Introduction to Soil Chemistry

130 extraction
3. What is the distribution of the component between the soil matrix and
the extractant?
The best answer to questions 1 and 2 both is affirmative, but if it is negative,
then the incompatibility must be removed. One method of accomplishing this
is to remove the initial extractant and redissolve or suspend the component
of interest in an analytically compatible solvent. Another approach would be
to sorb the component on a suitable sorbent and subsequently extract it from
the sorbent into a suitable, compatible solvent.The simplest and best approach
is simply to extract the component of interest into a compatible solvent
directly. Regarding question 3, extraction time and/or the need for multiple
extractions must be sufficient to extract all the component of interest. This
information is usually obtained by carrying the extraction out over varying
times or varying the number of times and determining which gives the highest
extraction efficiency.
7.2.1. Solvents
Caution: Common organic extracting solvents are both volatile and
flammable. Ignition can be caused by hot surfaces without the need
of sparks or flames. They will also form peroxides, particularly diethyl
ether, which are explosive when concentrated, especially when
heating is involved. Also, some solvents may be toxic or carcinogenic
or both. Always consult the U.S. Environmental Protection Agency
(USEPA) or Material Safety Data Sheets (MSDSs) before using any
solvent.
In all environmental work, particularly soil analysis, the purity of solvents used
is of extreme importance. This stems from three sources. Soil is extremely
complex in its composition, which can lead to solvent contaminants being confused
with or interfering with the separation and detection of the analyte or
analytes of interest; see Chapters 8 and 9 for more details on these types of
interference. The second source results when extractants are concentrated
prior to analysis. In this case any impurities present will also be concentrated,
but they may not all be concentrated proportionately. The third source is the
sensitivity of modern instrumentation, which results in detection of very low
solvent contaminant concentrations.
In soil analysis for agricultural purposes, micronutrients, such as iron, boron,
and copper, are important and as the name implies, are present in low concentration.
As these are micronutrients, sensitive methods of analysis are
needed to measure them. If present at low concentrations, they are not sufficient
for optimum plant growth. If present at high concentrations, they are
toxic. In environmental analysis determination of low concentrations is there-