Introduction to Soil Chemistry

halogen ion determination 123
6.7. CARBONATE DETERMINATION
Carbonates decompose under acidic conditions with the release of carbon
dioxide:
CaCO + 2HCl
Æ CaCl + CO ≠+ H O
3 2 2 2
CO + 2NaOH
Æ Na CO + H O
2 2 3 2
(6.2a)
(6.2b)
To determine the carbonate, a soil sample can be placed in an Erlenmeyer
flask and a 0.1 molar solution of hydrochloric acid (HCl) is added until no
more carbon dioxide is released. The amount of HCl consumed is used to calculate
the amount of carbonate present.The reaction shown is for calcium carbonate;
however, all carbonates in the soil will also be decomposed, and thus
this is a method for determining the total carbonate content, not just the
calcium carbonate. Additionally, inaccuracies can be caused by other components
in soil that can react with the HCl.
Another approach would be to measure the amount of carbon dioxide
produced either by measuring the volume of gas released or by reacting the
carbon dioxide, in a separate flask, with base and determining the amount of
base remaining after all the carbonate has precipitated and been removed
from the solution.
Alternatively, the weight lost when the carbonate is reacted with HCl can
be determined. Heating carbonates results in their decomposition. Thus soils
containing carbonate can be heated to the appropriate temperature and the
weight loss measured. In this approach the loss of organic matter and water
of hydration of various components in soil must be corrected for in order to
determine the weight of carbon dioxide lost [7].
6.8. HALOGEN ION DETERMINATION
Inorganic halogen containing compounds are usually very soluble salts. They
commonly occur as simple, single, negatively charged anions in soil. There are
two common exceptions to this generalization. First, fluorine is commonly
found bonded to phosphate in insoluble minerals called apatites, which are
calcium phosphate fluorides. The second are halogens, which are sigmabonded
to carbon.
The halogen anions are easily leached from soil with water and can be
determined using silver nitrate as a titrant. The letter X is commonly used to
represent halogens and thus may be interpreted to be any of them (i.e., F, Cl,
Br, or I) but is not generally used for At. The following reaction of silver with
halogen X - ignores other possible counterions, namely, nitrate and the cation
associated with the halide: