Introduction to Soil Chemistry

124 titrimetric measurement
- + X + Ag Æ AgClØ
(6.3)
Reaction (6.3) is a precipitation titration that can be done with or without
an indicator. Silver as a cation is very reactive and is an oxidizing agent. For
instance, it can oxidize aldehydes, including aldoses (sugars with an aldehyde
functionality). For this reason it is possible to obtain inaccurate or misleading
results with titrating a soil extract with silver nitrate.
The most common halogen in soil is chloride while both bromide and iodide
occur but are uncommon [8]. The occurrence of either of these anions in soil
would be cause for concern. Analysis for these other halides could be carried
out using either capillary electrophoresis or high-performance liquid chromatography
(HPLC) (see Chapters 8 and 9).
6.9. pH–STAT TITRATIONS
In another type of titration, termed pH–stat, 2 the system is maintained at one
fixed pH during a reaction. This type of titration has been applied to bioreactors
where neither the starting material nor the product of the reaction is
titrated but rather acidic or basic byproducts or coproduced acid or base are
measured. In biological reactions this may be CO 2,HCO 3 - , or CO3 2- .The system,
maintained at a fixed temperature during the reaction of interest, is titrated
with an automatic titrator set to maintain the specific pH. Bioreactors may be
sealed to ensure that no liquid or gas is lost or gained, or they may be open
to allow the exchange of gases.
The titrant used in pH–stat procedures is usually a dilute (perhaps 0.01
molar) acid or base. Two different sets of data can be obtained from a pH–stat
titration.The amount of a substrate consumed or product formed can be determined
by the total amount of titrant used. Because the titrant is added over a
period of time, the rate of reaction can also be determined.
Even a small soil sample can be considered as a bioreactor, and so the
pH–stat method is applicable to soil. However, although this method is applicable
to the study of processes occurring in soil, it is not particularly useful in
routine analysis of soil components [9].
6.10. CONCLUSIONS
The titrimetric determination of soil constituents is most commonly applied
to a limited number of soil analyses, namely, organic carbon, nitrogen compounds,
carbonates, and chlorides. Determination of acid content by titration
is seldom done because the titration curves are not amenable to typical titra-
2 Stat stands for static and derives from the fact that the pH is held static during the progress of
the reaction.