Geology Reference
In-Depth Information
Identification of N-, P-, and S-containing compounds is dicult, and
those that are identified tend to be metabolic products, such as nucleo-
tides and vitamins, released into the soil following the death of cells.
Broadly, nitrogen is an integral part of the humic molecule, and is
released as NH
4
1
ions when the humic material is degraded. Phosphorus
and sulfur are more commonly found as P and S esters, which can be
released as orthophosphate and sulfate ions by the action of phospha-
tase and sulfatase enzymes, respectively.
Worked example 5.2 - organic carbon content of soil
Organic carbon in soil can be measured by oxidation with acid
dichromate solution:
2K
2
Cr
2
O
7
þ
3C
þ
8H
2
SO
4
"
2Cr
2
(SO
4
)
3
þ
3CO
2
þ
2K
2
SO
4
þ
8H
2
O
(5.7)
The excess dichromate is measured by titration with ferrous sulfate
Cr
2
O
2
7
þ
6Fe
21
þ
14H
1
2Cr
31
þ
6Fe
31
þ
7H
2
O (5.8)
"
0.20 g soil was oxidized with 20 cm
3
of M/6K
2
CrO
7
in sulfuric acid.
The excess dichromate was determined by titration with 0.5 M FeSO
4
solution. A blank, no soil, control was carried out to assess loss of
dichromate by thermal decomposition. The titration values obtained
were 17.5 cm
3
for the soil sample and 39.0 cm
3
for the blank. What is
the % organic carbon in the soil?
Moles of Cr
2
O
7
2
added initially
¼
20
1/6
¼
3.334 mmol
Moles of Cr
2
O
7
2
remaining after reaction:
17.5
0.5
¼
8.75 mmol Fe
21
From Equation (5.8), 1 mol of Cr
2
O
7
2
reacts with 6 mol of Fe
21
Therefore, 8.75/6
¼
1.458 mmol of Cr
2
O
7
2
unused.
Blank titration
39.0
0.5
¼
19.5 mmol Fe
21
¼
3.25 mmol of Cr
2
O
7
2
Therefore, 3.334-3.25
¼
0.084 mmol of Cr
2
O
7
2
lost by thermal
decomposition.
Moles of Cr
2
O
7
2
used in oxidation
¼
3.334-1.458
¼
1.876 mmol
Subtract Cr
2
O
7
2
lost by thermal decomposition
¼
1.876-0.084
¼
1.792 mmol of Cr
2
O
7
2
From Equation (5.7) 2 mol of Cr
2
O
7
2
react with 3 mol of C
1.792
3/2
¼
2.688 mmol C
2.688
12
¼
32.26 mg carbon
(32.26 mg/200 mg)
100
¼
16.13% C in the soil.
