determined by the ratio [InB]/[InA]. As a general guide, the eye will register a complete change from one
colour to the other when this ratio changes from 10:1 to 1:10. Substitution in equation (5.4) enables the
concentration range of X over which the indicator will change colour to be calculated, i.e.
For example, the acid-base indicator methyl orange has a pKIn of 3.7 and will thus change colour over
the pH range 2.7–4.7. The ultimate sharpness of the end point will further depend upon the rate at
which pX is changing at the end point of the titration. The additional factors involved in determining
this rate of change are examined later in the discussions of specific titration methods. Because the
indicator competes with the analyte and reagent for X it is obvious that the indicator concentration must
be kept as low as possible in order to minimize interference with the analyte-reagent equilibrium. It
follows that the colours exhibited by an indicator must be of a high intensity.
Apparatus for Titrimetric Analysis
In general, the apparatus for titrimetric analysis is simple in construction and operation. A typical
analysis procedure would involve measurement of the amount of sample either by mass or volume, and
then addition of the titrant from a burette or micro-syringe. Apart from visual indication, the course of a
titration may be followed by electrochemical or photometric means; in neither is the equipment required
complex. A simple valve voltmeter or conductivity bridge will suffice on the one hand, and a simple
spectrophotometer or filter photometer with minor modifications on the other. Varying degrees of
automation may be incorporated.
Acid-base Titrations
Neutralization reactions between Lowry-Brønsted acids and bases are frequently employed in chemical
analysis. Methods based on them are sometimes termed acidimetric or alkalimetric.
Visual Indicators for Acid-base Titrations
Table 5.1 summarizes the details of some useful acid-base indicators. Exact agreement with the pH
range expressed by equation (5.5) is by no means always observed. This is because some colour
changes are easier to see than others and so the general approximation made in deriving equation (5.5)
is not uniformly close. Structurally, the indicators form three groups: phthaleins (e.g. phenolphthalein);
sulphonephthaleins (e.g. phenol red); and azo compounds (e.g. methyl orange).