FINAL REPORT

last two steps. If the weighs after heated soils have adifference of 0.0002g or less, then stop
the measurement.
3. Determination of organic matter
Methodology
Organic carbon is determined by means of a potassium dichromate back-titration. A known
excess of K 2 Cr 2 O 7 is added to the sample together with H 2 SO 4 and the organic carbon is
oxidized to CO 2 :
2Cr 2 O 2- 7 + 3C organic +16H + →4Cr 3+ + 3CO 2 + 8H 2 O
In this reaction, 2 moles of dichromate oxidize 3 moles of carbon. The unreacted excess of
dichromate remaining after reaction is determined by back-titration with ferrous sulfate:
Cr 2 O 2- 7 + 6Fe 2+ +14H + →2Cr 3+ + 6Fe 3+ + 7H 2 O
In this titration reaction, 1 mole of dichromate oxidizes 6 moles of Fe 2+ . Organic carbon is
calculated by difference. Ferrous iron (Fe 2+ ), if present in the soil. Will interfere by reacting
with chromate according to the same equations as that shown for the titration reaction above.
Materials
Hot plate
250ml and 100ml conical flasks
Burette
0.4N potassium dichromate solution
0.1N Standard ferrous ammonium sulfate solution
0.2% Phenanthroline indicator
Concentrated sulfuric acid
Experimental procedure
Weight 0.2g of soil, sediment or sludge and place in a conical refluxing flask. Add 10ml of
the potassium dichromate solution and swirl to mix. Carefully add 10ml concentrated sulfuric
acid. Dispense the acid a little at a time since it generates heat, and swirl gently to mix.
Connect the flask to the condenser and turn on cooling water. Cover open end of condenser
with a small beaker. Place on a hot plate and reflux for 5-10 minutes. Cooland rinse down the
condenser with distilled water, collecting the water in the flask. Disconnect flask from
condenser and add about 10ml water. Swirl to mix and add 4 drops of phenanthroline
indicator. Titrate with ferrous ammonium sulfate to the end point, at which the color changes
from blue-green to violet red
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