Formula Used
The main recipe uses the Henderson Hasselbalch equation:
pH = pKa + log10(A- / HA)
A- / HA = 10^(pH - pKa)
Total mmol = concentration in mM × volume in mL / 1000
Acid mmol = total mmol / (1 + ratio)
Base mmol = total mmol - acid mmol
Mass in grams = mmol × molecular weight / 1000
Stock volume in mL = mmol / stock molarity
For dilution, the calculator uses C1V1 = C2V2.
How to Use This Calculator
- Select a calculation mode.
- Enter the target pH, pKa, concentration, and final volume.
- Add molecular weights when solid reagent masses are needed.
- Add stock molarities when stock solution volumes are needed.
- Press Calculate to view the result below the header.
- Download the result as CSV or PDF for lab records.
Example Data Table
| Example buffer |
Target pH |
pKa |
Concentration |
Volume |
Use case |
| Acetate pair |
5.00 |
4.76 |
10 mM |
100 mL |
General acidic buffer planning |
| Phosphate pair |
7.40 |
7.20 |
50 mM |
250 mL |
Neutral assay buffer planning |
| Tris pair |
8.00 |
8.06 |
25 mM |
500 mL |
Basic buffer preparation |
Understanding Buffer Preparation
A buffer recipe must match pH, strength, and final volume. Small errors can change a method. This calculator helps plan a weak acid and conjugate base pair before weighing or pipetting. It is useful for training, records, and quick bench checks. It is not an official vendor tool, so always follow your validated protocol.
Why pKa Matters
The pKa value shows where the acid and base forms are balanced. When target pH equals pKa, both forms are equal. When pH rises, the base portion increases. When pH falls, the acid portion increases. A good buffer usually works best within one pH unit of its pKa. Outside that range, the recipe may still calculate, but the mixture resists pH change poorly.
How the Recipe Is Built
The calculator uses the Henderson Hasselbalch relationship. It converts the chosen pH and adjusted pKa into a base to acid ratio. Then it splits total buffer moles into acid and base portions. The final volume and concentration decide the total millimoles. Molecular weights convert millimoles into grams. Stock molarities convert millimoles into milliliters. The remaining volume is an estimated solvent quantity.
Advanced Checks
Temperature can shift pKa. The temperature coefficient field gives a simple correction when data are available. The capacity score shows how close the target pH is to the best buffering zone. A score near one hundred means the pH is close to pKa. A low score warns that another buffer pair may perform better.
Using Results Safely
Use high purity water and clean glassware. Add most solvent first, then dissolve measured reagents. Mix fully before checking pH. Adjust pH slowly with compatible acid or base. Bring the solution to final volume after pH adjustment. Record lot numbers, weights, volumes, temperature, and instrument calibration. Exported files help keep these details with the calculation. For regulated work, confirm every value against your standard operating procedure. For biological assays, also confirm osmolarity, ionic strength, and additive compatibility. Dispose of chemicals according to local rules and safety sheets.
Practical Example
For a ten millimolar acetate buffer at pH five, enter acetate pKa, volume, and concentration. The output separates acetic acid and sodium acetate needs. You can scale the same recipe for larger batches.
FAQs
1. Is this an official Sigma Aldrich tool?
No. It is an independent planning calculator. Use it for estimates, teaching, and preparation notes. Always verify values with your lab protocol, reagent label, and safety documentation.
2. Which equation does the calculator use?
It mainly uses the Henderson Hasselbalch equation. It also uses mole, mass, and dilution relationships. These formulas help estimate ratios, reagent weights, and stock volumes.
3. What does the capacity score mean?
The capacity score shows how close the target pH is to the pKa. A higher score means stronger buffering behavior. A low score suggests choosing another buffer pair.
4. Can I use stock solutions?
Yes. Enter acid and base stock molarities. The calculator converts the required millimoles into milliliters. Leave stock fields as zero when weighing solids.
5. Why is the solvent volume only estimated?
Stock additions, solids, pH adjustment, and temperature can change final volume. Add most solvent first. Adjust pH. Then bring the solution to the final mark.
6. What pKa should I enter?
Use the pKa for the buffer pair and working temperature. Reliable sources include product documents, chemical references, and validated internal methods.
7. Can this calculator adjust pH automatically?
It estimates acid and base requirements. Actual pH should be measured with a calibrated meter. Make final adjustment slowly using compatible acid or base.
8. Can I export the result?
Yes. After calculation, use the CSV or PDF button. These files help save formulas, quantities, and warnings with your lab notebook.