Buffer Solution Calculator

Mode

Two-stock (acid/base forms) Single-stock + titrant

Precision

3 sig figs 6 sig figs

Buffer system (optional pick)

Citrate has multiple sites; choose the site relevant to your pH.

Target pH

pKa at 25 °C

Temperature adjustment (optional)

Temperature (°C)

ΔpKa per °C

Total concentration C_tot

Final volume V_f

Stock base (A⁻ or B) concentration (M)

Stock acid (HA or BH⁺) concentration (M)

Choose stock type

Acid stock (HA) Base stock (A⁻)

Stock concentration (M)

Strong titrant concentration (M)

Use NaOH for acid stock or HCl for base stock.

Details: approximate buffer capacity near setpoint

Introduction

Buffers resist pH change by pairing a weak acid with its conjugate base (or, for basic buffers, a weak base with its conjugate acid). The most stable performance occurs when the working pH is close to the buffer’s pK_a, where the acid and base forms are present in comparable amounts. A practical rule is to plan within roughly pK_a ± 1 pH unit. This calculator supports two preparation routes. In the two-stock approach you mix known concentrations of the acid and base forms to hit a desired pH, total concentration, and final volume. In the single-stock route you start with either the acid or base form and add a strong titrant (NaOH or HCl) until the Henderson–Hasselbalch ratio is achieved.

The tool converts the target pH to a base-to-acid ratio using Henderson–Hasselbalch, splits the total concentration into fractions, and back-calculates volumes from your stock strengths. An optional temperature adjustment lets you shift the pK_a if you know a ΔpK_a/°C. Feasibility checks warn you when the computed stock volumes exceed the final vessel size or when any volume would be negative—both indicate stronger stocks or a different final volume are needed. Expand the capacity section to see an approximate buffer capacity near the setpoint, which provides a sense of how strongly the buffer will hold its pH during small additions of acid or base.

How to use

Pick a buffer system from the list or type a custom pK_a.
Enter the target pH, total concentration, and final volume.
Choose Two-stock or Single-stock + titrant and provide stock concentrations.
Set precision (3 or 6 significant figures) and calculate.
Review the volumes and the A⁻/HA ratio; adjust inputs if you see feasibility warnings.

Formula notes

Henderson–Hasselbalch: pH = pK_a,T + log10(A⁻/HA) ⇒ ratio = 10^(pH − pK_a,T).
Fractions: α_base = ratio/(1+ratio), α_acid = 1/(1+ratio).
n_total = C_tot·V_f (mL→L = ÷1000). n_A⁻ = α_base·n_total, n_HA = α_acid·n_total.
Two-stock: V_A⁻ = n_A⁻/C_A⁻, V_HA = n_HA/C_HA, V_diluent = V_f − (V_A⁻ + V_HA).
Single-stock (acid+base): V_stock = n_total/C_HA; V_titrant = n_A⁻/C_tit. (For base+acid: V_stock = n_total/C_A⁻; V_titrant = n_HA/C_tit.)

Disclaimer. Educational use only. Not a substitute for professional judgment.