Buffer pH Calculator

What Is the Henderson-Hasselbalch Equation, and What Does Buffer pH Depend On

A buffer's pH depends on two things only: the pKa of its weak acid, and the ratio of conjugate base to weak acid concentration in solution. The Henderson-Hasselbalch equation, pH = pKa + log₁₀([A⁻]/[HA]), captures that relationship directly, according to Wikipedia's entry on the equation, which traces it back to a simplified rearrangement of the acid dissociation equilibrium expression.

This calculator covers all three practical directions on that relationship: finding a buffer's resulting pH from known concentrations, working backward to find what ratio a target pH requires, and going a step further to work out the actual grams of acid and conjugate base needed to prepare that buffer at a specific volume.

Finding a Buffer's pH From Concentrations

Given a pKa and the molar concentrations of the conjugate base and weak acid, the pH follows directly from the equation. For an acetate buffer, pKa 4.76, with 0.15 M acetate and 0.10 M acetic acid, the pH works out to 4.76 + log₁₀(0.15/0.10), or 4.76 + 0.176, roughly 4.94.

Notice that only the ratio of the two concentrations enters the equation, not their absolute values, which is why diluting a buffer with water doesn't shift its pH even though both concentrations drop together, a point Pearson's buffer pH calculator reference also flags as the detail students most often miss when first learning the equation, covered in more detail further down.

Working Backward: The Ratio Needed for a Target pH

Rearranging the equation to solve for the ratio instead, [A⁻]/[HA] = 10^(pH − pKa), answers the more common real question: given a target pH and a chosen buffer system, what ratio of conjugate base to acid actually gets there? Omni Calculator's Henderson-Hasselbalch tool confirms this same rearranged form as the standard method for planning a buffer before mixing anything.

For a target pH of 5.00 using the same 4.76 pKa acetate system, that ratio works out to 10^(5.00 − 4.76), or roughly 1.74 parts conjugate base to 1 part acid.

Preparing a Buffer From Scratch: Grams, Moles, and Volume

Knowing the ratio alone doesn't tell you how much of each component to weigh out. Setting a total buffer concentration, C = [A⁻] + [HA], and combining it with the target ratio gives both individual concentrations directly: [A⁻] = C × ratio ÷ (1 + ratio), and [HA] = C ÷ (1 + ratio). Multiplying each by the target volume gives moles, and multiplying moles by molar mass gives the actual grams to weigh out.

For a 1 L, 0.20 M acetate buffer at pH 5.00, that works out to roughly 0.127 M sodium acetate (about 10.4 g at 82.03 g/mol) and 0.073 M acetic acid (about 4.4 g at 60.05 g/mol), a calculation Sigma-Aldrich's buffer preparation calculator runs the same way for lab-scale buffer prep.

Common Buffer Systems and Their pKa Values

Choosing a buffer system with a pKa close to the target pH is the single most important decision in buffer preparation, since buffering strength falls off sharply more than about one pH unit from pKa.

This calculator's preset dropdown pulls directly from this same table, cross-checked against MolBioTools' buffer calculator reference values, so a common system's pKa and typical component molar masses load automatically instead of requiring a separate lookup.

Why the Ratio Matters More Than Absolute Concentration

Because pH depends only on the log of the concentration ratio, a buffer diluted tenfold keeps very nearly the same pH even though both [A⁻] and [HA] have dropped by a factor of ten, since the ratio between them is unchanged. Chemistry LibreTexts' treatment of buffer pH notes this is exactly why dilution barely moves pH but does noticeably reduce buffer capacity, the two properties respond to different things even though they're often assumed to move together.

If you need to check how much that dilution actually costs in resistance to pH change, our Buffer Capacity Calculator covers that separate question directly.

Accuracy and Limitations

The Henderson-Hasselbalch equation is a dilute-solution approximation, it ignores activity coefficients and ionic strength effects that become significant in concentrated or high-salt buffers, and it stays reliable mainly within about pKa ± 1 pH unit, well outside that range the buffer has little real buffering capacity regardless of what the equation technically returns. ChemBuddy's technical notes on the equation add that at very low or very high concentrations, water's own autoionization can meaningfully skew the result away from what the simplified equation predicts. If your concentration data needs converting from another unit first, our Molarity Calculator handles that conversion before running the pH math.