Calculate buffer capacity, total acid/base amount, initial pH, or final pH from any three values using mol/L or mol/m³ units in any order.
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Buffer Capacity Formula
Buffer capacity is the amount of strong acid or strong base needed to change the pH of a solution by one pH unit. This calculator uses the total amount of acid or base per volume and the pH change.
- 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:
To solve for final pH:
To solve for initial pH:
The calculator lets you enter any three values and solves for the missing one. For buffer capacity and total amount, it uses the absolute pH change. For pH calculations, it follows the displayed direction of change, so a positive amount increases pH in the final pH calculation.
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.
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.
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 buffer capacity means the solution can absorb more added acid or base before its pH changes by one unit.
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.
