titrant

Meaning: The titrant is the standardized solution, delivered gradually from a buret, that reacts with the analyte (the solution of unknown concentration) in the flask. In acid-base work it is usually a strong acid or strong base, so the reaction goes essentially to completion.

Equation: At equivalence, moles of titrant × stoichiometric ratio = moles of analyte, with n = MV. For a monoprotic acid and a monoprotic base: MaVa = MbVb, where M is molarity (mol/L) and V is volume (L).

On the MCAT: Passages show a titration curve with pH on the y-axis and volume of titrant added on the x-axis. You locate the equivalence point at the steepest rise, calculate the unknown concentration, or read the pKa at the half-equivalence point of a weak acid (pH = pKa). A weak acid titrated with a strong base reaches equivalence above pH 7. A strong acid titrated with a strong base reaches pH 7 at 25 °C.

Common confusion: The titrant is defined by being the known solution that is added, not by being an acid or a base. The analyte can be either. The x-axis of the curve is titrant volume, not analyte volume. Also, the endpoint (the indicator color change) only approximates the equivalence point (the exact stoichiometric match).