Henderson_Hasselbalch_equation v.0.1.0
Estimates the pH of a buffer solution when the Ka numerical value of the acid dissociation constant is known.
To calculate the pH of a solution containing the acid and one of its salts, that is, of a buffer solution.
Assumptions: 1. It does not take into account the self-dissociation of water, which becomes increasingly important in dilute solutions. 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. 2. The acid is monobasic and dissociates according to the equation: HA = H+ + A- where H+ stands for hydrated hydronium ion. 3. The salt is completely dissociated in solution. A simple buffer solution consists of a solution of an acid and a salt of the conjugate base of the acid. The Henderson-Hasselbalch equation is derived from the definition of the acid dissociation constant and relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, Ka, and the concentrations of the two components. pH = pKa + log10([A−] / [HA]) Where: pKa = - log10 (Ka) When the concentrations of the acid and the conjugate base are the same, i.e, when the acid is 50% dissociated, the pH of the solution is equal to the pKa of the acid.
The Henderson-Hasselbalch equation is an approximation, with a certain region of validity.
Henderson LJ. Concerning the relationship between the strength of acids and their capacity to preserve neutrality. Am J Physiol. 1908;21(2):173–179.
openEHR-EHR-OBSERVATION.henderson_hasselbalch_equation
