How do I calculate pH from pKa and the ratio?

Add the base-10 logarithm of the conjugate base to acid ratio to the pKa. For a ratio of 1 the log term is 0, so pH equals pKa.

What ratio gives the best buffer capacity?

Buffer capacity is greatest when pH equals pKa, which occurs when [A-] equals [HA] and the ratio is 1. Buffers work well within about one pH unit of the pKa.

How do I find the ratio needed for a target pH?

Rearrange the equation: [A-]/[HA] = 10^(pH - pKa). For a target pH one unit above the pKa the ratio is 10; one unit below it is 0.1.

pKa / H-H

pKa & Henderson-Hasselbalch Calculator

Solve the Henderson-Hasselbalch equation for pH, pKa or the conjugate base/acid ratio.

Henderson-Hasselbalch

Solve for

pKa

[A-]/[HA]

pH = pKa + log₁₀([A-]/[HA])

Result

Fill in the two known values to compute the third.

Buffers are most effective within ±1 pH unit of the pKa; buffer capacity peaks when pH = pKa.

Description

The Henderson-Hasselbalch equation relates the pH of a buffer to the pKa of the weak acid and the ratio of its conjugate base to acid forms: pH = pKa + log10([A-]/[HA]). It is the standard tool for predicting buffer pH and designing buffer systems.

How to use

Choose which quantity to solve for — pH, pKa or the [A-]/[HA] ratio — then fill in the other two values; the disabled field is computed automatically. Buffers work best within ±1 pH unit of the pKa.

Learn more

What it does

The pKa and Henderson-Hasselbalch calculator solves the Henderson-Hasselbalch equation for pH, pKa, or the conjugate base to acid ratio. It is used by chemists and biochemists to predict buffer pH and to design buffer systems for experiments.

How it works

The Henderson-Hasselbalch equation is pH = pKa + log10([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] is the weak acid concentration. The tool can solve for any one of pH, pKa, or the ratio [A-]/[HA] when the other two are known. A buffer is most effective within plus or minus one pH unit of its pKa, and buffer capacity is greatest when pH equals pKa, that is when [A-] equals [HA] and the ratio is 1.

Worked example

Acetic acid has a pKa of 4.76. A buffer made with equal concentrations of acetic acid and acetate has ratio [A-]/[HA] = 1, so pH = 4.76 + log10(1) = 4.76. If the ratio is 10 (ten times more acetate than acid), pH = 4.76 + log10(10) = 5.76. To design a buffer at pH 5.76 with acetic acid (pKa 4.76), the required ratio is 10^(5.76 - 4.76) = 10, meaning ten parts conjugate base to one part acid.

When to use it

Use it to check the pH a buffer will have given known acid and base amounts. Use it to find the pKa of a weak acid from a measured buffer pH and ratio. Use it to determine the acid to base ratio needed to hit a target pH.

FAQ

What is the Henderson-Hasselbalch equation?: It is pH = pKa + log10([A-]/[HA]). It relates the pH of a buffer to the pKa of the weak acid and the ratio of its conjugate base to weak acid.
How do I calculate pH from pKa and the ratio?: Add the base-10 logarithm of the conjugate base to acid ratio to the pKa. For a ratio of 1 the log term is 0, so pH equals pKa.
What ratio gives the best buffer capacity?: Buffer capacity is greatest when pH equals pKa, which occurs when [A-] equals [HA] and the ratio is 1. Buffers work well within about one pH unit of the pKa.
How do I find the ratio needed for a target pH?: Rearrange the equation: [A-]/[HA] = 10^(pH - pKa). For a target pH one unit above the pKa the ratio is 10; one unit below it is 0.1.

pH CalculatorCalculate pH from [H⁺], or from pKa for weak acids and buffers (Henderson-Hasselbalch).Buffer CalculatorDesign a buffer solution using the Henderson-Hasselbalch equation. Compute the [A⁻]/[HA] ratio, concentrations, moles, and masses.Titration Curve CalculatorSimulate strong- and weak-acid titrations with a strong base — plot the pH curve and find the equivalence point.Molarity & Solution Preparation CalculatorSolve for molarity, solute mass, or solution volume from the other two values.

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