Diprotic Acid–Base Titrations: Equivalence Points and Buffer pH

Acid base titrations

Titrations involving the reaction of acids and bases.

Titrations in diprotic systems

Principles developed for titrations of monoprotic acids and bases extend directly to titrations of polyprotic acids and bases.

Volume of the second Ve

Must be 2Ve because the second reaction requires the same number of moles of HCl as the first.

Before titration begins

pH = -log(H). The pH is determined by: Kw = Ka1 * Kb2.

Solving for x in pH calculation

When given F-X, solve for x where x = H+ and pH = -log(x).

Any point between initial point and Ve

Solution is a buffer containing B and BH+.

pH at any point between initial point and Ve

pH = pKa2 + log(B/BH+); at ½ Ve1.

At Ve

All B has been converted into BH+ subtracted by the intermediate form of the diprotic acid BH^2+.

BH+ characteristics

BH+ is both an acid and a base (Amphiprotic); use BIG equation.

Dilution factor

V = Initial volume B / total volume in solution.

At any point between Ve1 and Ve2

Solution is a buffer containing the BH+ and the acid BH^2+.

pH at 1.5Ve1

pH = pKa1 + log(BH+/BH^2+).

At VE2

Solution is formally the same as a solution of BH^2+.

pH at VE2

Determined by: Kw = Ka1 * Kb2; when given F-X, solve for x where x = H+ and pH = -log(x).