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Buffer Solution pH Preparation Calculator (Henderson-Hasselbalch)

Preparing a buffer at a precise pH starts with the ratio of conjugate base to weak acid. This calculator applies the Henderson-Hasselbalch equation, then converts the ratio into moles and grams so you can weigh the acid and salt for any final volume.

Your numbers

pKa of the weak acid

Acetic acid is 4.76 at 25 C. Use the value for your temperature.

Target pH

Total buffer concentration

mol/L

Acid plus conjugate base.

Final volume

Molar mass of acid form

g/mol

Acetic acid 60.05.

Molar mass of conjugate base salt

g/mol

Anhydrous sodium acetate 82.03.

Your results

Mass of acid

21.93

Grams of weak acid to add.

Mass of conjugate base salt

52.07

Grams of salt to add.

Base to acid ratio

1.7378

10 raised to (pH minus pKa).

Base share of total buffer

63.5%

Total moles of buffer

Moles of acid

0.3653

Moles of conjugate base

0.6347

The Henderson-Hasselbalch equation is an approximation that works best for dilute solutions and pH within about one unit of pKa. Ionic strength, temperature and activity effects shift the real pH, so always trim with a calibrated pH meter.

Save results:

Base share of buffer vs target pH

Sweeps Target pH from 3.5 to 6, holding your other inputs fixed.

Recipe and reagent traceability

Ask how buffer and reagent preparations can be tied to lot-controlled inventory and batch records.

Optional. The tool and every export work without this. No spam.

How to use the calculator

Enter the pKa of the weak acid, the pH you want and the total buffer concentration, meaning acid plus conjugate base, and the final volume in liters. Add the molar masses of the acid form and the salt form. The calculator returns the base to acid ratio, the moles of each component and the mass to weigh. Dissolve in about 80 percent of the final volume, adjust pH with a small amount of strong acid or base if needed, then make up to volume.

Use the molar mass of the exact form you weigh, such as a hydrate
Match the pKa to your working temperature
Check the pH meter calibration before final adjustment

The formula behind it

Henderson-Hasselbalch states that pH equals pKa plus the log of the base to acid ratio. Rearranged, the ratio equals 10 raised to the difference between pH and pKa. When the pH equals the pKa the ratio is one and the two forms are present in equal amounts. With the total concentration fixed, acid moles equal total moles divided by one plus the ratio, and base moles are the rest. Buffer capacity is highest near the pKa and falls quickly outside about one pH unit either side.

Worked example and mistakes

For 10 L of 0.1 mol/L acetate buffer at pH 5.00 with pKa 4.76, the ratio is about 1.74, so around 63.5 percent of the buffer is acetate. That means roughly 0.37 mol of acetic acid and 0.63 mol of sodium acetate, or about 22 g and 52 g. Common mistakes include using the wrong salt hydrate mass, ignoring temperature effects on pKa, and choosing a buffer whose pKa is far from the target pH. For polyprotic acids, use the pKa closest to your target.

Pick a buffer with pKa within one unit of the target pH
Do not calibrate the meter with the buffer you are making
Record lot numbers and final measured pH on the batch record

Frequently Asked Questions

What is the Henderson-Hasselbalch equation?

It is pH = pKa + log10([A-]/[HA]), relating a buffer's pH to the ratio of conjugate base to weak acid. It is a good approximation for dilute buffers when pH lies within about one unit of the pKa.

Why does my measured pH differ from the calculated value?

Ionic strength, temperature, activity coefficients and the accuracy of the pKa all shift the real pH. Prepare to the calculated amounts, then trim to the target using a calibrated meter and small additions of acid or base.

What buffer concentration should I use?

It depends on the capacity you need. Higher concentration resists more added acid or base but changes ionic strength and cost. Many process buffers fall between 0.01 and 0.5 mol/L, so follow your method or process specification.

Can I use it for ammonium or phosphate buffers?

Yes, if you enter the right pKa and molar masses. For ammonium use the acid form NH4+ with pKa about 9.25, and for phosphate use the pKa nearest your target pH and the correct salt forms.

If buffer and reagent preparations are part of your batch records, Netray can discuss how to link them to lot-controlled inventory and quality documents in your ERP.

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