Quantitative Analysis – Absorptiometry
The use of visible and UV spectrometry for quantitative analysis by comparing the absorbance of
standards and samples at a selected wavelength is one of the most widespread of all analytical
techniques. It is also one of the most sensitive. The analysis of mixtures of two or more components is
facilitated by the additivity of absorbances. This has been discussed earlier (p. 356). Other applications
include measurement of the absorption of complexes as a function of solution conditions or time to
establish their composition, and to determine thermodynamic and kinetic stability for analytical
purposes or for more fundamental studies.
Choice of Colorimetric and Spectrophotometric Procedures
Not all chromogenic compounds are suitable for quantitative measurements, and the choice of system
and procedure depends largely on the chemistry of the species to be determined. Points to be considered
in the selection of a procedure include:
(1) Stability of the absorbance with respect to time (30 minutes should be the minimum) and to minor
variations in pH, ionic strength and temperature.
(2) Degree of selectivity of a complexing agent including the effect of other species likely to be present
and the effect of an excess of the reagent. Calculations based on conditional constants (p. 40) may help
to establish optimum conditions.
(3) Conformity to the Beer–Lambert law and the value of the molar absorptivity.
One of the major problems lies in the extent of interference from other constituents of a sample. This
can often be obviated by a prior separation using chromatography or solvent extraction (p. 55) or by the
use of masking agents (p. 40), pH control or changes in oxidation state. Standards should always be
matched to the gross composition of the sample as closely as possible, and calibration curves frequently
checked. The precision of absorption measurements has already been discussed (p. 361).
A valuable means of comparing two or more methods with regard to precision, conformity to the Beer–
Lambert law and the range of concentrations usefully measured is to plot calibration data in the form of
a Ringbom 2 curve, i.e. transmittance versus log concentration. The straight line portions of the S-
shaped curves, (Figure 9.10) indicate optimum concentration ranges, and the relative precision,
assuming a 1% constant instrumental error, is 230/S, where S is the slope of the straight line portion of
the curve, sometimes called the sensitivity. Hence, in the figure, method 1 is seen to be more sensitive
and to give better precision than method 3, but has a narrower optimum range. Method 2 is useful at
higher concentration levels only.