Page 35 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

COMMON TECHNIQUES 1.6

1.6 COMMON TECHNIQUES

The main techniques employed in quantitative analysis are based upon (a) the
quantitative performance of suitable chemical reactions and either measuring
the amount of reagent needed to complete the reaction, or ascertaining the
amount of reaction product obtained; (b) appropriate electrical measurements
(e.g. potentiometry); (c) the measurement of certain optical properties (e.g.
absorption spectra). In some cases, a combination of optical or electrical
measurements and quantitative chemical reaction (e.g. amperometric titration)
may be used.
The quantitative execution of chemical reactions is the basis of the traditional
or 'classical' methods of chemical analysis: gravimetry, titrimetry and volumetry.
In gravimetric analysis the substance being determined is converted into an
insoluble precipitate which is collected and weighed, or in the special case of
electrogravimetry electrolysis is carried out and the material deposited on one
of the electrodes is weighed.
In titrimetric analysis (often termed volumetric analysis in certain books),
the substance to be determined is allowed to react with an appropriate reagent
added as a standard solution, and the volume of solution needed for complete
reaction is determined. The common types of reaction which are used in
titrimetry are (a) neutralisation (acid-base) reactions; (b) complex-forming
reactions; (c) precipitation reactions; (d) oxidation-reduction reactions.
Volumetry is concerned with measuring the volume of gas evolved or
absorbed in a chemical reaction.
Electrical methods of analysis (apart from electrogravimetry referred to
above) involve the measurement of current, voltage or resistance in relation to the
concentration of a certain species in solution. Techniques which can be included
under this general heading are (i) voltammetry (measurement of current at a
micro-electrode at a specified voltage); (ii) coulometry (measurement of current
and time needed to complete an electrochemical reaction or to generate sufficient
material to react completely with a specified reagent); (iii) potentiometry
(measurement of the potential of an electrode in equilibrium with an ion to be
determined); (iv) conductimetry (measurement of the electrical conductivity of
a solution).
Optical methods of analysis are dependent either upon (i) measurement of
the amount of radiant energy of a particular wavelength absorbed by the sample,
or (ii) the emission of radiant energy and measurement of the amount of energy
of a particular wavelength emitted. Absorption methods are usually classified
according to the wavelength involved as (a) visible spectrophotometry
(colorimetry), (b) ultraviolet spectrophotometry, and (c) infrared spectro-
photometry.
Atomic absorption spectroscopy involves atomising the specimen, often by
spraying a solution of the sample into a flame, and then studying the absorption
of radiation from an electric lamp producing the spectrum of the element to be
determined.
Although not strictly absorption methods in the sense in which the term is
usually employed, turbidimetric and nephelometric methods which involve
measuring the amount of light stopped or scattered by a suspension should also
be mentioned at this point.
Emission methods involve subjecting the sample to heat or electrical treatment