Buffer pH Calculator

Q: How do I prepare a buffer at a specific pH?

Choose a weak acid whose pKa is within 1 unit of your target pH. Use the Henderson-Hasselbalch equation to find the required [A - ]/[HA] ratio: ratio = 10 (pH − pKa) . Mix the weak acid and its conjugate base (or its salt) in that ratio at your desired total concentration. For pH 7.4 with phosphate buffer (pKa 7.20), the ratio is 10 0.20 ≈ 1.58 base to acid.

Frequently Asked Questions

What is the Henderson-Hasselbalch equation?

pH = pKa + log10([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] is the weak acid concentration. It approximates the pH of a buffer solution from the pKa of the weak acid and the ratio of its ionized to un-ionized forms. The approximation works well when both concentrations are above ~0.001 M and pKa is between about 3 and 11.

What is the effective buffering range of a buffer?

A buffer works most effectively within one pH unit of its pKa (pKa − 1 to pKa + 1). Outside this range, the ratio of acid to conjugate base becomes extreme (>10:1 or <1:10), and the buffer has little capacity to resist pH changes. For example, an acetate buffer (pKa 4.76) is effective from about pH 3.76 to 5.76.

How do I prepare a buffer at a specific pH?

Choose a weak acid whose pKa is within 1 unit of your target pH. Use the Henderson-Hasselbalch equation to find the required [A-]/[HA] ratio: ratio = 10(pH − pKa). Mix the weak acid and its conjugate base (or its salt) in that ratio at your desired total concentration. For pH 7.4 with phosphate buffer (pKa 7.20), the ratio is 100.20 ≈ 1.58 base to acid.

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How This Calculator Works

This calculator applies the Henderson-Hasselbalch equation to find the pH of a buffer solution or to determine the acid-to-base ratio needed to reach a target pH. A buffer is a mixture of a weak acid and its conjugate base that resists changes in pH when small amounts of strong acid or base are added.

Select "Calculate pH" to find the resulting pH from known concentrations. Select "Calculate ratio needed" to find the [A^-]/[HA] ratio your buffer must have to achieve a specific pH.

The Henderson-Hasselbalch Equation

The equation is derived from the acid dissociation equilibrium expression:

pH = pK_a + log₁₀([A^-] / [HA])

log₁₀([A^-] / [HA]) = pH_target − pK_a

Ratio [A^-] / [HA] = 10^{(pH_target − pK_a)}

When [A^-] = [HA], the ratio is 1, log₁₀(1) = 0, and pH = pK_a. This is the midpoint of the buffer range and the point where buffer capacity is greatest.

Buffer Capacity and Range

A buffer is most effective when the desired pH falls within one unit of the pK_a (pK_a − 1 to pK_a + 1). Outside this range, one component dominates so completely that the buffer offers little resistance to pH change. For best performance, choose a weak acid whose pK_a is as close as possible to the target pH.

Buffer capacity also depends on total concentration. A 1 M buffer can absorb far more acid or base than a 0.01 M buffer at the same pH before the pH shifts significantly.

Common Buffer Systems

Acetic acid / acetate: pK_a 4.76 (effective range pH 3.8 to 5.8)

Phosphate (H₂PO₄^- / HPO₄^2-): pK_a 7.20 (effective range pH 6.2 to 8.2)

Bicarbonate / carbonate (HCO₃^- / CO₃^2-): pK_a 10.33 (effective range pH 9.3 to 11.3)

TRIS (Tris-H⁺ / Tris): pK_a 8.06 at 25 °C (effective range pH 7.1 to 9.1)

HEPES: pK_a 7.55 (effective range pH 6.6 to 8.6)

TRIS and HEPES are especially popular in biochemistry because their pK_a values fall near physiological pH and they do not interfere with many biological assays.

Choosing a Buffer for Your pH

Match the buffer pK_a to your target pH. If your target is pH 7.4 (physiological), phosphate or HEPES are good choices. For pH 5.0, acetate works well. For pH 9.0, TRIS or carbonate are appropriate. Always verify buffer compatibility with your reagents: some metals precipitate with phosphate, and TRIS absorbs carbon dioxide over time, shifting the pH.

Does temperature affect buffer pH?

Yes. The pK_a of most buffers shifts with temperature. TRIS is particularly sensitive, changing about −0.028 pH units per degree Celsius. Phosphate buffers are more stable. Always calibrate your pH meter and prepare buffers at the temperature at which they will be used.

What happens outside the effective buffer range?

When pH differs from pK_a by more than one unit, one form (acid or base) is present at less than 9% of the total buffer concentration. Adding even a small amount of strong acid or base consumes that minority form quickly and the pH changes rapidly. The buffer has not failed completely, but its capacity is severely reduced.

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