Enter the target pH, the total concentration, and the volume, and the tool tells you how much of each reagent to take. You can mix two reagents or titrate a single one with a strong acid or base, and you pick the reagent you actually own, hydrate by hydrate. Give it the concentration of a stock solution and it switches from a mass to a volume to pipette.
The panel names at the top right open and close each panel. Drag a panel by its header to move it: panels snap to each other and to the screen edge, and a panel resting on an edge drops the rounded corner on that side. Double-click a panel title to switch between floating panels and a single fixed panel at the top right.
The calculation solves the full mass balance and charge balance of the polyprotic acid, including the self-ionization of water. It is not the Henderson–Hasselbalch approximation. Ionic strength is corrected with the Davies equation, log γ = −A z²(√I/(1+√I) − 0.3 I), and the mixed constant pKa′ = pKa + D(2z − 1) treats the pH as an activity and the buffer species as concentrations, where z is the charge of the conjugate acid. Temperature enters as pKa(T) = pKa(25 °C) + (dpKa/dT)(T − 25); A(T) comes from the Malmberg–Maryott permittivity and pKw(T) from Harned–Owen. The buffer capacity is β = C·dn̄/dpH + 2.303([H⁺] + [OH⁻]).
The pale band on the colour scale is pKa ± 1, the range where a buffer actually buffers. Outside it the capacity collapses, so pick a system whose effective pKa sits near your target pH. The list of systems that fit the pH is sorted by that distance.
The dpKa/dT values are representative literature figures and can be 10 to 20 % away from what you measure. Always check the final pH with a pH meter and adjust.