Calculate the pH of any buffer solution using the Henderson-Hasselbalch equation. Supports common buffer systems and custom pKa values.
The Henderson-Hasselbalch equation is the cornerstone of buffer chemistry, relating the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to acid concentrations: pH = pKa + log([A⁻]/[HA]). This elegant relationship allows chemists and biologists to predict and control solution pH with remarkable precision.
Buffer solutions resist pH changes when small amounts of acid or base are added, making them indispensable in biochemistry, cell culture, pharmaceutical formulation, and analytical chemistry. The effectiveness of a buffer depends on three factors: the pKa of the weak acid relative to the target pH, the total concentration of buffer components, and the ratio of conjugate base to weak acid.
This calculator computes the pH of any buffer system from its component concentrations, shows the effect of adding strong acid or base, and displays the buffer capacity and effective range. Whether you're preparing phosphate-buffered saline for cell culture, an acetate buffer for HPLC, or investigating the bicarbonate system in blood chemistry, this tool provides instant, accurate results with full Henderson-Hasselbalch breakdown.
Manually applying Henderson-Hasselbalch with acid/base additions requires careful stoichiometry. This calculator handles the math instantly and warns you when pH drifts outside the effective buffering range. This buffer ph calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
pH = pKa + log₁₀([A⁻]/[HA]) (Henderson-Hasselbalch equation). After adding n moles of strong acid: new [HA] = [HA]₀ + n/V, new [A⁻] = [A⁻]₀ − n/V.
Result: pH = 4.51
pH = 4.76 + log(0.085/0.15) = 4.76 + log(0.567) = 4.76 + (−0.247) = 4.51. This is within the effective range of 3.76–5.76.
Starting from the acid dissociation equilibrium HA ⇌ H⁺ + A⁻, the equilibrium expression is Ka = [H⁺][A⁻]/[HA]. Taking the negative logarithm of both sides gives −log(Ka) = −log[H⁺] − log([A⁻]/[HA]), which rearranges to pH = pKa + log([A⁻]/[HA]). This derivation assumes that the concentrations equal the activities, which is a good approximation for dilute solutions.
Phosphate buffers (pKa2 = 7.2) are workhorses for near-neutral pH, but they precipitate with calcium and inhibit some enzymes. Tris (pKa = 8.07) is ubiquitous in molecular biology for DNA/RNA work. HEPES (pKa = 7.55) and MOPS (pKa = 7.20) are zwitterionic "Good's buffers" designed specifically for biological research, offering minimal metal binding and membrane impermeability.
To prepare a buffer at a target pH: (1) Choose a buffer with pKa near your target, (2) Calculate the required [A⁻]/[HA] ratio from Henderson-Hasselbalch, (3) Dissolve the acid form in water, (4) Add NaOH (or HCl for the base form) to reach the target pH while monitoring with a calibrated pH meter, (5) Bring to final volume with water, and (6) Verify the pH. This empirical adjustment accounts for activity coefficients, impurities, and temperature effects.
It is pH = pKa + log([A⁻]/[HA]), relating buffer pH to the pKa of the weak acid and the concentration ratio of conjugate base to weak acid. This keeps planning practical and lowers the chance of preventable errors.
It breaks down when concentrations are very low (< 0.001 M), when pH is far from pKa (ratio > 100:1 or < 1:100), or for polyprotic acids where multiple equilibria overlap.
Select a buffer whose pKa is within ±1 pH unit of your target. At pH = pKa, the buffer has maximum capacity. The closer to pKa, the better the pH control.
Ideally, no — the ratio [A⁻]/[HA] stays the same upon dilution. In practice, very dilute buffers may shift slightly due to the water equilibrium contribution.
Phosphate-buffered saline (PBS) is a phosphate buffer at pH 7.4 containing NaCl and KCl to match physiological osmolarity. It is the most widely used buffer in biology.
This calculator treats each acid-base pair independently. For phosphate buffer, use the pKa relevant to your pH range: pKa1 = 2.15, pKa2 = 7.20, pKa3 = 12.35.