How this calculator works
Observed capacity is the amount of strong acid or base added per solution volume per unit pH change. The finite-difference value averages the response across the entered pH interval; it is not necessarily the differential capacity at one pH. Use equivalents, so one mole of transferred H+ or OH- is one mole here. The volume is treated as constant. Reagent direction controls inverse pH modes; measured modes use the absolute observed change.
Formula
beta = added equivalents / (solution volume x absolute pH change). Change in pH = added equivalents / (beta x volume).
Worked example
Adding 0.01 mol equivalents to 1 L while pH changes from 5 to 6 gives 0.01 mol/(L pH). At capacity 0.02 mol/(L pH), 2 L and a change of 0.5 need 0.02 mol.
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Frequently asked questions
Why include volume?
Capacity is an intensive quantity. The same pH response in twice the volume needs twice the added amount.
Can initial and final pH be equal?
A zero change makes the finite-difference calculation undefined.
Does this predict buffer exhaustion?
No. A single observed capacity does not describe the complete titration curve.