Henderson-Hasselbalch Calculator

The Henderson-Hasselbalch equation estimates the pH of a buffer made from a weak acid and its conjugate base. Its familiar form is pH = pKa + log10(base concentration divided by acid concentration). The equation connects an acid's intrinsic tendency to dissociate, expressed by pKa, with the composition of the prepared solution. This calculator evaluates that relationship directly and also displays the base-to-acid ratio used in the logarithm. When acid and conjugate base concentrations are equal, their ratio is one and the logarithm is zero. The pH then equals the pKa. Adding more conjugate base raises the pH, while adding more weak acid lowers it. A tenfold increase in the ratio raises pH by one unit; a tenfold decrease lowers pH by one unit. This logarithmic behavior makes buffers effective near the pKa of the weak acid. A practical buffer usually works best within roughly one pH unit of its pKa. In that interval, the concentration ratio lies between about one tenth and ten, so meaningful amounts of both members of the conjugate pair are present. Buffer capacity is not determined by the ratio alone. Two buffers can have the same predicted pH while the more concentrated one resists added acid or base more strongly. Diluting both components equally preserves the ideal ratio and pH, but decreases capacity and can make activity effects more important. The equation is an approximation derived from the acid dissociation equilibrium. It assumes that equilibrium concentrations can be represented adequately by the entered values and that activity coefficients are near one. Accuracy can decline in very dilute or highly concentrated solutions, at high ionic strength, near the limits of buffer range, or when additional equilibria are significant. Polyprotic acids require choosing the pKa associated with the relevant conjugate pair. Use analytical concentrations in matching units, such as mol/L for both components. Because only their ratio enters the equation, mmol/L can also be used if both fields share that unit. Temperature matters because pKa values change with temperature, so use a value measured or tabulated near the experiment's conditions. The calculator is suitable for coursework, initial laboratory planning, biological buffers, and formulation estimates, while precise preparation should be confirmed with calibrated pH measurement.