the ionization 

 is far less complete 



CH3COOH + HoO ^ H3O+ + CH3COO- 



for the reason that the conjugate base, the acetate ion, is a rather strong 

 base. Nevertheless, in a more basic solvent, such as liquid ammonia, 

 this same acid is effectively completely ionized, acting as a strong acid : 



CH3COOH + NH3 ^ NH4+ + CH3COO- 



It is obvious that the behavior of a substance as an acid or base 

 depends greatly upon the solvent medium. Many substances considered 

 to be strong acids because of their behavior in water may well be weak 

 acids in another solvent or may even be bases. For example, hydro- 

 chloric acid is only slightly ionized in benzene while nitric acid and 

 water act as strong bases with sulphuric acid as solvent, 



HNO3 4- H2SO4 ^ H2NO3+ + HSO4- 

 H2O + H2SO4 ^ H3O+ + HSO4- 



This broader view of the acid-base relationships has an important 

 advantage in understanding the catalysis of many reactions. Many 



