End-point Detection
A prerequisite for a precise and accurate titration is the reproducible identification of an end point
which either coincides with the stoichiometric point of the reaction or bears a fixed and measurable
relation to it. An end point may be located either by monitoring a property of the titrand which is
removed when the stoichiometric point is passed, or a property which can be readily observed when a
small excess of the titrant has been added. The most common processes observed in end-point detection
are change of colour; change of electrical cell potential; change of electrical conductivity; precipitation
or flocculation. (Electrochemical methods are discussed in Chapter 6; precipitation indicators find only
limited use.)
Visual Indicators
The use of a colour change to indicate the end point is common to a wide variety of titrimetric methods.
Visual detection of end points is a major factor in maintaining the simplicity of titrimetry, hence the
capability of the human eye to detect colour change plays an important role in these techniques.
In general terms a visual indicator is a compound which changes from one colour to another as its
chemical form changes with its chemical environment
where X may be H+, Mn+ or eā, and the colour of the indicator is sensitive to the presence of H+, Mn+,
oxidants or reductants. An indicator constant is defined as
If the indicator is present in an environment where a titration reaction generates or consumes the X
species, the indicator will change with the concentration of X in the solution and the colour of the
solution will be