(b) Dichromate and chromate ions in aqueous solutions interconvert to a degree which is pH
dependent:Unless standards are prepared in buffered media, positive or negative deviations may result from
measurements made at 348 nm or 372 nm respectively Alternatively, measurements can be made at
the isosbestic point, i.e. where the absorbance curves of each form intersect, and where absorbance
is not a function of equilibrium concentrations but only of the overall concentration. Solutions of
weak acids and bases should also be measured at their isosbestic points for the same reason.(3) Negative deviations occur if the radiation used is polychromatic, as in the case of filter-photometers,
and if measurements are made where the value of ε changes appreciably across the width of the band
(Figure 9.3). However, measurements made at the maximum of a broad band show little or no
deviation.
Figure 9.3
Effect of bandpass and choice of wavelength on a Beer's law plot.
Curve A represents a calibration curve using a narrow bandpass
monochromator at λ 2. Curve B represents a calibration curve using a^
wide bandpass filter at λ 2 or a narrow bandpass monochromator at λ 1.(4) 'Stray' light passing through the optical system is generally not absorbed by the sample to the same
extent as that of the selected wavelength. Its presence leads to negative deviations.
Precision of Absorption Measurements
The precision of absorption measurements depends upon the degree of sophistication of the
instrumentation and on the chemical species involved, both of which can affect the apparent validity of
the Beer-Lambert law. Where a single absorbing species exhibiting a broad flat maximum is to be
determined, adequate results can often be achieved with a simple filter-photometer. In the visible
region, this technique is known as colorimetry. The inherent disadvantage of colorimetric procedures
using simple filter instruments with a broad bandpass lies in the invalidation of the Beer-Lambert law
and the lack of compatibility between results from different