Buffer pH Calculator
Use Henderson-Hasselbalch for buffer systems. This guide explains what the topic is used for, the chemistry ideas behind it, and how students should approach related problems manually.
What Makes a Solution a Buffer
A buffer is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) in solution together. Because both the proton donor and proton acceptor are present at once, the mixture can neutralize small additions of strong acid or strong base without a large swing in pH.
A buffer only resists pH change within a working range; add enough strong acid or base to consume nearly all of one component, and the mixture stops behaving like a buffer.
The Henderson-Hasselbalch Equation
The relationship is pH = pKa + log([A-] / [HA]), where pKa is the acid dissociation constant of the weak acid and the ratio compares conjugate base to remaining acid. When the acid and conjugate base concentrations are equal, the log term is zero and pH equals pKa exactly, which is the center of a buffer's effective range.
To use it manually: identify the weak acid and its pKa, find the moles of acid and conjugate base actually present, take their ratio, and add its base-10 logarithm to pKa.
Choosing a Buffer Pair
Pick a weak acid whose pKa sits within about one pH unit of the target pH. Outside that range, the buffer still works chemically but resists pH change much more weakly, since one of the two components is present in only a small amount relative to the other.
Common lab buffer pairs include acetic acid and acetate near pH 4.5-5.5, and dihydrogen phosphate and hydrogen phosphate near pH 6.5-7.5, chosen because their pKa values fall near those target ranges.
Worked Example
An acetate buffer made with 0.20 mol acetic acid (pKa 4.76) and 0.30 mol sodium acetate in the same volume has pH = 4.76 + log(0.30/0.20) = 4.76 + 0.18 = 4.94. Doubling both amounts while keeping the same ratio would not change the pH, only the buffer capacity, which is the key distinction between concentration and ratio in this equation.
Buffer capacity peaks when the acid and conjugate base concentrations are equal, since that is where the mixture can absorb the largest addition of strong acid or base before the ratio shifts significantly.
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Buffer pH Calculator is an educational chemistry tool for use henderson-hasselbalch for buffer systems.
It uses standard textbook equations and atomic masses. For laboratory or clinical decisions, verify with official protocols.
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