UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 206

spectrometer are given below. What is the concentration of KMnO 4 and K 2 Cr 2 O 7 in the
sample?

Solution Measured at l ¼ 440 nm Measured at l ¼ 525 nm

0.02 M KMnO 4 standard A ¼ 0:021 A ¼ 0:880

0.04 M K 2 Cr 2 O 7 standard A ¼ 0:645 A ¼ 0:010

Unknown sample A ¼ 0:450 A ¼ 0:576

SOLUTION: From Beer's law (Eqs 8.4 and 8.5), we can use a lump parameter k to account for
the product of two constants (e and l), that is, A ¼ kC, where k has a unit of L/mol. Recall
that k is also the slope of the UV-VIS calibration curve. For KMnO 4 standard solution,
we have

0:021 ¼ k 440; Mn 0:02; k 440; Mn ¼ 1:05; and 0:880 ¼ k 525; Mn 0:02; k 525; Mn ¼ 44:0

For K 2 Cr 2 O 7 standard solution, we can similarly calculate the k values as

0:645 ¼ k 440; Cr 0:04; k 440; Cr ¼ 16:13; and 0:010 ¼ k 525; Cr 0:04; k 525; Cr ¼ 0:250

For the unknown sample, two simultaneous equations can be generated at two different
wavelengths as

0:450 ¼ 1:05C Mn þ16:13C Cr
0:576 ¼ 44:0C Mn þ0:250C Cr

Solving these two equations will give C Mn ¼ 0 : 0129M and C Cr ¼ 0 : 0268M.

8.2 UV-VISIBLE SPECTROSCOPY

8.2.1 UV-Visible Instrumentation

Spectroscopic instruments differ considerably depending on the wavelength regions
(i.e., visible, UV, IR). Regardless of the variations, however, all spectrometers (spec-
trophotometers) require five major components: (1) a source of continuous rad-
iation over the wavelengths of interest, (2) a monochromator for selecting a narrow
band of wavelength from the source spectrum, (3) a sample cell, (4) a detector for
converting radiant energy into an electrical signal, and (5) a device to read out the
response of the detector (absorbance, transmittance, etc.) (Fig. 8.14).
A student version spectrometer in the visible wavelength range is shown in
Figure 8.15. A brief description of each component is as follows.

Radiation source : Usually a low-pressure hydrogen or deuterium lamp is
used for UV (185-375 nm) and a tungsten filament incandescent lamp for
visible measurements. The tungsten filament emits approximately 325 nm
to 3

m m, so it can also be used in the near-UV and near-IR regions.