Buffer pH Calculator

A buffer solution contains meaningful amounts of a weak acid and its conjugate base. Together they moderate pH changes when small quantities of strong acid or base are introduced. Added hydrogen ions react mainly with the conjugate base, while added hydroxide ions react mainly with the weak acid. This paired response keeps the hydrogen ion activity more stable than it would be in an unbuffered solution. The Henderson-Hasselbalch equation relates buffer pH to the acid dissociation constant and the ratio of conjugate base to weak acid. In logarithmic form, pH equals pKa plus the base-10 logarithm of the base concentration divided by the acid concentration. When both concentrations are equal, the ratio is one, its logarithm is zero, and pH equals pKa. A tenfold excess of base raises pH one unit above pKa; a tenfold excess of acid lowers it one unit. This calculator accepts concentrations in moles per liter, but any common concentration unit gives the same ratio when both inputs use that same unit. The entered values should represent the acid and conjugate base after mixing and any neutralization reaction. If a strong acid or base is added during buffer preparation, first use stoichiometry to determine how many moles of each buffer component remain, then divide by the common final volume. When both species share that volume, their mole ratio can also be used directly. The Henderson-Hasselbalch relationship is an approximation based on a simple equilibrium treatment. It works best when both forms are present in appreciable amounts, typically when their ratio lies between one tenth and ten. That corresponds to a useful buffering range of approximately pKa minus one through pKa plus one. Very dilute solutions, high ionic strength, extreme ratios, polyprotic equilibria, and activity effects can produce noticeable deviations. Temperature also changes pKa and should match the source of the constant. Use this result to plan laboratory buffers, understand titration regions, check formulation calculations, or study acid-base chemistry. The calculation predicts pH under the stated ideal assumptions; sensitive analytical, biological, or manufacturing work should confirm pH with a calibrated meter and adjust the prepared solution carefully.