In 1908, Lawrence Joseph Henderson derived an equation to calculate the pH of a buffer solution. In 1917, Karl Albert Hasselbalch re-expressed that formula in logarithmic terms, resulting in the Henderson–Hasselbalch equation.
Theory
A simple buffer solution consists of a solution of an acid and a salt of the conjugate base of the acid. For example, the acid may be acetic acid and the salt may be sodium acetate. The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, Ka, and the concentrations of the species in solution. To derive the equation a number of simplifying assumptions have to be made. The mixture has the ability to resist changes in pH when a small amount of acid or base is added, which is the defining property of a buffer solution. Assumption 1: The acid is monobasic and dissociates according to the equation It is understood that the symbol H+ stands for the hydrated hydronium ion. The Henderson–Hasselbalch equation can be applied to a polybasic acid only if its consecutive pK values differ by at least 3. Phosphoric acid is such an acid. Assumption 2. The self-ionization of water can be ignored. This assumption is not valid with pH values more than about 10. For such instances the mass-balance equation for hydrogen must be extended to take account of the self-ionization of water. and the pH will have to be found by solving the two mass-balance equations simultaneously for the two unknowns, and . Assumption 3: The salt MA is completely dissociated in solution. For example, with sodium acetate Assumption 4: The quotient of activity coefficients,, is a constant under the experimental conditions covered by the calculations. The thermodynamic equilibrium constant,, is a product of a quotient of concentrations and a quotient,, of activity coefficients. In these expressions, the quantities in square brackets signify the concentration of the undissociated acid, HA, of the hydrogen ion H+, and of the anion A−; the quantities are the corresponding activity coefficients. If the quotient of activity coefficients can be assumed to be a constant which is independent of concentrations and pH, the dissociation constant, Ka can be expressed as a quotient of concentrations. Rearrangement of this expression and taking logarithms provides the Henderson–Hasselbalch equation
Application
The Henderson–Hasselbalch equation can be used to calculate the pH of a solution containing the acid and one of its salts, that is, of a buffer solution. With bases, if the value of an equilibrium constant is known in the form of a base association constant, Kb the dissociation constant of the conjugate acid may be calculated from where Kw is the self-dissociation constant of water. pKw has a value of approximately 14 at 25 °C. If the "free acid" concentration, , can be taken to be equal to the analytical concentration of the acid, TAH an approximation is possible, which is widely used in biochemistry; it is valid for very dilute solutions. The effect of this approximation is to introduce an error in the calculated pH, which becomes significant at low pH and high acid concentration. With bases the error becomes significant at high pH and high base concentration.
