Calculate the pH of a buffer solution containing a weak acid and its conjugate base.
Buffer Composition
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M
Calculated pH
Enter pKa and concentrations to calculate pH
About this calculator
Overview
The Henderson-Hasselbalch equation mathematically describes the relationship between the pH of a solution containing a weak acid and its conjugate base, and the acid dissociation constant (Ka) of that acid. It provides a quantitative way to understand buffer systems and how they resist pH changes when small amounts of acid or base are added. The equation shows that pH depends on both the strength of the acid (pKa) and the ratio of conjugate base to acid concentrations.
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Pro Tips
The core formula is pH = pKa + log₁₀([Base]/[Acid]).
pKa is a measure of acid strength; the lower the pKa, the stronger the acid.
A buffer's effective range is generally considered to be pH = pKa ± 1 unit.
Be careful with units: concentrations [HA] and [A⁻] must be in the same units (usually Molarity).
The equation assumes that the 'x is small' approximation holds, which is true for most weak acid/base buffer systems.
Temperature affects pKa, so pH calculations for biological systems should ideally use pKa values at 37°C.
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Fun Facts
"Human blood is buffered by the bicarbonate system to stay between pH 7.35–7.45; deviation outside 6.8–7.8 is usually fatal."
"The equation was originally developed by Lawrence Henderson (1908) and reformulated into logs by Karl Hasselbalch (1917)."
"Buffer capacity is at its maximum when pH = pKa, meaning the concentrations of acid and conjugate base are equal."
"Lake Natron's extreme pH (9–10.5) is maintained by a natural sodium carbonate/bicarbonate buffer system."
"Pharmacists use this equation to determine how much of a drug will be ionized in the stomach (low pH) vs. the intestines (higher pH)."
"Most biological enzymes only function within a very narrow pH range, which is why cell cytoplasm is heavily buffered."
"Citric acid and its salts are common food buffers, used to keep the pH of sodas and candies consistent for flavor and safety."
Formula: pH = pKa + log₁₀([A⁻]/[HA])
Buffer Capacity vs pH Relationship
Buffer capacity peaks when pH = pKa ([A⁻] = [HA])
Maximum resistance to pH changes occurs at the inflection point
