What this calculator does
Buffer capacity (Van Slyke equation) works out how resistant a buffer solution is to pH change, from its total concentration and pKa. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
Formulaβ = 2.303·([H+] + THA·Ka·[H+]/(Ka+[H+])² + Kw/[H+]), Kw = 1.0×10⁻¹⁴ (Van Slyke, 1922)
The inputs explained
| Field | What to enter |
|---|---|
| Buffer pH | A number. Starts at 4.76. |
| pKa of the weak acid | A number. Starts at 4.76. |
| Total buffer concentration (THA) (mol/L) | A number, measured in mol/L. Starts at 0.1. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with buffer ph
Every other input is held at the calculator’s starting values while buffer ph varies. Select any row to load that scenario into the calculator.
How the answer changes with buffer ph
| Buffer pH | Buffer capacity β | [H+] used | Ka used |
|---|---|---|---|
| 2.38 | 0.0106 mol/(L·pH) | 0.00416869 mol/L | 0.00001738 mol/L |
| 3.57 | 0.0137 mol/(L·pH) | 0.00026915 mol/L | 0.00001738 mol/L |
| 4.76 | 0.0576 mol/(L·pH) | 0.00001738 mol/L | 0.00001738 mol/L |
| 7.14 | 0.000953 mol/(L·pH) | 7.2444e-8 mol/L | 0.00001738 mol/L |
| 9.52 | 0.000080 mol/(L·pH) | 3.0200e-10 mol/L | 0.00001738 mol/L |
| 14.28 | 4.388 mol/(L·pH) | 5.2481e-15 mol/L | 0.00001738 mol/L |