Introduction to Soil Chemistry

soil titration 113
6.1. SOIL TITRATION
Typically acid soils are titrated with a sodium or calcium hydroxide [NaOH or
Ca(OH) 2] solution and basic soils with hydrochloric acid (HCl) and are most
commonly followed using a pH meter. Carbonates in basic soils release CO 2
during any treatment with HCl, thus making the titration more difficult. For
this reason carbonates are often determined by other methods. It is important
to keep in mind that basic solutions react with carbon dioxide in air and form
insoluble carbonates. This means that either the basic titrant is standardized
each day before use or the solution is protected from exposure to carbon
dioxide in air. Specific descriptions of titrant preparation, primary standards,
and the use of indicators and pH meters in titrations can be found in the texts
by Harris [see Bibliography], Harris and Harris [1], and Skoog et al. [2].
The complex nature of soil makes hydrochloric acid the preferred acid
titrant because the other common acids can be involved in reactions, which
are preferably avoided. Both sulfuric and phosphoric acids are di- and triprotic,
respectively, and each proton has a different pK a. In addition, in high concentration
both are dehydrating agents and in low concentration are hydrating
agents and thus can be involved in other than the desired titration reactions,
potentially leading to erroneous results. Nitric acid, on the other hand, is
monoprotic but is also an oxidizing reagent and can react with organic matter
to produce nitro compounds and thus produce erroneous results. The use of
sulfuric, phosphoric, and nitric acids may not be a problem with well-defined
systems but can be a problem when applied to undefined systems, especially
soils.
Oxidation–reduction reactions and titrations are often easy to carry out,
and in many respects oxidation–reduction titrations are the same as an
acid–base titration. For instance, a standardized oxidizing solution, often permanganate,
is added to a solution of an easily oxidized species of interest.
Permanganate is dark purple in color and colorless when reduced, making the
endpoint easy to determine. Other oxidizing reagents can be used, and strongly
colored molecules can be used as indicators in the same way as in acid–base
titrations. Also, a platinum electrode coupled to a reference electrode can be
used to determine the endpoint using most pH meters.
Most oxidation reactions are between specific metal cations or metal oxyanions
and cations. The problem that arises when applying oxidation–reduction
reactions to soils is that all soils contain a complex mixture of oxidizable and
reducible cations and anions and organic matter, which means that it is impossible
to determine which is being titrated. An exception to this is the oxidation
of organic matter where an oxidation–reduction titration is routinely
carried out. Organic matter determination will be discussed in Section 6.3.
Precipitation titration are typified by the titration of chloride with silver
or vice-versa. In this case interferences with the precipitation reaction may
occur because of components in the soil, and the soil itself may interfere with
detection of the endpoint. Thus precipitation reactions are rarely applied