Phosphate Buffer Calculator

Calculate precise phosphate buffer compositions for pH 5.8-8.0 using Henderson-Hasselbalch equation

NaH₂PO₄·H₂O (Monobasic)

5.338
grams

Na₂HPO₄ (Dibasic Anhydrous)

8.704
grams

Na₂HPO₄·7H₂O (Dibasic Hydrated)

16.436
grams (alternative)

Acid:Base Ratio

1:1.58
H₂PO₄⁻:HPO₄²⁻

Ionic Strength

222.6
mM

Effective pKa

7.20
at 25°C

What is a Phosphate Buffer Calculator?

A phosphate buffer calculator determines the exact amounts of monobasic (NaH₂PO₄) and dibasic (Na₂HPO₄) sodium phosphate needed to prepare a buffer solution at any desired pH between 5.8-8.0. It uses the Henderson-Hasselbalch equation with phosphate's pKa₂ = 7.20.

How to Use This Phosphate Buffer Calculator

Quick guide to prepare your buffer:

  1. Enter your target pH (5.8-8.0)
  2. Set buffer concentration and volume needed
  3. Choose between solid salts or stock solutions
  4. Follow the calculated amounts precisely

When to Use Phosphate Buffers

Phosphate buffers are ideal for:

  • Cell culture media (pH 7.2-7.4)
  • Enzyme assays in physiological pH range
  • Western blot and immunohistochemistry
  • DNA/RNA extraction protocols
  • Protein purification and crystallization

Example Calculation

For 1L of 100mM phosphate buffer at pH 7.4:

NaH₂PO₄·H₂O: 2.78 g
Na₂HPO₄: 11.45 g
Ratio: 1:3.98 (acid:base)

Temperature and ionic strength corrections included.

Henderson-Hasselbalch Equation

The calculation is based on:

pH = pKa + log([HPO₄²⁻]/[H₂PO₄⁻])
pKa₂ = 7.20 (at 25°C)

Accounts for temperature dependence of pKa values.

FAQ

Q: What's the useful pH range for phosphate buffers?
A: pH 5.8-8.0, with optimal buffering around pKa₂ = 7.20.

Q: Should I adjust pH after mixing?
A: Theoretical calculations are very accurate, but final pH check is recommended.