Calculate grams of monobasic and dibasic sodium or potassium phosphate for a phosphate buffer from your target pH, final volume and total concentration.

Total concentration = monobasic + dibasic phosphate.

Formula weights used for the masses. Pick Moles only if your salts differ.

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Phosphate Buffer Formula

The Henderson-Hasselbalch equation is used to calculate the ratio of dibasic to monobasic phosphate.

pH = pKa + log(([Dibasic]) / ([Monobasic]))
  • Where pH is the desired pH level of the buffer solution
  • pKa₂ is the negative base-10 logarithm of the second acid dissociation constant (approximately 7.20 near 25 °C)
  • [Dibasic] represents the concentration of HPO₄²⁻; its mole ratio is the same when both forms share the final volume
  • [Monobasic] represents the concentration of H₂PO₄⁻

To prepare the buffer, use the Henderson-Hasselbalch equation to determine the ratio of the two phosphate forms and calculate the required moles of each.

What is a Phosphate Buffer?

Definition:

A phosphate buffer is a solution designed to resist changes in pH, commonly used in biochemical and laboratory applications. It is prepared by mixing monobasic and dibasic phosphate salts.

How to Prepare a Phosphate Buffer?

Example Problem:

The following example outlines the steps and information needed to prepare a phosphate buffer solution.

First, determine the desired pH of the buffer. For instance, if you aim for a pH of 7.40, enter this value into the calculator.

Next, specify the total buffer volume and the total buffer concentration. In this example, consider preparing 1 L of buffer at a concentration of 0.1 M.

Finally, calculate the required moles of monobasic and dibasic phosphate using the equation above:

pH = pKa + log([Dibasic] / [Monobasic])

The calculated values give approximate starting moles of each phosphate form, not guaranteed final pH or salt masses. Prepare to the specified final volume, measure pH at the working temperature and adjust as needed. This tool is scoped to pH 6.2–8.2 around pKa₂ = 7.20.

FAQ

What factors can affect the pH of a phosphate buffer?

The pH of a phosphate buffer can be influenced by temperature, ionic strength, and the purity of the phosphate salts used.

Can this calculator be used for buffers with different pKa values?

This calculator is designed for phosphate buffers with a pKa of 7.20; adjustments are needed when working with other buffering systems.

How do I modify the buffer preparation for different volumes or concentrations?

Simply enter the desired total buffer volume and concentration into the calculator to recalculate the required moles of each phosphate component.