Sørensen’s Phosphate Buffer Calculator

Enter your target pH and ionic strength. The calculator determines the phosphate concentration and gives you the exact mixing volumes and weights to prepare your buffer.

What Do You Need?

Target pH

Target ionic strength (M)

0.15 M is typical for physiological conditions

Final volume (mL)

Dibasic reagent form

Buffer Recipe

Required total phosphate concentration

0.0674 M

Calculated from your target ionic strength of 0.15 M at pH 7.40

Solution A (monobasic): 38.7%Solution B (dibasic): 61.3%

Solution A — Monobasic

38.7 mL

NaH₂PO₄·H₂O

Solution B — Dibasic

61.3 mL

Na₂HPO₄ (anhydrous)

Disclaimer:This calculator is provided as a guide only. Always verify your buffer pH with a calibrated meter before use. Results assume 25 °C and ideal behaviour — actual pH may vary with temperature, ionic effects, and reagent purity.

Preparation Protocol

1. Prepare Stock A (0.0674 M monobasic): Dissolve 0.930 gof NaH₂PO₄·H₂O (MW 137.99) in deionised water. Make up to 100 mL in a volumetric flask.
2. Prepare Stock B (0.0674 M dibasic): Dissolve 0.956 gof Na₂HPO₄ (anhydrous, MW 141.96) in deionised water. Make up to 100 mL.
3. Mix: Combine 38.7 mL of Stock A with 61.3 mL of Stock B to give 100 mL of buffer at pH 7.40 and ionic strength 0.15 M.
4. Verify: Check the pH with a calibrated meter. Adjust with a small amount of Stock A (lower pH) or Stock B (raise pH) if needed.

Parameter	Value
pH	7.40
Ionic strength	0.15 M
Phosphate concentration	0.0674 M
Final volume	100 mL
NaH₂PO₄·H₂O to weigh	0.930 g
Na₂HPO₄ to weigh	0.956 g

About Sørensen’s Phosphate Buffer

Sørensen’s phosphate buffer is one of the most widely used buffer systems in biological and biochemical research. Developed by S.P.L. Sørensen in 1909, it uses a mixture of monobasic sodium phosphate (NaH₂PO₄) and dibasic sodium phosphate (Na₂HPO₄) to maintain a stable pH between approximately 5.8 and 8.0.

How This Calculator Works

You specify the pH and ionic strength you need. The calculator uses the Henderson-Hasselbalch equation to find the ratio of monobasic to dibasic phosphate, then works backwards from your target ionic strength to determine the total phosphate concentration:

pH = pK_a+ log([HPO₄²−] / [H₂PO₄−])

I = c × (f_A + 3·f_B)

where c is the total phosphate concentration, f_A and f_B are the fractions of monobasic and dibasic species, and I is the ionic strength. The pK_a2of phosphoric acid is 7.20 at 25 °C, placing phosphate buffers right in the physiological range.

Why Use Phosphate Buffers in Microfluidics?

Phosphate buffers are particularly well-suited for microfluidic applications because they are non-toxic, have minimal interaction with most polymer substrates (COC, COP, PMMA), and maintain stable pH across the temperature fluctuations typical of lab-on-chip devices. They are commonly used as carrier fluids, wash buffers, and reaction media in droplet-based and continuous-flow microfluidic systems.

Limitations

The effective buffering range is pH 5.8–8.0 (within ±1 unit of pK_a2). Outside this range, buffer capacity drops sharply. Phosphate buffers can also interfere with assays involving divalent cations (Ca²⁺, Mg²⁺) due to precipitation of insoluble phosphate salts. They are not recommended for use with calcium-dependent enzymes.

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