DETERMINATION OF ORGANIC MATTER BY WALKEY- BLACK WET OXIDATION METHOD
OBJECTIVE: This method determines the percentage of organic carbon and organic matter in the soil.
The soil sample is suspended in standard potassium dichromate (K2Cr2O7) solution to which concentrated sulphuric acid is then added. The heat generated on adding the sulphuric acid and the presence of chromic acid creates strong oxidizing conditions. The excess chromic acid is back titrated in the presence of a suitable indicator. 2Cr2O72- + 16 H+ + 3C = 4Cr3+ + 8H20 +3CO2
CHEMICALS AND REAGENTS:
i) 1N K2Cr2O7 : Dissolve 49.04 g K2Cr2O7 (dried at 105ºC) in water and make it to 1 Litre ii) Concentrated sulphuric acid, Concentrated Phosphoric Acid iii) Diphenylamine Indicator: Dissolve 1 g of Diphenylamine in 50ml of concentrated sulphuric acid. iv) 1N Ferrous Sulphate( FeSO4.7H2O): Dissolve 278 g of Ferrous Sulphate in approximate 800 mL of Deionized water and 15 mL Concentrated sulphuric acid and make it to1 litre with Deionized water.
PROCEDURE 1. Grind the soil sample thoroughly. 2. Place 1g of air oven dry soil in a 500 mL conical flask. 3. Add exactly 10ML of 1N K2Cr2O7 solution and swirl gently to wet the sample thoroughly. 4. Carefully add 20mL of concentrated sulphuric acid and swirl thoroughly to mix. 5. Allow the mixture to stand for 30 minutes. 6. Add 200mL Distilled water to the flask. 7. Filter and collect the filterate in 250mL conical flask. 8. Add 10 mL of Concentrated Phosphoric Acid and 5 drops of Diphenylamine Indicator. ( Add 0.2g sodium fluoride to the filterate if available) 9. Firstly Blank potassium dichromate is titrated with 1N Ferrous Sulphate using small increment additions. Continue adding until the color of indicator changes to green. 10. Record the volume needed to reach the end point. 11.