Choose average buffer capacity, acid/base added per solution volume, initial pH, or final pH. Enter the requested values and, for pH estimates, choose acid or base addition.

Estimate average capacity over a measured pH interval using added strong acid/base per solution volume.

Nonnegative acid/base equivalents per solution volume, not the stock reagent concentration or total moles.

pH may lie outside 0–14; this arithmetic model supports −100 to 100.

Buffer Capacity Formula

This calculator estimates average buffer capacity over a finite pH interval: added strong acid/base equivalents per solution volume divided by the absolute pH change. This average differs from the local differential capacity; reverse pH estimates assume constant capacity and negligible dilution.

β = (C) / (|pHf - pHᵢ|)
  • β = buffer capacity, in mol/L/pH or mol/m³/pH
  • C = total amount of acid or base added per volume, in mol/L or mol/m³
  • pHi = initial pH
  • pHf = final pH
  • |pHf – pHi| = absolute pH change

To solve for the total amount of acid or base:

C = β × |pHf - pHᵢ|

To estimate final pH (use + for base addition and − for acid addition):

pHf = pHᵢ ± (C) / (β)

To estimate initial pH (use − for base addition and + for acid addition):

pHᵢ = pHf ∓ (C) / (β)

Select the quantity to solve for. Capacity and added amount per volume use the absolute pH change. For pH estimates, enter a nonnegative added amount and choose strong acid (pH decreases) or strong base (pH increases).

Buffer Capacity Units and Common Reference Values

Use consistent units when checking your result. The calculator converts mol/m³ to mol/L internally.

Quantity Unit Conversion
Total acid/base amount per volume mol/L Base unit used by the calculator
Total acid/base amount per volume mol/m³ 1 mol/m³ = 0.001 mol/L
Buffer capacity mol/L/pH Base unit used by the calculator
Buffer capacity mol/m³/pH 1 mol/m³/pH = 0.001 mol/L/pH

Buffer Capacity Result Meaning
Low value A small amount of acid or base causes a larger pH change.
Higher value More acid or base is needed to change the pH by the same amount.
Zero pH change Buffer capacity cannot be calculated because the formula would divide by zero.

Example Calculations

Example 1: Calculate buffer capacity

You add 0.020 mol/L of acid or base, the initial pH is 7.20, and the final pH is 6.80.

Δ pH = |6.80 - 7.20| = 0.40
β = (0.020) / (0.40) = 0.050 mol / L / pH

The buffer capacity is 0.050 mol/L/pH.

Example 2: Calculate total amount of acid or base

A solution has a buffer capacity of 0.075 mol/L/pH. The pH changes from 5.50 to 5.10.

Δ pH = |5.10 - 5.50| = 0.40
C = 0.075 × 0.40 = 0.030 mol / L

The total amount of acid or base is 0.030 mol/L.

FAQs

What does buffer capacity mean?

Buffer capacity measures how strongly a solution resists pH change. A larger average capacity means more acid or base per solution volume is associated with the same pH change over the measured interval.

Why does the formula use the absolute value of pH change?

Buffer capacity is usually reported as a positive quantity. The pH may increase when base is added or decrease when acid is added, but the size of the change is what matters for buffer capacity. That is why the calculator uses |final pH – initial pH| when calculating buffer capacity or total amount.

Can buffer capacity be calculated if the initial and final pH are the same?

No. If the initial and final pH are the same, the pH change is zero. The buffer capacity formula would require division by zero, so the result is undefined.