Page 104 - Geochemistry of Oil Field Waters

92 ANALYSIS OF OILFIELD WATERS

or
where R is the isotope ratio such as 180/160 D/H, and the delta values are
expressed in per mil like salinity, and &MOW = O%,.

COLORIMETRIC METHODS

The instrumental measurement of the absorption of radiant energy at a
certain wavelength involves spectrophotometry. The essential components of
a spectrophotometer include:
(1) Radiant energy source such as a tungsten-filament incandescent lamp
for the visible region, while hydrogen or deuterium discharge lamps usually
are used for the ultraviolet region.
(2) A monochromator, which is a device that isolates a narrow band of the
radiant energy.
(3) Containers, cells, or cuvettes usually made of glass to hold the solution
being analyzed.
(4) A detector, which is a device (usually a phototube) that measures the
radiant energy passed through the solution.
In the application of spectrophotometric analysis the two terms “trans-
mittance” and “absorbance” are important. Transmittance is:

T =- I2
I1
where T = transmittance; II = radiant energy incident upon the first surface
of the sample; and I2 = radiant energy leaving the sample.
The term absorbance is defined as:
1
A = -1ogIJ” = lOg1,T

or the negative logarithm of the transmittance.
In the preparation of spectrophotometric curves of light-intensity ratio
plotted against concentration, it is preferable, for convenience, to use ab-
sorbance as the basis of the plot. Under these conditions a system that
conforms to Beer’s law gives a straight-line plot, and the commonly used
colorimetric systems that do not conform will usually show only a moderate
curvature (Willard et al., 1965). Extreme curvature, when the curve is
plotted on the basis of absorbance data, is sometimes a sign that the system
is not sufficiently stable for analytical purposes. Semicolloidal suspensions of
colored substances often give extreme curvatures. When transmittance data
are used for plotting, a curve is always obtained unless semilogarithmic
coordinates are used. The modern. spectrophotometers have an absorbance
calibration as well as the conventional “percent transmittance”, and it is
common practice to use the absorbance scale. The relations between trans-