The hydrolysis of salts of weak acids and weak bases

by P. R. Skelton

Institution: University of Canterbury
Department: Chemistry
Year: 1930
Record ID: 1521786
Full text PDF: http://hdl.handle.net/10092/7940


It has shown, from theoretical considerations, by R. O. Griffiths (Trans. Faraday Soc. 17,525, 1922) that the classical derivation of an equation connecting the degree of hydrolysis of salts formed from weak acids and weak bases with the dissociation constants of the acid and base respectively, is based on an assumption which in valid for moderately dilute solutions and for moderately weak acids and bases only. In very dilute solutions, or when the acid or base is very weak, the classical equation breaks down. Tizard’s deduction (Trans. Chem. Soc. 97, p.2477, 1910) that the concentration of hydrogen ions in a solution of a weak salt is constant, whatever the dilution, is, for the same reason, in error when the concentration is very low, or when hydrolysis is nearly complete. In the classical treatment it is assumed that, on the hydrolysis of a weak salt (B.A.) the concentration of free undissociated acid, and of free undissociated base in solution are equal, the ionisation of these two components being regarded as negligible, wherein lies the fallacy. The existence of the above erroneous assumption in the classical treatment has also been pointed out by E.W. Hullett (in a thesis presented last year – as yet unpublished). Hullett arrived at the same conclusions as Griffiths (loc. cit.) but, whereas Griffiths merely showed the necessity for defining the degree of hydrolysis of a weak salt more explicitly, Hullett did assign a definite meaning to the degree of hydrolysis, and, moreover, derived equations with which, from electrometric measurements of hydrogen ion concentration, and from knowledge of the dissociation constant of either acid or the base, it is possible to calculate the degree of hydrolysis of weak salt.