Henderson Hasselbalch Calculator - pH from pKa or Ka

The Henderson-Hasselbalch equation pH = pKa + log10([A-]/[HA]) lets the Henderson Hasselbalch calculator return pH, pOH, [H+], [OH-], and the base/acid ratio from a pKa (or Ka) and the molar concentrations of HA and A-.

• • General chemistry homework: Solve Henderson-Hasselbalch problems on acetate, phosphate, carbonate, or citrate buffers with the pKa table values taught in class.
• • Biochemistry buffer prep: Set phosphate or HEPES buffer pH near 7.4 for enzyme assays before weighing reagents on the bench.
• • Reverse ratio solve: Pick a target pH and back out the [A-]/[HA] ratio you need to mix.
• • Analytical method development: Plan mobile-phase pH for HPLC or sample-prep buffers where pH must stay within a tight window around the analyte pKa.

Buffers are central to any chemistry workflow that depends on a stable pH. Most users start with a Ka or pKa from a reference table and the molarities of the acid and conjugate base they plan to mix. The tool turns those numbers into a working pH so you can decide whether the buffer is worth preparing at that ratio. Aqueous buffer solutions at 25 C with Kw equal to 1.0e-14 are assumed; assays at 37 C will see a small shift.

This tool handles the same equation as the dedicated buffer pH calculator, but adds a Ka input mode and an explicit base/acid ratio so you can work from a published Ka or solve for the ratio directly.

The Henderson Hasselbalch calculator reads your input mode, looks up the pKa and the two molar concentrations, and solves the equation in one step. If you entered Ka directly it converts to pKa first, then returns pH, pOH, [H+], [OH-], and the base/acid ratio.

• pKa: Negative base-10 logarithm of the acid dissociation constant Ka. The calculator also accepts Ka directly and converts to pKa via pKa = -log10(Ka).
• [HA] (mol/L): Molar concentration of the weak acid; for basic buffers this is the conjugate acid BH+ concentration.
• [A-] (mol/L): Molar concentration of the conjugate base; for basic buffers this is the weak base B concentration.
• [H+] (mol/L): Equilibrium hydrogen ion concentration derived from the calculated pH via [H+] = 10^(-pH).
• [OH-] (mol/L): Equilibrium hydroxide ion concentration derived from pOH via [OH-] = 10^(-pOH), with pOH = 14 - pH at 25 C.

In basic buffer mode the Henderson Hasselbalch calculator applies pH = pKa + log10([B]/[BH+]) directly, where pKa is the conjugate acid pKa (9.25 for ammonium) and the conjugate base field holds the weak base B (ammonia). A buffer with more weak base than conjugate acid sits above the conjugate acid pKa, mirroring the acid branch.

According to Chemistry LibreTexts, the Henderson-Hasselbalch approximation pH = pKa + log10([A-]/[HA]) applies to buffer solutions where the analytical concentrations approximate the equilibrium concentrations and extends in the same log form to the weak base / conjugate acid pair (B / BH+) of a basic buffer.

If you already know [H+] or [OH-] from an electrode reading and want to skip straight to pH or pOH, the pH and pOH calculator handles the conversion in either direction.

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

Pick whether you have pKa or Ka, enter the buffer type and the two molar concentrations, then read the working pH.

1. 1 Pick the acid constant input: Select pKa if you know the negative log of Ka, or Ka if you only have the dissociation constant from a literature source.
2. 2 Enter pKa or Ka: Type the value from your reference table; 4.76 for acetic acid, 7.20 for phosphate H2PO4-, 9.25 for ammonium.
3. 3 Choose 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 (NH3 / NH4+).
4. 4 Enter the weak acid concentration: Type the molarity of HA (acid buffers) or the conjugate acid BH+ (basic buffers, e.g., ammonium NH4+) in mol/L from 1e-6 to 10.
5. 5 Enter the conjugate base concentration: Type the molarity of A- (acid buffers) or the weak base B (basic buffers, e.g., ammonia NH3); the ratio drives the pH shift.
6. 6 Read the result and adjust the ratio: Note the buffer pH, pOH, [H+], [OH-], and base/acid ratio; raise or lower the conjugate base to bring the buffer toward its working range if needed.

For a 0.10 M acetate buffer at pKa 4.76 with [HA] = [A-] = 0.10 mol/L, the calculator returns pH = 4.76 with a base/acid ratio of 1.00. Raise the conjugate base to about 0.346 mol/L to reach pH 5.30.

After the tool returns a target molarity, the grams to moles calculator converts the grams you weigh on the bench into moles for the same buffer.

A Henderson Hasselbalch calculator turns the rule that buffers resist pH change into a working pH you can plan an experiment around.

• • Two input modes for the same equation: Enter pKa from a textbook table or Ka from a literature source; the calculator handles the log conversion automatically.
• • 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 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.

Pair this tool with a grams-to-moles calculator when you need to plan how much of each reagent to weigh out on the bench.

Once the working pH is confirmed, the buffer capacity calculator tells you how much strong acid or base that buffer can absorb before the pH moves by one unit.

Five variables drive the buffer pH this calculator returns and explain why similar buffers behave differently in the lab.

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

A buffer that looks perfect on paper can drift in the lab: the calculator returns the analytical-concentration prediction, not the activity-corrected value.

According to Wikipedia, the approximation holds when analytical concentration approximates equilibrium concentration and breaks down outside roughly pKa plus or minus 1.

According to Chemistry LibreTexts, a buffer resists pH change within a working range of about pKa plus or minus 1 because the log term dominates outside that window.

If you need to push the buffer pH by a known amount, the stoichiometry reaction calculator sets up the limiting-reactant side of that titration calculation.