How to make a buffer at a target pH by hand

The Henderson-Hasselbalch equation is pH = pKa + log([A⁻]/[HA]). Rearrange for the ratio you need: [A⁻]/[HA] = 10^(pH − pKa). Split your total concentration into the two forms using fraction A⁻ = ratio/(1+ratio) and fraction HA = 1/(1+ratio), then multiply each concentration by the volume (in liters) to get moles — and by the molar mass for grams. A buffer works best within about pKa ± 1; outside that range it has little capacity, so pick a buffer whose pKa is close to your target pH.

pKa values shown are standard 25 °C textbook values; some sources differ by a few hundredths, and phosphate/bicarbonate shift with temperature and ionic strength (blood bicarbonate is often quoted as 6.1).