Temperature-controlled oven, preset to 60oC
+ 5
Balance capable of weighing to 0.0001g
Magnetic stirrer
2 x 1litre volumetric flasks
2 x 100ml burettes graduated to 0.1ml
4 x 10ml pipettes with suction bulb
2 x beakers 0.5litre capacity
1 x 25ml measuring cylinder
A desiccator containing anhydrous silica
gel
Washbottle/spray filled with distilled
water
Distilled water
The following reagents need to be of
analytical reagent quality
Potassium dichromate N solution. Dissolve
49.035 g of potassium dichromate in distilled water to make 1 litre of
solution.
Ferrous sulphate, approximately 0.5N
solution. Dissolve approximately 140g of ferrous sulphate in 0.5N
sulphuric acid to make 1 litre of solution (add 14ml of concentrated
sulphuric acid to distilled water to make 1 litre of 0.5N sulphuric acid
solution). NOTE this solution is unstable in air and should be kept
tightly stoppered. It needs to be standardised against the dichromate
solution each day it is used.
Sulphuric acid, concentrated, relative
density of 1.84.
Orthophosphoric acid, 85%, relative
density of 1.70 to 1.75.
Indicator solution. Dissolve 0.25g of
diphenylaminesulphonate in 100ml of distilled water.
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Standardisation of Ferrous sulphate.
10ml of the N potassium dichromate solution is run from a burette into
a 0.5litre beaker. 20ml of concentrated sulphuric acid is then added
very carefully using a 25ml measuring cylinder. Swirl and allow to
cool for some minutes. Add 200ml of distilled water, followed by 10ml
of orthophosporic acid and 2ml of indicator (using different 10ml
pipettes). Set to stir on a magnetic stirrer. Ferrous sulphate is then
added from a second burette in 0.5ml increments (or as a rapid drip),
until the colour of the solution changes from blue to green. A further
0.5ml of potassium dichromate is then added, turning the colour back
to blue. Ferrous sulphate is then added drop by drop, with the stirrer
operating, until the single drop when the colour changes from blue to
green. The total volume of ferrous sulphate used (x) then needs
to be recorded, to the nearest 0.05ml.
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Prepare a batch of up to twelve samples
for analysis.
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Grind the sample in a mortar and
pestle to remove all aggregations.
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Place approximately 5g of the powder into
a pre-weighed (4dp) 30ml Pyrex beaker.
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Oven dry (105oC) for half a day. With a stainless steel spatula/spoon
place about 1g of sediment into a pre-weighed 25ml beaker. Weigh to
0.0001g. Add all powder to a 0.5litre beaker. Repeat for all samples,
placing each weighed powder into a new beaker.
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Removal of carbonates (method of Shaw).
Add 25ml of sulphurous acid to each 25ml beaker (sufficient for 1g of
carbonate, although checking is recommended). Swirl thoroughly and
leave for 3 hours minimum. Then evaporate the beakers to dryness in
the oven at 60oC. NOTE this acid is gives an extremely
unpleasant vapour, use a a fume cupboard.
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Sample titrations. 10ml of the N
potassium dichromate solution is run from a burette into the 0.5litre
beaker. 20ml of concentrated sulphuric acid is then added very
carefully using a 10ml pipette. Swirl thoroughly for about 1 minute
then allow to cool on a wooden surface for 30 minutes. Add 200ml of
distilled water, followed by 10ml of orthophosporic acid and 2ml of
indicator (using different 10ml pipettes). Set to stir on a magnetic
stirrer. Ferrous sulphate is then added from a second burette in 0.5ml
increments increments (or as a rapid drip), until the colour of the
solution changes from blue to green. A further 0.5ml of potassium
dichromate is then added, turning the colour back to blue. Ferrous
sulphate is then added drop by drop, with the stirrer operating, until
the single drop when the colour changes from blue to green. The total
volume of ferrous sulphate (y)used then needs to be recorded,
to the nearest 0.05ml.
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IMPORTANT NOTE: The total volume of
potassium dichromate used to oxidise the organic matter in the soil is
determined by
10.5 x (1-y/x) ml
The correct results are only given if
between 5 and 8ml of potassium dichromate are used. Analyses should be
repeated if a widely different volume has been used, adjusting the
weight of sample used to give the correct volume of potassium chromate
oxidised. If the sample is green after the addition of the
orthophosphoric acid, too much carbon is present and the sample size
needs to be decreased.
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Enter the standardisation values and the
results from each titration the EXCEL file for calculating the organic
carbon content of the samples. The assumption is implicit in the
calculations that approximately 77% of the carbon in the organic
matter is oxidised using this method.