All calculatorsBuffer Preparation Calculator
Prepare a buffer at a target pH and final volume. Pick a buffer system (or enter your own pKa), choose weigh solids or mix stock solutions, and get a clean, step-by-step lab recipe (moles, grams, mL, and water-to-add).
Background
A buffer is a mixture of a weak acid (HA) and its conjugate base (A⁻) that resists pH change. The key relationship is the Henderson–Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).
How to use this calculator
- Pick a buffer system (or choose Custom and enter your own pKa).
- Enter your target pH, total buffer concentration Ctotal, and final volume.
- Choose a preparation method: Mix stocks (mL) or Weigh solids (g).
- Click Calculate to get a complete recipe + optional step-by-step.
How this calculator works
- Compute the needed ratio: R = [A⁻]/[HA] = 10^(pH − pKa).
- Split the total concentration: [HA] = Ctotal/(1+R) and [A⁻] = Ctotal·R/(1+R).
- Convert to moles using final volume V(L): n = C × V.
- Stocks mode: compute volumes: Vstock = n / Mstock.
- Solids mode: compute grams: g = n × MW.
Formula & Equation Used
Henderson–Hasselbalch: pH = pKa + log([A⁻]/[HA])
Ratio: R = [A⁻]/[HA] = 10^(pH − pKa)
Total concentration: Ctotal = [HA] + [A⁻]
Split: [HA] = Ctotal/(1+R), [A⁻] = Ctotal·R/(1+R)
Moles: n = C × V (with V in L)
Stocks: Vstock(L) = n / Mstock
Solids: g = n × MW
Example Problems & Step-by-Step Solutions
Example 1 — Mix stocks (acetate)
Prepare 500 mL of 0.10 M acetate buffer at pH = 4.76 (assume pKa = 4.76) using 1.0 M HA stock and 1.0 M A⁻ stock.
- R = 10^(pH − pKa) = 10^0 = 1 → equal parts HA and A⁻.
- [HA] = 0.10/(1+1) = 0.05 M, [A⁻] = 0.05 M.
- V = 0.500 L → moles: n(HA)=0.05×0.5=0.025 mol and same for A⁻.
- Stocks are 1.0 M → V(HA)=0.025 L=25 mL, V(A⁻)=25 mL, top up with water to 500 mL.
Example 2 — Weigh solids (generic)
Prepare 250 mL of 0.10 M buffer at pH = 8.10 with pKa = 8.06. The calculator finds the needed moles of HA and A⁻, then converts to grams using your entered molar masses.
Frequently Asked Questions
Q: Why does “pH = pKa” give a 50/50 buffer?
Because when pH = pKa, the ratio [A⁻]/[HA] = 10^0 = 1. That means equal concentrations of acid and base forms.
Q: Will this recipe always hit the pH perfectly?
It’s a strong starting point, but real lab pH can shift due to temperature, ionic strength, reagent purity, and hydrate form. Many labs prepare the buffer, then do a small final pH adjustment with dilute acid/base.
Q: What if my required stock volumes are larger than my final volume?
That means your stocks are too dilute for the concentration/volume you want. Use more concentrated stocks, or reduce Ctotal, or increase final volume.
