1. PRINCIPLE
Samples are digested with an excess of acidic potassium dichromate solution. The chromium
is reduced from the Cr (VI) oxidation state to the Cr (III) state by the oxidisable organic
carbon present in the sample. The unreacted Cr (VI) can then be titrated with ferrous
sulfate to determine the amount of Cr (VI) consumed. The oxidisable organic carbon, organic
matter and total organic carbon can be calculated.
2. INTERFERENCES
2.1 Underestimation of the organic matter/organic carbon may result when higher oxides of manganese are present in substantial quantities. The possible effects of these
interferences should be taken into account in the analysis of some types of poorly aerated
soils.
2.2 Over estimation of organic matter/carbon may occur due to the presence of large amounts
of chloride or metallic or ferrous iron or sulfides in the soil sample.
The interference due to chlorides may be minimised by either washing the sample with
water or replacing the concentrated sulphuric acid with COD digestion reagent. The silver
in the reagent precipitates the chlorides in the sample.
The sulfides can be destroyed prior to crushing by the addition of dilute sulfuric acid. Add
acid until no further evolution of hydrogen sulfide occurs. The sample should be dried prior
to crushing.
3. REAGENTS
3.1 Sulphuric acid (H2SO4), 98%, AR
3.2 Silver sulphate (Ag2SO4), AR
3.3 Sulfuric acid reagent: refer to QWI-EN/EP026 or prepare as below: add 25.3 g of silver sulphate (3.2) to a 2.5 L Winchester of sulphuric acid (3.1). Let stand for 24 hours
to dissolve. Mix.
3.4 Potassium dichromate (K2Cr2O7), AR
3.5 Potassium dichromate, 0.1667 M: Accurately weigh 49.035 g of K2CR2O7 (5.4) and dissolve in water. Make up to 1 L in a volumetric flask. Verify by running x 200 on ICP-AES.
The expected Cr result I 17320 mg/L. For acceptance/rejection criteria refer to QWI-EN/50.
3.6 Sulfuric acid, 1 N: Carefully add 28 mL of sulfuric acid (3.1) to 800 mL of reagent grade water. Allow to cool and make to 1 L. This may be done in a beaker.
3.7 Sulfuric acid, 0.1 N: Carefully add 100 mL of 1 N sulfuric acid (3.1) to 800 mL of reagent grade water. Allow to cool and make to 1 L.
3.8 Ferrous sulphate heptahydrate (FeSO4.7H2O), AR
3.9 Standard ferrous sulfate titrant, ~0.5 M: Dissolve 140 g FeSO4.7H2O (3.8) in ~800 mL of water. Carefully add 14 mL of H2SO4 (3.1), cool and dilute to 1 L. Standardise on use.
3.10 Sodium diphenylamine-sulfonate, AR
3.11 Indicator solution: Dissolve 0.25 g of sodium diphenylamine-sulfonate (3.10) in 100 mL of water.
3.12 Phosphoric acid, 85%, AR
3.13 Organic Matter Standard – Garden soil. In-house.
4. APPARATUS
4.1 Mortar and pestle
4.2 Drying oven, 104 +/- 10oC
4.3 Balance (0.0001 g accuracy)
4.4 Volumetric flask, 100 and 1000 mL
4.5 Beaker, capable of holding 1 L
4.6 Pipettes, 1 and 10 mL
4.7 Measuring cylinders, glass, 20 and 200 mL
4.8 Erlenmeyer flasks, 250 or 500 mL
4.9 Burette, 25 Ml, 0.1 mL increments
4.10 Hotplate or similar heat insulating surface.
5. PROCEDURE
5.1 Sample Pre-treatment
5.1.1 Dry sample at 104 +/- 10oC and crush using a mortar and pestle.
5.1.2 If sample contains sulfides or chlorides, the following procedure is to be employed.
Carefully (wearing glasses, gloves and laboratory coat) wash about 10 g of sample
with COD digestion 0.1 N sulphuric acid reagent (5.6) until all effervescence ceases. The sample is then washed with reagent grade water, dried and crushed again.
5.2 Weigh 0.2-5.0 g (+/-0.01 g)* of sample and standard (5.13) into a 250 mL conical flask.
Note: The size of sample for chemical analysis will vary with the amount of organic matter
present in the soil. The most suitable size of sample to be used is that it is giving a total of
5 mL to 8 mL dichromate solution reduced.
