Sodium Phosphate Buffer Calculator
Calculate monobasic/dibasic sodium phosphate amounts from target pH, concentration, and final volume.
What this Na phosphate buffer calculator does
This calculator helps you prepare a sodium phosphate buffer using the monobasic and dibasic sodium phosphate pair. You set your desired pH, total phosphate concentration, and final volume, and it returns:
- Required acid/base ratio from the Henderson–Hasselbalch equation
- Final concentration of each phosphate species
- Moles needed for each component
- Mass to weigh for common hydrate forms
- Optional volumes to pipette from stock solutions
The chemistry behind sodium phosphate buffers
Conjugate pair used near neutral pH
Near neutral pH, phosphate buffering is dominated by: H2PO4- (acid) and HPO42- (base). Their pKa2 is around 7.2 at room temperature, making phosphate a useful buffer around pH 6.2–8.2.
Core equation
[base]/[acid] = 10^(pH - pKa)
Once that ratio is known, total phosphate concentration is split between acid and base: Ctotal = [acid] + [base].
How to use the calculator
- Enter target pH.
- Confirm or adjust pKa (temperature dependent).
- Enter total phosphate concentration in mM.
- Enter final volume in mL.
- Select the exact hydrate forms in your lab bottle labels.
- Optionally enter stock molarities to get pipetting volumes.
Worked example (50 mM, pH 7.40, 1 L)
With pKa = 7.21, the base/acid ratio is 10(7.40 − 7.21) ≈ 1.55. For 50 mM total phosphate, that gives roughly:
- Acid (monobasic) ≈ 19.6 mM
- Base (dibasic) ≈ 30.4 mM
In 1.0 L, that corresponds to about 0.0196 mol acid and 0.0304 mol base. The exact gram values then depend on whether you use anhydrous or hydrated salts.
Practical lab notes
1) Temperature shifts pH
pKa changes with temperature. If your protocol requires strict pH control, calibrate your pH meter at working temperature and fine-adjust after dissolution.
2) Ionic strength and additives matter
Added salts (e.g., NaCl), proteins, and solvents can shift measured pH. Check pH in the final matrix, not only in water.
3) Adjust to final volume last
Dissolve salts first in ~80–90% of target volume, adjust pH if needed, then bring to final volume.
4) Sterility and storage
For cell culture or sensitive workflows, filter sterilize (0.22 µm) and store under clean conditions.
FAQ
Is this the same as PBS?
Not exactly. PBS is a phosphate-buffered saline formulation with defined NaCl/KCl content. This calculator only handles the phosphate buffer pair.
Can I use this for potassium phosphate?
The acid/base ratio math is the same, but molecular weights differ. Use correct salt MW values if switching systems.
Why can calculated and measured pH differ slightly?
Real solutions deviate from ideal assumptions due to activity effects, temperature, hydration state errors, and instrument calibration.
Final reminder
This tool is intended for fast lab planning. Always verify pH experimentally, confirm chemical identity and hydrate state from bottle labels, and follow your institution’s SOP and safety guidelines.