Equivalence point

Equivalence point or titration curve exhibits an inflection point at the equivalence point. A striking fact about equivalence is that in a reaction the equivalence of the reactants as well as products is conserved.

Acid-base titration example

Equivalence point occurs during an Brønsted-Lowry Theory of acids and bases.

When performing a manual titration, it may be difficult or impossible to detect when the equivalence point is reached. Often a pH indicator is added to the reaction vessel with an endpoint that is very close to the equivalence point. This causes a visible color change at the equivalence point and therefore at the point that no more titrant should be added.

Acid-base titrations are commonly taught in school and are the most familiar form of titration, however, it is only one of numerous forms of titration. See titration for more detail.

Methods to determine the endpoint

Different methods to determine the endpoint include:

Redox indicators are also frequently used. A drop of indicator solution is added to the titration at the start; when the colour changes the endpoint has been reached.

Potentiometer

A potentiometer can also be used. This is an instrument which measures the electrode potential of the solution. These are used for titrations based on a redox reaction; the potential of the working electrode will suddenly change as the endpoint is reached.

ion selective electrode. This allows the pH of the solution to be measured throughout the titration. At the end point there will be a sudden change in the measured pH. It can be more accurate than the indicator method, and is very easily automated.

Conductance

The ionic strength). Thus, predicting the change in conductivity is harder than measuring it.

Colour change

In some reactions, the solution changes colour without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours.

Precipitation

If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag+ and Cl- to form the very insoluble salt AgCl. Surprisingly, this usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations often have to be done as "back" titrations (see below).

Isothermal titration calorimeter

An isothermal titration calorimeter uses the heat produced or consumed by the reaction to determine the endpoint. This is important in enzymes.

silver nitrate, nickel by titration with dimethylglyoxime and fluoride by titration with aluminium (as K₂NaAlF₆) Because the temperature probe does not need to be electrically connected to the solution (as in potentiometric titrations), non-aqueous titrations can be carried out as easily as aqueous titrations. Solutions which are highly colored or turbid can be analyzed by thermometric without further sample treatment. The probe is essentially maintenance-free. Using modern, high precision stepper motor driven burettes, automated thermometric titrations are usually complete in a few minutes, making the technique an ideal choice where high laboratory productivity is required.

Spectroscopy

Spectroscopy can be used to measure the absorption of light by the solution during the titration, if the spectrum of the reactant, titrant or product is known. The relative amounts of the product and reactant can be used to determine the endpoint. Alternately, the presence of free titrant (indicating that the reaction is complete) can be detected at very low levels.

Amperometry

Amperometry can be used as a detection technique (amperometric titration). The current due to the oxidation or reduction of either the reactants or products at a working electrode will depend on the concentration of that species in solution. The endpoint can then be detected as a change in the current. This method is most useful when the excess titrant can be reduced, as in the titration of halides with Ag+. (This is handy also in that it ignores precipitates.)