Analytical Methods:Wet Oxidation

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SASSA Home Page ⇒ Analytical Methods Home Page ⇒ Laboratory Analytical Techniques ⇒ Organic Matter Analysis ⇒ Wet Oxidation Method

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Contents 1 Organic Carbon Analysis (wet oxidation) 1.1 Safety 1.2 Method 1.2.1 Equipment 1.2.2 Chemicals and reagents 1.2.3 Analysis

[edit] Organic Carbon Analysis (wet oxidation)

This approach is much more time consuming and costly than loss-on-ignition but is more accurate for clay rich soils, or soils containing limestone and other carbonates. This methods should be used on such soils when accurate soil carbon data is required. In most cases loss-on-ignition is a perfectly acceptable alternative. More information on loss-on-ignition methods can be found here.

In wet oxidation, potassium dichromate is used to oxidise off the organic carbon in the soil. The carbon is given off as carbon dioxide (CO₂). By adding a known amount of potassium dichromate and then measuring how much was used at the end of reaction by carrying out a titration with a reducing agent (ammonium ferrous sulphate) the amount of carbon in the soil can be calculated.

[edit] Safety

Some of the chemicals used in the method are toxic or highly corrosive. Always consult chemical safety data sheets and follow appropriate health and safety protocols before handling or using chemicals.

Many chemical safety data sheets are available here from this Oxford University based database.

[edit] Method

[edit] Equipment

• 3 x reagent bottles.
• 2 x 1 litre beakers
• 2 x 1 litre volumetric flasks
• 1 x 100 ml beaker
• 1 x 100 ml volumetric flask
• 1 x burette
• 2 x 10 ml pipettes
• 1 x 5 ml pipette
• 1 x 2 ml pipette
• pipette bulbs or fillers
• 1 x 25 ml measuring cylinder
• 1 x 100 ml measuring cylinder
• Scientific balance
• 500 ml volumetric flasks
• Distilled water

[edit] Chemicals and reagents

• Reagent 1 - Oxidising agent(oxidises the organic carbon in the soil), 0.167M Potassium dichromate. Transfer approximately 800ml distilled water to a 1000ml beaker. Dissolve 49.04g oven-dried potassium dichromate in the distilled water. Transfer the solution to a 1000 ml volumetric flask and make to volume (the mark on the kneck of the volumetric flask) with distilled water.

• Reagent 2 - Indicator (marks with a colour change when all the remaining reagent 1 has reacted with reagent 3), Barium diphenylamine sulphonate. Transfer approximately 75 ml distilled water to a 100ml beaker. Dissolve 0.16g reagent in the water, warming gently to aid dissolution. Transfer the solution to a 100ml volumetric flask and make to volume with distilled water.

• Reagent 3 - Reducing agent (reacts with reagent 1 left over after its reaction with the soil), 0.5M ammonium iron(II) sulphate. Transfer approximately 750 ml distilled water to a 1000ml flask. Dissolve in this, 196g ammonium iron (II) sulphate then add, with care, 5 mls concentrated sulphuric acid. Mix the solution then transfer to a 1000 ml volumetric flask and make to volume with distilled water.

• Concentrated sulphuric acid

• Orthophosphoric acid

[edit] Analysis

1. Grind a 5g subsample of air-dry fine earth to pass through a 0.5mm sieve. Dry in the oven at 105^oC for 4 hours and store in a dessicator prior to use.
2. Weigh 0.10 - 0.70 g of oven dry soil into a 500ml conical flask. Record the actual weight of soil (W1) to 0.01 g.

NOTE: Use 0.10-0.20g for peaty soils, 0.20-0.40g for A horizons (topsoils) and 0.50-0.70g for B horizons (subsoils).

1. Pipette 10ml of reagent 1 into the flask and swirl gently to ensure mixing. Be carfeul not to get any of the reagent on your skin.
2. Place the flask in the fume cupboard and carefully add 20 ml of concentrated sulphuric acid from a measuring cylinder, running the acid down the inside of the flask. Swirl gently and leave the mixture to stand for 30 minutes.
3. Add 200 ml distilled water, then pipette carefully 10 ml of orthophosphoric acid and 2 ml indicator (reagent 2) into the mixture.
4. The flask will be hot, so cool it under some running water.
5. Fill a burette with reagent 3 and titrate the solution against reagent 3 until a bright green colour develops in the solution in the flask; this is the end-point. As the end-point is approached the colour turns to a purplish-blue. At this point add the ammonium ferrous sulphate dropwise so as not to miss the end-point. Note the titre volume A (the amount of reagent 3 that you have run out of the burette).
6. Carry out a reagent blank using all the reagents as above but without adding any soil and note the titre volume (B).

Calculate the percentage organic carbon as:

% Organic Carbon = (4 x (B - A)) ÷ (B x W1)