Introduction to Soil Chemistry

66 soil basics iii
the respective oxide and carbon dioxide, which is lost, resulting in a loss of
weight in the sample.
For these and many other reasons, direct oxidation of soil organic matter
using high temperature and atmospheric air is not the best procedure for
determining soil organic matter content.
3.7.2.2 Oxidation Using Dichromate
The most common method used in determining soil organic matter content is
oxidation using dichromate. In this procedure a mixture of potassium or
sodium dichromate with sulfuric acid is prepared and mixed with a soil sample.
The heat released from mixing the soil with the dichromate and other accessory
solutions heats the mixture to around 100°C for a specific period of time.
Alternatively, external heating may be used or required.After cooling, an indicator
is added and the unused dichromate titrated. The amount of dichromate
remaining can be related back to the amount of carbon and organic matter
present (see also Chapter 6, Section 6.3). Because of the relative low temperatures
and the reagents used, this is a reliable method, which does not lead to
loss of other components except carbonates. The loss of carbonate does not
involve reduction of dichromate but can cause interferences that must be
taken into account [21].
Caution: It should be noted that this procedure involves the use of
hazardous chemicals by laboratory personnel. It also results in hazardous
waste that must be disposed of carefully and properly.
3.7.2.3 Other Oxidative Procedures
Potentially any oxidizing reagent can be used to determine soil organic matter.
Of the many other possibilities, the use of catalytic oxidation in pure oxygen
and or application of hydrogen peroxide are the two more common methods.
Any organic chemical can be oxidized to carbon dioxide and water using catalyst,
pure oxygen, and heating. A known amount of material is placed with a
catalyst in a stream of oxygen and heated, producing carbon dioxide and
water, which are trapped and weighed. From this the amount carbon and
hydrogen in the original organic matter can thus be determined. As might be
surmised, this method can also be used to determine the amount of nitrogen
and sulfur in soil organic matter. This procedure is done on a one-by-one basis
and so is slow and time-consuming. For these reasons it is not used where a
significant number of samples must be analyzed.
A soil sample can be mixed with hydrogen peroxide and heated to decompose
organic matter. This procedure is often used when the objective is to
remove organic matter and the amount present is not determined. Problems
with this procedure include frothing due to decomposition of hydrogen per-