Usually,
| Soil Type | Appropriate Amount of Soil |
|---|---|
| Organic horizon* | 0.1-0.2 g |
| Heating rate | 5 to 6°C/min |
| Surface Soils | 0.5 g |
| Subsoils | 2.0 g |
* Surface layer of decomposing material not significantly mixed with the mineral soil.
5.3 Run 10 mL of the 0.1667 M K2Cr2O7 solution (3.5) into the conical flask containing the sample, standard or blank from a burette and, very carefully, add 20 mL sulphuric acid
reagent (3.3) using a measuring cylinder.
Note: Standardise FeSO4 by running 3 blanks through steps 5.3-5.9.
5.4 Swirl the mixture thoroughly for about 1 min and then stand it on a heat insulating surface for 30 minutes to allow oxidation of the organic matter to proceed.
5.5 Add 200 mL of water followed by 10 mL of phosphoric acid (3.12) and 1 mL of the indicator (3.11) to the flask, shake the resultant mixture thoroughly. If the indicator is absorbed by
the soil, add a further 1 mL of the solution.
Note: The heat transfer affects the extent and rate of reaction. Therefore it is crucial that
the flask is cooled prior to the titration in order to obtain uniformity of results.
5.6 Titrate with 0.5 M of FeSO4 (3.9) in 0.5 mL increments, swirling the contents of the flask until the colour of the solution changes from blue to green.
Note: If the sample background is high, filter the suspension through an acid-washed glass
fibre filter.
5.7 Add a further 0.5 mL of K2Cr2O7 solution (3.5) changing the colour back to blue.
5.8 Titrate with 0.5 M of FeSO4 (3.9) drop by drop, with continued swirling until the colour changes from blue to green.
5.9 Record the total volume of FeSO4 titrant to the nearest 0.05 mL.
6. CALCULATIONS
6.1 Calculation Equation for Organic Matter and TOC
V (mL) x 0.67
OM% = -------------------
Wt. Sample (g)
TOC% = OM% x 0.58 OR
V (mL) x 0.39
TOC% = -------------------
Wt. Sample (g)
Where
OM% = organic matter present in the oven dry sample, in percent
TOC% = total organic carbon present in the oven dry sample, in percent
V = volume of K2Cr2O7 needed to oxidise the organic matter in the soil (mL)
= 10.5 x (1-V2/V1)
Wt = mass of sample soil used in the test (oven-dried) (g)
6.2 Relationship Between Organic Matter (OM%), Total Organic Carbon (TOC%) and Oxidisable
Organic Carbon (OOC%)
The procedure is based on the determination of the oxidisable organic carbon (OOC) content of the soil and assumption that:
(a) Soil organic matter contains an average of 58% of carbon by mass.
(b) 77% of the carbon in the organic material is oxidised.
(c) 1 mL of volume of K2Cr2O7 consumed is equivalent to 3 mg of carbon (Piper, 1943, Soil
and Plant Analysis).
The percentage of organic matter (OM) present in the oven-dried sample is calculated
below:
OM% = TOC% / 0.58
OOC%
OM% = ---------------
0.58 x 0.77
TOC% = OM% x 0.58
= OOC% x 1.3
Because the Oxidisable Organic Carbon in the oven-dried sample is calculated below:
(V1-V2) x NFeSO4 x 0.003 (g/mL) x 100%
OOC% = -------------------------------------------------
Wt. Sample (g)
10.5 x (1-V2/V1) x 6 x 0.1667 x 0.3
= -------------------------------------------------
Wt. Sample (g)
= V x 0.3 / Wt. Sample (g)
Therefore:
V (mL) x 0.67
OM% = ---------------------
Wt. Sample (g)
V (mL) x 0.39
TOC% = ---------------------
Wt. Sample (g)
Where
OM% = organic matter present in the oven dry sample, in percent
TOC% = total organic carbon present in the oven dry sample, percent
OOC% = oxidisable organic carbon present in the oven dry sample, in percent
V = volume of K2Cr2O7 required to oxidise the organic matter in the soil (mL)
= 10.5 x (1-V2 / V1)
Wt = mass of sample soil used in the test (oven-dried) (g)
N = normality of FeSO4 (N)
V2 = total volume of FeSO4 used in the sample (mL)
V1 = average of total volume of FeSO4 used in blank (mL)
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