Buffer pH Calculator

Why Some Solutions Resist a Change in pH

Add a drop of acid to plain water and the pH swings immediately. Add that same drop to a buffer — a mixture of a weak acid and its conjugate base — and the pH barely moves. That resistance to change is the entire point of a buffer, and the Henderson-Hasselbalch equation is what lets you calculate exactly where that stable pH sits before you ever mix the solution.

The Formula

pKa is a fixed property of the weak acid being used. [A−] is the concentration of the conjugate base component, and [HA] is the concentration of the remaining weak acid — when they're equal, the log term drops to zero and the buffer pH equals the pKa exactly.

Where This Calculation Matters

• Cell culture and biochemistry — growth media and enzyme assays require a stable pH, usually maintained with a phosphate or Tris buffer.
• Pharmaceutical formulation — injectable and oral drug formulations are buffered to a pH that keeps the active ingredient stable and tolerable.
• Environmental and soil chemistry — natural bicarbonate buffering in soil and water resists pH swings from acid rain or runoff.
• Analytical method development — many chromatography and titration methods specify an exact buffer pH for reproducible results.

Buffer pH from an Acetic Acid / Acetate System (pKa = 4.76)

A buffer works best within about ±1 pH unit of its pKa, where both components are present in comparable amounts.

How to Use This Calculator

1. Enter the pKa of Weak Acid for the buffer system in use.
2. Enter the Conjugate Base Concentration [A-] (mol/L).
3. Enter the Weak Acid Concentration [HA] (mol/L).
4. Select Calculate to get the resulting buffer pH.

Related Calculations

Need to titrate a strong acid or base to a specific endpoint instead? See the Acid-Base Titration Calculator. For converting a raw hydrogen ion concentration into pH without a buffer system, use the pH Calculator.