Titration involves gradually adding a solution of known concentration into a sample of a solution of unknown concentration until the solute of unknown concentration is used up, or reacted away. The quantity of known concentration used determines the concentration of solvent. The solution of unknown concentration is called the “titer.” The titrating solution is called the “titrant.” Usually, a chemical indicator is used in the titer, changing color when the titer has been used up in reaction.
In acid-base titration, the titrant is an acid or base, and the titer is the opposite. The titrant is added until the titer is fully neutralized and the water solution has become neutral (pH 7.0). A pH meter will show a steep change in pH (when graphed in a “titration curve”) when the titer is passing through the equivalence point because the concentration of hydronium (H3O+) and hydroxide ions (OH-) in neutral water is 10?? molals (moles per liter) each. Addition of a drop or two of acid or base to neutral water would therefore increase the H3O+ or OH- concentrations by orders of magnitude.
Another type of titration curve graphs conductivity against titrant. Such titrations can be used on acids and bases because they conduct electricity, but can also be used on a wider range, e.g., reactions that use ions in general. Furthermore, an indicator is not needed, eliminating the need to find one that changes near the equivalence point’s pH. The titration curve will again change precipitously at the equivalence point as in a pH titration curve. In the case of an acid-base reaction, the equivalence point would have a noticeable dip in conductivity where neutralization occurs, since neutral water doesn’t conduct electricity as well as bases and acids. In the case of an oxidation or reduction reaction, the conductivity curve may be two straight lines meeting at a bend as the solution transitions from one set of ions to another.
In oxidation-reduction, or redox, titration, the neutralizing reaction involves transfer of an electron from one molecule to another, as opposed to the transfer of a hydrogen, as in an acid-base titration.
Aside from this difference in what is transferred, there are similarities between acid-base and redox titrations. The ions in the solution of a redox reaction allow electrical conductivity, allowing potentiometric titration. The changing of the ion concentration in a redox reaction also leads to a change of pH at the equivalence point, allowing use of indicators and a titration curve, though perhaps not with as precipitous of a result.
Sometimes an indicator is not needed at all for a redox reaction. For example, potassium permanganate (VII) changes from purple to clear when it is reduced.
Complexometric titrations are used to study metal ions. The titer is often ethylenediaminetetraacetic acid (EDTA). The equivalence point occurs when all the metal ions have been bonded to. An indicator can be used to pinpoint the equivalence point. Calmagite is a common complexometric indicator.
In precipitation titration, the solute of unknown concentration is precipitated out of solution by the titrant. The significance of the precipitation is that there are no longer ions in solution with which the titrant can react. The endpoint is therefore detected with the rapid increase in concentration of an ion from the titrant, due to not having titer ions with which to react.