Henderson-Hasselbalch Calculator

pH = pKa + log10([A-]/[HA]), where pKa is the negative log of the acid dissociation constant, [A-] is the conjugate base concentration, and [HA] is the weak acid concentration. This equation is derived from the Ka expression and allows rapid calculation of buffer pH without solving equilibrium expressions directly.

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (or a weak base and its conjugate acid) at comparable concentrations. Buffers are essential in biological systems (blood pH 7.4), pharmaceutical formulations, and analytical chemistry.

Buffer capacity is the amount of acid or base a buffer can absorb before its pH changes significantly. It is maximum when [A-] = [HA] (pH = pKa) and decreases as the ratio moves away from 1:1. Practically, a buffer works well within 1 pH unit of its pKa (ratio between 0.1 and 10). Higher total buffer concentration also increases capacity.

1. Choose an acid whose pKa is within 1 unit of your target pH. 2. Use Henderson-Hasselbalch to calculate the required [A-]/[HA] ratio: [A-]/[HA] = 10^(pH - pKa). 3. Weigh out the acid and its salt at that molar ratio. Common buffers: acetate (pKa 4.76) for pH 3.8-5.8, phosphate (pKa 7.20) for pH 6.2-8.2, Tris (pKa 8.07) for pH 7.1-9.1.

The equation assumes: (1) the acid is weak enough that [HA] is approximately equal to the nominal concentration; (2) activity coefficients equal 1 (valid at low ionic strength); (3) the autoionization of water is negligible. It is less accurate for very dilute buffers (total concentration less than 1 mM), very acidic or basic pH, or high ionic strength solutions.