Introduction to Soil Chemistry

158 spectroscopy
placed in the sample cell, which is placed in the sample compartment of the
instrument and a spectrum obtained. Many compounds have large molar
absorptivities, 1 and thus only small amounts are needed to obtain a spectrum.
Most UV–Vis spectrophotometers function best between 0 and 1 absorbence.
Often a sample will have an absorbence above 1, as is the case illustrated in
Figure 8.6. In these cases it is usually most useful to dilute the sample by a
factor of 10 to bring the absorption below 1 before using the absorbence data.
8.7. THE VISIBLE REGION
The visible region of the spectrum, with wavelengths 400nm (violet) and
900nm, is used extensively in soil analysis in the colorimetric determination of
components extracted from soil (see Section 8.8). Once extraction is complete
and the extract has been filtered and otherwise cleaned as needed, it is analyzed
for the components of interest by treating it with a reagent to produce a colored
product. The amount of color is directly related to the amount of component
present.
An excellent example of this type of analysis involves the determination
of phosphate in soil extracts. Soil is extracted with an appropriate extractant
and added to a solution of acid molybdate, with which the phosphate reacts
to produce a purple or blue, phosphomolybdate solution. Standard phosphate
solutions are prepared and reacted with acid molybdate, and the intensity of
the phosphomolybdate produced is measured. A standard of calibration curve
is prepared from which the intensity of the color is directly related to the
concentration of phosphate in the extract.
For this type of analysis to be accurate, three characteristics must exist:
1. The soil extract must not contain any components that absorb light at
the same wavelength as the phosphomolybdate; this includes suspended
material that will refract light. Refracted light does not get into the detector
and so is recorded as being absorbed by the sample, thus giving an
inaccurate result.
2. The color producing reagent must not react with any other commonly
occurring component in the extract to form a similarly colored product.
3. The soil must not contain any compound that inhibits or interferes with
the production of the colored compound.
As described in previous chapters, soil contains many different inorganic
and organic elements, ions, and compounds plus both inorganic and organic
colloids. Thus it cannot be assumed that the soil being investigated does not
contain any of the types of interference mentioned above. Some soils high in
1 Absorbtivity is defined as A =ebC, where A = absorbance, e=molar absorption coefficient (liter
mol -1 cm -1 ), b = pathlength of radiation through sample (cm), and C = molar concentration.