Introduction to Soil Chemistry

118 titrimetric measurement
Because of the complex nature of soil and the soil solution, it is rarely possible
to directly determine specific soil constituents by titrating soil or soil solutions
using a pH meter, selective ion electrode, or a platinum electrode (with
appropriate reference electrode (see Chapter 5 for more details on this
subject) [3].
6.3. ORGANIC MATTER
Soil organic matter can be divided into many fractions; however, the first distinction
between fractions is the active fraction, that is, the fraction undergoing
active decomposition and the stable fraction, namely, the fraction that is
relatively stable, mostly humus. The most common method of determining soil
organic matter does not differentiate between these two types. All organic
matter is oxidized using a strong oxidizing agent, most often potassium or
sodium dichromate in sulfuric acid. To effect complete oxidation, heating,
which can be done using a hotplate or by mixing the acidic and dichromate
solutions, is required. When the reaction is completed, unreacted dichromate
is titrated, in an oxidation–reduction titration, and the difference is used as the
amount of organic matter present.
This titration uses an indicator, the color of which is difficult to see because
of the soil present. The indicator, however, is much easier to see with natural
rather than fluorescent lighting.
Other methods for determination of soil organic matter are available [4];
however, they are not as commonly used as is dichromate oxidation, commonly
called the Watley–Black method. Usually these methods are both more
time-consuming and less accurate than is the dichromate oxidation titration
method. Keep in mind that the dichromate oxidation of organic matter is the
standard by which all other methods of determining soil organic matter must
be compared [4,5].
Caution: Chromates, including potassium and sodium dichromate,
are hazardous materials, as are sulfuric and phosphoric acid used in
the oxidation of soil organic matter. Great care must be exercised in
using these chemicals.
6.4. AMMONIA
Ammonia is a gas that reacts with water to form ammonium as follows:
H 3N + H 2O
NH 4 + + – OH
(6.1)