water-soluble Cl– (5A1 or 5A2; % Cl)], both expressed on the same soil moisture status and allowing for the weight of sample.The chemical basis of this procedure is similar to that of Method 6A1, except external heating on a hotplate digester is included. The method is that of Heanes (1984). It does not require expensive apparatus and avoids or minimises many interferences (e.g. carbonates, wood, charcoal, coke, Fe2+, readily reducible Mn). Correction for positive Cl– interference in saline soils (>0.5% Cl–) is recommended.
Analogous to Na2Cr2O7.2H2O or CrO3 used in Method 6A1, K2Cr2O7 is a powerful oxidising agent (+6 oxidation state) and a systemic poison, the more so when heated with concentrated H2SO4. Accordingly, avoid external and internal contact and keep the reagents away from combustible materials due to risk of fire. The oxidation step should be carried out in a fume cabinet. Further note that the concentration of Cr2O72– specified in this method is one-third that used in 6A1. Accordingly, the likelihood that >75% of Cr2O72– will be reduced during measurement of soils high in TOC needs close attention.
Thermostatically controlled Al hotplate digester, with a mounted and insulated Al block, able to operate at 135 ± 2°C. For example, 66 holes of 32 mm dia and depth 60 mm can fit within a heating block with dimensions 450 × 250 × 70 mm.
Reagent Water
As for Method 6A1.
0.167 M Potassium Dichromate
Dissolve 49.1 g analytical grade K2Cr2O7 in Reagent Water and dilute to 1.0 L.
Sulfuric Acid
H2SO4 as for method 6A1.
Standard Sucrose Solution
1 mL contains 5 mg of C. Prepare as for Method 6A1.
Prepare a series of standards for each set of analyses by dispensing 0, 0.5, 1, ---- 4.0 mL (as required) of the Standard Sucrose Solution into 100 mL graduated borosilicate test tubes (200 × 30 mm). These standards contain 0, 2.5, 5, . . 20 mg C, corresponding to 0–2% C for 1.0 g soil samples and 0–10% C for 0.2 g soil samples. Evaporate the dispensed sucrose solutions to dryness in an oven, at ≤65°C, then cool to room temperature.
Transfer finely-ground (<0.15 mm; as specified by Heanes 1984), air-dry soil, according to the expected C content, into 100 mL graduated test tubes as used for sucrose standards. No soil sample used in the measurement should contain >20 mg oxidisable C; usually 0.1–2 g containing 20–2.5 mg C. To a blank, dry sucrose standards and soils, add 10.0 mL 0.167 M K2Cr2O7 and agitate carefully while adding 20 mL H2SO4 gradually to avoid the loss of soil and chromic acid from localised boiling. Agitate for a further 30 sec before inserting the test tube and contents into a hole in the preheated (135°C) Al block of the hotplate digester, located in a fume cabinet. Remove tubes from the digester after 30 min and allow them to cool to room temperature. Add Reagent Water to the 50 mL graduation mark (approximately) on the tube and swirl the contents. Next vigorously fill the test tube to near the 100 mL mark, using a wash bottle with Reagent Water, to mix contents and to release the heat of dilution. Allow to cool to room temperature, make to volume (100 mL), seal, invert several times to mix well, and allow the suspension to settle.
Centrifuge to clear the supernatant and measure absorbance at 600 nm. Construct a calibration line by plotting absorbance of the standard sucrose assays against their known contents of C (or use a microprocessor controlled equivalent). Expect the calibration line to be slightly curvilinear. 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.
Calculate total % C – Heanes, allowing for the weight of sample. Repeat the determination with less soil if >75% of Cr2O72– is reduced (see Note 1).
Report total OC – Heanes (% 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.
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 total OC – Heanes, as described for non-saline soils. Repeat the soil C determination with less soil if >75% of Cr2O72- is reduced (see Note 1).
TOC (Heanes) = [Apparent TOC% – water-soluble Cl– (5A1 or 5A2; % Cl)], both expressed on the same soil moisture status and allowing for the weight of sample.
Report results as described for non-saline soil. 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.
1. 0.167 M K2Cr2O7 (i.e. 1.0 N K2Cr2O7) is the reagent concentration described by Heanes (1984). As 75% of Cr2O72– is required for 22.5 mg organic-C, it follows that for a 1.0 g sample, the 75% reagent ‘barrier’ is reached at a soil concentrations of about 2.25% C. For a 0.2 g sample, the corresponding soil concentration is 11.25% C.