6A1 Organic C – W&B

Concentrated H₂SO₄ is added to soil wetted with a Cr₂O₇^2– solution. Heat of dilution raises the temperature to 110–120°C, which is sufficient to induce substantial oxidation. The reaction is as follows:

Table 6.2. Summary detaÕ of method codes, technologies, method titles and notes on soÕ C tests described in this chapter.

In the absence of interferences, the chromic ions (Cr³⁺) produced should be in reasonable proportion to the OC oxidised. In aqueous acidic solution, Cr³⁺ has absorption maxima at 450 nm and 600 nm. Since Cr₂O₇^2– does not absorb at 600 nm, the absorbance at this wavelength can be used to estimate Cr³⁺ and hence OC (Sims and Haby 1971; Metson et al. 1979). An advantage over the original titrimetric procedure and the colorimetric estimation of unreacted Cr₂O₇^2– solution is that accurate standardisation of the Cr₂O₇^2– solution is not required.

Since Cl^– reacts to form chromyl chloride (CrO₂Cl₂), correction for positive Cl^– interference in saline soils (>0.5% Cl) is recommended. Ferrous iron (Fe²⁺) may also produce high results but this possibility is usually ignored (PASS would be an exception) as air-dry soils do not normally contain significant reactive Fe²⁺ (Anon 1983). Elemental C (e.g. coal and charcoal) is usually little attacked by this method (except if present as very fine particles), while carbonates, even when present at high concentration (e.g. 50%) have inconsequential impacts.

Note that the sodium dichromate (Na₂Cr₂O₇.2H₂O) or chromium trioxide (CrO₃) used in the procedure are powerful oxidising agents and systemic poisons. Accordingly, avoid external and internal contact and keep these chemicals away from combustible materials due to risk of fire. These are preferred to potassium dichromate (K₂Cr₂O₇), simply for reasons of cost (Metson et al. 1979).

Reagents

Reagent Water

This water should be in equilibrium with the atmosphere with respect to CO₂ concentration, it should have an EC of <1.5 × 10^-3 dS/m, and it should be devoid of soluble OM sufficient to affect the blank.

0.5 M Sodium Dichromate

Dissolve ≈149 g L.R. sodium dichromate (Na₂Cr₂O₇.2H₂O) in Reagent Water and make to 1.0 L. Filter through sintered glass or glass-fibre filter material to ensure the solution is free of particles prior to use.

1.0 M Chromium Trioxide (alternative to 0.5 M sodium dichromate)

Dissolve ≈100 g L.R. chromium trioxide (CrO₃) in Reagent Water and make volume to 1.0 L. Filter through sintered glass or glass-fibre filter material to ensure the solution is free of particles prior to use.

Standard Sucrose Solution

1 mL contains 5 mg of C.

Dissolve 11.8745 g sucrose [(C₁₂H₂₂O₁₁), previously dried for at least 24 h in a desiccator over sulfuric acid (H₂SO₄; 18 M)], and dilute to 1.0 L with Reagent Water in a volumetric flask.

Sulfuric Acid

Commercial sulfuric acid (H₂SO₄; sg. 1.84), free of suspended matter.

Procedure – Non-saline Soils

Prepare a series of standards for each set of analyses by dispensing 0, 1.0, 2.0, 3.0, 4.0, 5.0, ... 10.0 mL (as required) of the Standard Sucrose Solution into 250 mL conical beakers. These standards contain 0, 5, to 50 mg C, corresponding to 0–5% C for a 1.0 g soil sample and 0–25% C for a 0.2 g sample. Evaporate the dispensed sucrose solutions to dryness in an oven at not greater than 65°C then cool to room temperature.

Weigh samples of finely-ground (<0.5 mm), air-dry soil, according to the expected C content: 1.00 g for expected values of <5% C, and 0.20 g for expected values >5.0% C. Transfer soils to dry 250 mL conical beakers (all should be of similar manufacture and dimensions and located equidistance apart). To the standards and soils add 10 mL of 0.5 M sodium dichromate or 1.0 M chromium trioxide solution and swirl gently to ensure all soil particles are wetted. Wait 10 min with occasional swirling then carefully but quickly add across 10–15 sec (use gentle swirling to avoid the loss of soil and chromic acid from localised boiling) 20 mL concentrated H₂SO₄. Wait a further 30 min with occasional swirling then add 170 mL Reagent Water from a dispenser, stir or swirl to mix thoroughly and set aside to cool and for particles to settle. (Consistent timing across the additions of H₂SO₄ and dilutant water helps ensure constant reaction conditions, including heat generation.)

After cooling, centrifuge if the supernatant is not clear. Determine absorbance of the supernatant at 600 nm with Reagent Water set at zero. Construct a standard curve by plotting absorbance of the standard sucrose assays against their known contents of C (or use a microprocessor-controlled equivalent). Thereafter, dispose of spent reagents and treated soils in a safe and environmentally responsive manner, noting that Cr is a heavy metal that is both toxic and environmentally persistent.

Repeat the determination with less soil if >75% of Cr₂O₇^2– is reduced (only likely with soils of very high C content).

Report OC_W&B (% C) on an oven-dry basis, allowing for the weight of sample. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.

Procedure – Saline Soils (>0.5% Cl)

Determine water-soluble Cl (Method 5A1 or 5A2) and express results for Cl as a percentage, noting that 10 000 mg Cl/kg = 1% Cl. Also determine apparent OC_W&B as described for non-saline soils. Again, repeat the determination with less soil if >75% of Cr₂O₇^2- is reduced.

Calculation – Saline Soils

both expressed on the same soil moisture status and allowing for the weight of sample.

Report OC_W&B (% C) on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.