Determining an unknown concentration by titration

The problem to solve

Imagine you have a hydrochloric acid solution whose exact concentration you don't know. How can you determine it precisely, without very sophisticated equipment? That is exactly the role of titration (also called volumetric analysis): a technique that measures the concentration of a chemical species by reacting it, quantitatively, with a solution of known concentration.

The general principle

The solution of unknown concentration is called the titrated solution, and the solution of known concentration the titrant. The titrant is progressively added to a precise volume of the titrated solution, until the reaction between the two is exactly complete: neither excess acid nor excess base. This precise moment is called the equivalence point.

Why an acid-base reaction

An acid-base titration relies on a reaction between an acid and a base, which exchange H+ ions (protons). For example, hydrochloric acid HCl reacts with sodium hydroxide NaOH according to:

H3O+ (aq) + HO- (aq) -> 2 H2O (l)

(the oxonium and hydroxide ions react to form water)

This equation is balanced: 3 H atoms and 1 O atom on each side, and a total charge of zero on both sides (+1 - 1 = 0 on the left, 0 on the right).

What we're really looking for

By precisely noting the volume of titrant added at the equivalence point, we can then calculate the unknown concentration, as you'll see in the second chapter. Titration thus turns a chemistry problem (a concentration invisible to the naked eye) into a volume measurement problem, much simpler to carry out precisely.

Common mistake

Don't confuse the titrated solution (the one whose concentration we're looking for, placed in the beaker) with the titrant (the one whose concentration we know, placed in the burette): swapping the two makes any calculation inconsistent.