Henderson-Hasselbalch Calculator
The Henderson-Hasselbalch equation is the standard formula for calculating the pH of a buffer solution. A buffer resists changes in pH when small amounts of acid or base are added, which makes buffers essential in biochemistry, analytical chemistry, pharmaceutical formulation, food science, and any process where maintaining a stable pH matters. The equation is pH = pKa + log([A minus] divided by [HA]), where pKa is the acid dissociation constant of the weak acid expressed on a log scale, [A minus] is the molar concentration of the conjugate base (the deprotonated form), and [HA] is the molar concentration of the weak acid (the protonated form). When the two concentrations are equal, their ratio is 1, log(1) = 0, and pH equals pKa. This is the point of maximum buffering capacity. When [A minus] is ten times [HA], log(10) = 1 and pH = pKa + 1. When [HA] is ten times [A minus], log(0.1) = minus 1 and pH = pKa minus 1. Useful buffers are generally limited to a pH range of pKa plus or minus 1. This calculator accepts either the direct ratio of base to acid, or the individual concentrations, and returns the pH, the fraction of the acid that is ionised, and the useful buffer range around your pKa. A commonly used example is acetic acid / sodium acetate buffer, with pKa 4.75. When the concentrations of acetic acid and acetate are equal, the buffer pH is 4.75.
Henderson-Hasselbalch applies to dilute aqueous buffer solutions and assumes ideal behaviour. Ionic strength and temperature affect the actual pKa; use as a planning guide.
How it works
The pH is calculated as pH = pKa + log([A-] / [HA]). The ratio is [A-] divided by [HA]. The fraction of the acid that is ionised (in the base form) is alpha = [A-] / ([A-] + [HA]) = ratio / (1 + ratio). The useful buffer range is shown as pKa minus 1 to pKa plus 1, the region where buffering capacity is adequate. Outside this range, the pH changes significantly with small additions of acid or base.
Worked example
Acetic acid has a pKa of 4.75. A buffer is made with equal concentrations of acetic acid and sodium acetate: [A-] = 1 mol/L and [HA] = 1 mol/L. The ratio is 1 / 1 = 1. pH = 4.75 + log(1) = 4.75 + 0 = 4.75. The fraction ionised is 1 / (1 + 1) = 50.0%. The useful buffer range is 4.75 minus 1 = 3.75 to 4.75 plus 1 = 5.75. These match the default values above.
Related calculators
- pH Calculator: calculate pH from hydrogen ion concentration or vice versa.
- Molar Mass Calculator: find the molar mass of a compound from its formula.
- Ideal Gas Law Calculator: solve PV = nRT for any gas law variable.
- Parts Per Million Calculator: convert between ppm, mg/L and other concentration units.