If the soil organic matter contains an element in a fairly constant percentage, then the determination of that element should enable deduction of the organic matter. This has been attempted by determining nitrogen, but this was found to vary too widely in organic soil materials. More commonly, carbon has been determined. It is assumed that soil organic matter contains on average 58% carbon, so that multiplication of the value for carbon by 1.72 (100/58) will give the percentage organic matter. The other factor is the percentage organic matter to be oxidized by the chromic acid produced in situ, with or without external heat. The average for the classic Walkley and Black (1934) method is 77%, which may not be valid for subsoils (Allison, 1960), whereas Tinsley's wet combustion method (Tinsley, 1950) oxidizes most of the organic matter, facilitated by the presence of perchloric acid. Carbon in graphite and coal may not be oxidized. It is therefore preferable to report the 'percentage easily oxidizable organic C' rather than the 'percentage organic matter'.
Errors. Chromic acid digestion can incur errors from any significant presence of chlorides, which can reduce the dichromate ion to give high results. The addition of silver sulphate to the digestion acid, or prior leaching of the soil could eliminate this effect. Any ferrous ion (from an anaerobic soil situation) would also reduce dichromate, but the aerobic soil drying process would normally oxidize this to ferric ion. Higher oxides of manganese could compete with dichromate in oxidizing organic matter, but this is not usually significant. Some dichromate will be reduced by organic hydrogen, but this is approximately compensated for by the loss of carbon as CO2 from organic oxygen:
Safety. The addition of concentrated sulphuric acid on sodium (or potassium) dichromate is to produce chromium trioxide, which is a powerful oxidizing agent capable of oxidizing carbon to carbon dioxide. The solution is loosely called chromic acid, but although true chromic acid H2CrO4 has not been isolated, the aqueous solution contains dichromic acid, H2Cr2O7. The acid-dichromate reagent (hexavalent chromium) is corrosive to skin, respiratory and gastrointestinal tract, and may create a cancer risk. There may be restrictions on disposal into municipal sewerage systems, therefore storage for professional chemical waste disposal may be required. Adequate PPE should be employed to protect eyes, skin and lungs. With adequate supervision, the procedure has been used in practical classes for many years with no reportable incident.
Reaction equations. Oxidation step: dichromate (Cr2O72-) and organic carbon (Co):
2Cr2O72- + 3Co + 16H ^ 4Cr3+ + 3CO2 + 8H2O Titration step: ferrous ion (Fe2+) with excess dichromate:
• Acid-dichromate mixture, 66.7 mM - dissolve 20.0 g sodium dichromate dihydrate (Na2Cr2O7.2H2O, mol. wt 298.00, equivalent wt 49.67) in about 50 ml water in a 2-l beaker and carefully add 400 ml sulphuric acid (98% m/m) with stirring, and allow to cool. Next add 140 ml perchloric acid 70% (1.70 g ml-1) or 163 ml of 60%. Make up to 1 l with water.
• Ferrous ammonium sulphate, approximately 0.4 M - carefully add 5 ml sulphuric acid (98% m/m H2SO4) to 1.5 l water, stir slowly to dissolve, then add 314 g ammonium ferrous sulphate [(NH4)2SO4.FeSO4.6H2O, mol. wt 392.14] stir to dissolve and dilute to 2 l.
• Ferroin indicator - slowly dissolve 3.71 g of o-phenanthroline and 1.74 g FeSO4.7H2O in 250 ml water.
Procedure. Grind a sample of soil, previously sieved to ^2 mm, in a pestle and mortar, and sieve to ^0.5 mm, or ^30 mesh (US methods use the 60 mesh Market Grade Testing Sieve, 0.23 mm mesh opening). Sufficient soil to contain 10-20 mg carbon should be accurately weighed into a 500 ml conical (Erlenmeyer) flask with ground glass neck to fit a cold finger condenser. (Typical amounts of sample are: surface soil, 1 g; subsoil, 4 g; organic soil, 0.1 g; compost, 0.1 g and 40 ml dichromate mixture.)
Pipette, using a suitable safety pipette filler, 25 ml acid-dichromate mixture into the flask, fit the cold finger dispenser and slowly turn on the water supply. Place on a hotplate and simmer for 2 h.
While the sample is refluxing, standardize the ferrous ammonium sulphate against the dichromate mixture. This must be done daily. Carry out two successive blank digestions with heating for 10 min using 25 ml acid-dichromate, but no soil. Cool and add approximately 100 ml water followed by 4 drops of Ferroin indicator. Titrate with the 0.4 M ferrous ammonium sulphate until the indicator changes from blue-green to reddish-grey. If this is overshot, pipette 1 ml acid-dichromate and titrate dropwise to the end-point, allowing for the extra dichromate in the calculation step.
After digesting the soil samples, allow to cool and titrate in the same way. If more than 20 ml of acid-dichromate have been reduced, repeat using less sample.
1. Calculate the number of mol of K2Cr2O7 in 25 ml acid-dichromate reagent using the equation: no. mol x concn. (mol l-1) x vol. (l)
= 0.0667 x 25/1000 = 1.668 x 10-3 mol, or 1.668 mmol.
2. If the standardization titration was 26 ml ferrous ammonium sulphate solution, it had reacted with 25 ml acid-dichromate containing 1.668 mmol. If the back-titration of the residual acid-dichromate was 10 ml ferrous ammonium sulphate solution, then the residual acid-dichromate amounted to 10 x 1.668/26 = 0.642 mmol. The amount used to oxidize the organic carbon is therefore
3. From the reaction equation for the oxidation step, it can be seen that 3 mol of carbon react with 2 of dichromate. The number of mol of C oxidized is therefore 1.026 x 3/2 = 1.539 mmol. Since the molar mass of C is 12.0, the mass of C oxidized is therefore 1.539 x 10-3 mol x 12 g mol-1, or 18.5 mg. If this is in 1 g soil, the easily oxidizable organic C content becomes 18.5 mg g-1 air-dry soil. This can be expressed for oven-dry soil as in Method 5.2, Calculation (2).
4. The calculation above reduces to the following equation: C content (mg C g-1 air-dry soil) = 30(1-x/y)/soil mass, where x = residual acid-dichromate titre, and y = standardization titre.
5. The organic matter may be approximately obtained by multiplying by 1.74 as explained above. In the calculation example, this would become:
18.5 x 1.72 = 31.82 mg organic matter g-1, or 3.18%.
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