If you need to prepare a potassium phosphate buffer in the lab, this calculator helps you quickly determine how much monobasic and dibasic phosphate you need for your target pH, concentration, and final volume.
Potassium Phosphate Buffer Calculator
How this potassium phosphate buffer calculator works
Potassium phosphate buffer is usually made from two components:
- KH2PO4 (monobasic; acidic component)
- K2HPO4 (dibasic; basic component)
The pH is set by the ratio of base to acid, while the total concentration determines how much buffering capacity you have.
pH = pKa + log10([base]/[acid])
From this relation, the calculator finds the base:acid ratio, then splits your total phosphate concentration into each component, and finally reports masses and optional stock volumes.
What you get in the output
- Base/acid ratio for your selected pH
- Concentration of each species (mM)
- Moles required for final volume
- Mass of KH2PO4 and K2HPO4 to weigh
- Volumes to pipette if using stock solutions
Step-by-step preparation notes
1) Decide your target pH and concentration
A common choice is 10–100 mM total phosphate near neutral pH. For many biochemical workflows, 50 mM and pH 7.2–7.4 is typical.
2) Prepare with solids or stocks
You can either weigh powders directly or mix concentrated stocks. Direct weighing is good for one-off batches; stock solutions are often better for routine prep and reproducibility.
3) Dissolve in about 80–90% of final volume
After adding salts (or stock solutions), mix thoroughly before final pH check.
4) Fine-tune pH if needed
Real solutions can deviate slightly due to temperature, ionic strength, or hydration state of salts. Adjust with small additions of acid/base if needed.
5) Bring to final volume
Adjust with purified water to exactly your target volume, then re-check pH.
Important practical considerations
- Temperature matters: pKa shifts with temperature, so pH may move when solutions warm or cool.
- Check hydration state: some phosphate salts are hydrates and have different molecular weights.
- Autoclaving can shift pH slightly: verify pH after sterilization when precision is important.
- Compatibility: phosphate can precipitate with divalent cations (e.g., calcium, magnesium) at higher concentrations.
Quick FAQ
What pH range is potassium phosphate buffer best for?
Typically around the second phosphate pKa, roughly pH 6.2 to 8.2, with strongest buffering near pH 7.2.
Can I use sodium phosphate values for potassium phosphate?
The acid-base chemistry is the same, but masses are different due to molecular weight differences. Use correct MW for the exact salts you prepare.
Why does measured pH differ from calculated pH?
Small differences happen due to meter calibration, temperature, ionic strength, salt purity, and hydration state. Treat calculator output as an excellent starting point, then verify experimentally.
Lab disclaimer
This tool is for educational and planning use. Always verify calculations, check your reagent labels, and confirm final pH in the actual lab workflow before use in sensitive experiments.