Buffer pH Calculator - Henderson-Hasselbalch pH Result

A buffer pH calculator applies the Henderson-Hasselbalch equation to estimate the pH of a weak acid and its conjugate base (or a weak base and its conjugate acid) from pKa and two molar concentrations. It returns pH together with pOH, hydrogen ion concentration, and the base/acid ratio so you can compare the buffer to your target working pH without rebalancing the chemistry each time.

• • General chemistry homework: Check Henderson-Hasselbalch problems on acetate, phosphate, carbonate, or citrate buffers.
• • Biochemistry buffer prep: Set phosphate or HEPES buffer pH near 7.4 for enzyme assays before weighing reagents.
• • Analytical method development: Plan mobile-phase pH for HPLC or sample-prep buffers where pH must stay within a tight window.
• • Food and dye chemistry: Size citric or acetic acid buffers used in food formulations and dye baths.

Buffers are central to any chemistry workflow that depends on a stable pH, from classroom titrations to clinical chemistry analyzers. Most users only have two numbers at the bench: pKa from a reference table and the molarities of the acid and conjugate base they actually plan to mix. A buffer pH calculator takes those three numbers and returns the equilibrium pH directly so you can decide whether the buffer is worth preparing at that ratio.

The page assumes aqueous buffer solutions at 25 C with Kw equal to 1.0e-14, which is the same assumption underlying most general-chemistry textbooks. If your assay runs at 37 C, expect a small shift from the value the calculator returns; the Henderson-Hasselbalch equation itself is the same shape, only the underlying water dissociation constant changes.

Once you know the buffer pH, Buffer Capacity Calculator tells you how much strong acid or base the same buffer can absorb before the pH drifts by one unit.

The calculator reads your buffer mode (acid or basic), looks up the pKa and the two molar concentrations, and solves the Henderson-Hasselbalch equation in one step. For weak-acid buffers it uses the familiar log-ratio form; for weak-base buffers it derives pH from pKb and the conjugate pair at 25 C.

• pKa: Negative base-10 logarithm of the acid dissociation constant Ka. For basic buffers, enter the conjugate acid pKa and the calculator uses pKa = 14 - pKb.
• [HA] (mol/L): Molar concentration of the weak acid. For basic buffers this is the protonated base BOH concentration.
• [A-] (mol/L): Molar concentration of the conjugate base. For basic buffers this is the conjugate acid B+ concentration.
• [H+] (mol/L): Equilibrium hydrogen ion concentration derived from the calculated pH via [H+] = 10^(-pH).

When you switch to basic buffer mode the calculator converts your entered pKa into pKb with pKa = 14 - pKb, which is exact at 25 C. It then computes pH from pH = 14 - pKb - log10([B+]/[BOH]), the standard base-buffer form taught alongside Henderson-Hasselbalch in undergraduate analytical chemistry.

According to Omni Calculator, the buffer pH equation is pH = pKa + log([A-]/[HA]) and an equal-concentration carbonate buffer gives pH = 6.4

When you only have a hydrogen or hydroxide ion concentration and need the matching pH or pOH, pH & pOH Calculator handles the conversion in either direction.

Four ideas explain every number the buffer pH calculator returns, from the role of pKa to the limits of the approximation.

These four ideas are enough to interpret every buffer pH result the calculator returns. If the base/acid ratio is far from 1 the working pH will sit more than one unit away from pKa, and the buffer will resist pH change less effectively.

To turn the molarity you entered into a mass to weigh on the bench, Grams to Moles Calculator converts grams and molar mass into moles first.

Enter the chemistry of your buffer and the calculator returns the working pH, pOH, hydrogen ion concentration, and base/acid ratio.

1. 1 Pick the buffer type: Select Acid buffer for HA / A- pairs such as acetic acid / acetate, or Basic buffer for B / BH+ pairs such as ammonia / ammonium.
2. 2 Enter pKa: Use the published pKa of the weak acid (4.76 for acetic acid, 6.4 for carbonate, 7.20 for phosphate H2PO4-, 9.25 for ammonium).
3. 3 Enter the weak acid concentration: Type the molarity of HA (or BOH for basic buffers) in mol/L. The calculator accepts values from 1e-6 to 10 mol/L.
4. 4 Enter the conjugate base concentration: Type the molarity of A- (or B+ for basic buffers). Keep both concentrations positive; the ratio drives the pH shift.
5. 5 Read the buffer pH: Note the pH, pOH, hydrogen ion concentration, and the base/acid ratio returned by the calculator.
6. 6 Adjust the ratio if needed: If the result is outside pKa +/- 1, raise or lower the conjugate base concentration to bring the buffer toward its working range before you mix the reagents.

For a 0.10 M acetate buffer at pKa 4.76 with [HA] = [A-] = 0.10 mol/L, the calculator returns pH = 4.76, [H+] = 1.74e-5 mol/L, and a base/acid ratio of 1.00. To shift the same buffer to pH 5.30 you would raise the conjugate base to roughly 0.346 mol/L so that log10(0.346 / 0.10) = 0.54.

When you have the molecular formula of the acid or conjugate base and want the molar mass that pairs with Mole & Molar Mass Calculator, the molar mass flows directly into the bench-scale buffer prep.

A buffer pH calculator turns the qualitative rule 'buffers resist pH change' into a number you can plan an experiment around.

• • Quantitative buffer design: Know the working pH before you weigh any reagents, so you only prepare buffers that match your assay.
• • Faster ratio selection: Compare acetate, phosphate, citrate, and Tris at the pH your reaction actually needs, instead of guessing from a table.
• • Direct support for homework: Match the worked Henderson-Hasselbalch numbers in general-chemistry and analytical-chemistry problems in seconds.
• • Quick sanity check: Confirm that a stock buffer someone else prepared actually sits at the pH the label claims.
• • Reusable for many buffer pairs: Switch between acid and basic buffer modes without re-entering the formula by hand.

If you also need to plan how much of each reagent to weigh out on the bench, pair the buffer pH calculator with the grams-to-moles tool to convert molarity into a mass to weigh.

Five variables drive the buffer pH the calculator returns, and the same variables explain why two ostensibly similar buffers behave differently in the lab.

• • The Henderson-Hasselbalch approximation assumes the analytical concentration equals the equilibrium concentration; it ignores the small correction from water autoionization and from the acid's own dissociation.
• • The basic-buffer branch uses pKa + pKb = 14, which holds at 25 C; outside that temperature the water term must be updated before the result is reliable.
• • Polyprotic acids such as phosphate or citrate contribute more than one dissociation step at certain pH values; the calculator treats each buffer as a single HA / A- pair, so for accurate work near a second pKa use a dedicated polyprotic buffer tool.

These factors are why a buffer that looks perfect on paper can still drift in the lab; the calculator returns the analytical-concentration prediction, not the activity-corrected value the electrode will read.

According to Wikipedia, Henderson-Hasselbalch applies to weak-acid buffers and, via pKa = pKw - pKb, to weak-base buffers

According to Chemistry LibreTexts, a buffer resists pH change within a working range and capacity that limits how much the pH shifts

When you have to figure out how much strong acid or base a buffer can neutralize without leaving the working pH window, Stoichiometry Reaction Calculator sets up the limiting-reactant side of that calculation.