The Investigation of Protolytic Equilibria in Aqueous Solution

  • R. P. Bell


Since the free proton is never encountered in solution, the equation A ⇌ B + H+, defining an acid-base pair, does not represent a realizable equilibrium, and all actual acid-base equilibria are of the form, A1 + B2 ⇌ B1 + A2. This type of reaction is known as a protolytic or proton-transfer reaction. Any qualitatively sensible concept of acid-base strength would imply that the equilibrium conversion of A1 + B2 into B1 + A2 will be more complete the stronger the acid A1 and the base B2 and the weaker the acid A2 and the base B1. Quantitatively, the equilibrium constant [B1][A2]/[A1][B2] is equal to the ratio of the (hypothetical) constants [B1][H+]/[A1] and [B2][H+]/[A2], and it will therefore measure the ratio of the acid strengths of A1 and A2, or the ratio of the base strengths of B2 and B1. Since these two ratios are equal, there is no point in considering the acid and base strengths separately, and it has become usual to describe the properties of any acid-base pair A−B in terms of the acid strength of A. Thus for the pair CH3COOH-CH3COO we speak of the acid strength of acetic acid rather than the base strength of acetate ion, and for the pair NH 4 + -NH3 of the acid strength of the ammonium ion rather than the base strength of ammonia.


Dissociation Constant Acid Strength Acidity Function Acid Constant Weak Electrolyte 
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Copyright information

© R. P. Bell 1973

Authors and Affiliations

  • R. P. Bell
    • 1
  1. 1.University of StirlingScotland

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