Page 285 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 10
TlTRlMETRlC ANALYSIS

10.1 TlTRlMETRlC ANALYSIS
The term 'titrimetric analysis' refers to quantitative chemical analysis carried
out by determining the volume of a solution of accurately known concentration
which is required to react quantitatively with a measured volume of a solution
of the substance to be determined. The solution of accurately known strength
is called the standard solution, see Section 10.3. The weight of the substance to
be determined is calculated from the volume of the standard solution used and
the chemical equation and relative molecular masses of the reacting compounds.
The term 'voluinetric analysis' was formerly used for this form of quantitative
determination but it has now been replaced by titrimetric analysis. It is
considered that the latter expresses the process of titration rather better, and
the former is likely to be confused with measurements of volumes, such as those
involving gases. In titrimetric analysis the reagent of known concentration is
called the titrant and the substance being titrated is termed the titrand. The
alternative name has not been extended to apparatus used in the various
operations; so the terms volumetric glassware and volumetric flasks are still
common, but it is better to employ the expressions graduated glassware and
graduated flasks and these are used throughout this book.
The standard solution is usually added from a long graduated tube called a
burette. The process of adding the standard solution until the reaction is just
complete is termed a titration, and the substance to be determined is titrated.
The point at which this occurs is called the equivalence point or the theoretical
(or stoichiometric) end point. The completion of the titration is detected by
some physical change, produced by the standard solution itself (e.g. the faint
pink colour formed by potassium permanganate) or, more usually, by the
addition of an auxiliary reagent, known as an indicator; alternatively some
other physical measurement may be used. After the reaction between the
substance and the standard solution is practically complete, the indicator should
give a clear visual change (either a colour change or the formation of turbidity)
in the liquid being titrated. The point at which this occurs is called the end point
of the titration. In the ideal titration the visible end point will coincide with the
stoichiometric or theoretical end point. In practice, however, a very small
difference usually occurs; this represents the titration error. The indicator and
the experimental conditions should be so selected that the difference between
the visible end point and the equivalence point is as small as possible.