TL;DR — Pick the species. Pick the compounds. Set the concentrations. Done.
Buffer design feels like a dark art. It isn’t. It’s three steps.
1. Pick the buffering species
Your target pH chooses it for you: pick a species whose pKa is within ~2 units of your target. Need pH 7.2? Phosphate (pKa₂ ≈ 7.2) is your friend.
2. Pick the compounds
Choose the acid and base forms that carry your species and counter-ion — e.g. sodium phosphate monobasic + dibasic. Using a pure species? Pair it with a strong acid or base for charge balance.
3. Set the concentrations
Enter the target buffer concentration (plus an optional salt for ionic strength) and the target pH. Solve for the proportions that land on it.
Or skip all three
Type “50 mM phosphate, 0.1 M NaCl, pH 7.4” and let the engine pick the compounds and proportions for you.
Bottom line
Species → compounds → concentrations. Same three steps, every buffer.