Agriculture Reference
In-Depth Information
Table 6.1. Titrametric Methods Used by USEPA
Method
Species Titrated
Titrant
9014
Cyanide
Silver nitrate
9034
Sulfides
Iodine
9253
Chloride
Silver nitrate
directly to soil; however, they can be applied to soil extracts. Common envi-
ronmental titration methods described in the USEPA methods are summa-
rized in Table 6.1 [1,2].
6.1.1.
Backtitration
In a backtitration an excess amount of standardized reagent is taken and
reacted with an unknown amount of a component of interest. When the reac-
tion is completed, the remaining unused reagent is titrated and the amount of
component of interest is determined by difference. In the kjeldahl procedures
described below, freed ammonia is distilled into an acid of known concentra-
tion. When all the ammonia has been distilled, the remaining unreacted acid is
titrated, and one can calculate the amount of ammonia distilled (and thus in the
original material being investigated) from the difference between the amount
of original acid present at the start and the amount remaining at the end.
Backtitrations are common in soil analysis, as they are used in both nitro-
gen and organic matter determinations. Backtitrations are highly valuable ana-
lytical techniques and are applicable to other environmental analysis as well.
6.2.
pH TITRATION OF SOIL
Titration of soil pH is an old method that is not widely used today. Basically,
a soil suspension is prepared and titrated with a standardized base, often
sodium hydroxide, although various basic calcium compounds such as calcium
oxide (CaO) and calcium hydroxide [Ca(OH)
2
] can and have also be used.
Because of the dark color of many soils, they are often titrated using a pH
meter as the indicator of the endpoint. A setup for the titration of soil is shown
in Figure 6.2. Titration is slow in that it takes some time, after the addition of
titrant for the semblance of equilibrium to be reached. Once this happens, a
reading or simply another addition of titrant can be made.
A titration curve for an acid soil suspension to which 1 mL of a calcium
hydroxide titrant is added and the change in pH followed for 2.3 min is shown
in Figure 6.3. As can be seen, the pH initially increases and then falls back
toward the original pH. The curve not only has a sawtooth pattern but is also
curved in the reverse direction from a standard acid-base titration curve such
as that shown in Figure 6.1.
