Phosphate Buffer Calculator

A phosphate buffer is a salt solution used in labs and biology to maintain a stable pH — this calculator tells you exactly how much of each salt to weigh out. Enter your buffer volume, target pH (5.8–8.0), and buffer concentration, then select your phosphate salt type to get the grams of monosodium phosphate and disodium phosphate required. Secondary outputs include the concentration of each component in mM and the base to acid ratio.

mL
M

Results

Monosodium Phosphate Required

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Disodium Phosphate Required

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Monosodium Phosphate Concentration

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Disodium Phosphate Concentration

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Base to Acid Ratio

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Frequently Asked Questions

What is the pH range for phosphate buffers?

Phosphate buffers are most effective in the pH range of 5.8 to 8.0, with optimal buffering capacity around pH 7.2. This range corresponds to the second dissociation constant (pKa2) of phosphoric acid.

How does the Henderson-Hasselbalch equation apply to phosphate buffers?

The Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) determines the ratio of dibasic phosphate (HPO4²⁻) to monobasic phosphate (H2PO4⁻) needed to achieve the desired pH.

What's the difference between anhydrous and hydrated phosphate salts?

Anhydrous salts contain no water molecules, while monohydrate contains one water molecule and dihydrate contains two per salt molecule. The hydrated forms have higher molecular weights, affecting the mass calculations.

Why do phosphate buffers have high buffering capacity?

Phosphate buffers have high buffering capacity because phosphoric acid is triprotic with three dissociation constants, providing multiple buffering regions and high salt concentrations.

Can phosphate buffers be used in biological experiments?

Yes, phosphate buffers are widely used in molecular biology and biochemistry. However, they can inhibit some enzymes and may precipitate in the presence of divalent cations or ethanol.

How do I prepare the buffer solution once I have the calculated amounts?

Dissolve the calculated amounts of both phosphate salts in approximately 80% of the final volume of distilled water, adjust pH if needed, then dilute to the final volume.

What factors affect the accuracy of phosphate buffer calculations?

Temperature, ionic strength, and the presence of other ions can affect the actual pH. The calculations assume ideal conditions and may need slight adjustments in practice.

How stable are phosphate buffer solutions?

Phosphate buffers are generally stable when stored properly at room temperature. However, they should be protected from microbial growth and contamination, especially in biological applications.