Page 105 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 93

mittance and absorbance plots for potassium permanganate solutions at
three wavelengths are illustrated by Mellon (1950, p.95).
Several other terms for light absorption are given in the literature and are
still found on the printed scales of some photometers. “Optical density” is
often used; it is the same as absorbance.

Interferences

In spectrophotometric determinations, interferences often result from the
presence in the sample of dissolved or suspended foreign material that either
absorbs radiant energy or reacts with the color reagent to form a complex
that absorbs radiant energy. In either case, the absorbance of the sample is
decreased. Where the interference results from the formation of an absorbing
complex by ions in solution, dilution of the sample can eliminate the inter-
ference if the sensitivity of the color reagent for the element sought is
sufficiently greater than for the interfering ions. If this is not the case, other
methods must be found to increase the selectivity of the method. Among
such methods are:
(1) pH adjustment: if pH is an important factor in complex ion formation,
its adjustment can favor the formation of the complex of the element
desired instead of the interfering ions.
(2) Masking: compounds such as EDTA (ethylenediaminetetraacetic acid)
are added to the sample to form a stable complex with interfering ions, thus
preventing their reaction with the color reagent.
(3) Solvent extraction: preferential solubility of some ions in organic
solvents permits the removal of interfering ions.
Another common source of interference in spectrophotometry is the use
of color reagents that absorb at the wavelength at which the complex of the
element desired is measured. Such interference usually can be reduced or
eliminated by the use of a reagent blank.
In some samples a significant source of interference results from the
presence of natural color. The natural color in water samples often gives
appreciable absorbance and requires either compensation or elimination. In
some cases it is possible to select a spectrophotometric reagent of sufficient
sensitivity that the absorbance of the constituent sought will exceed the
absorbance of the natural color by a large factor. If this factor is 50 or higher
the error caused by the natural color is 2% or less. Knowledge of the relative
sensitivity of the constituent to be determined relative to the natural color in
the sample is necessary before such a factor can be used. If the relative
sensitivity is unknown the natural color of the sample should be com-
pensated for or removed. This can be done by determining the absorbance of
the test sample versus the blank specified for the procedure. Determine the
absorbance of the naturally colored sample versus distilled water. The differ-
ence is the corrected absorbance and is used to determine concentration
values.