Method 512a Determination of extractable sulphur manual method

Instrumentation. The determination of extracted sulphate-S may be carried out by ICP, ion chromatography or turbidimetry. The ICP procedure measures both organic and inorganic S present in the extract, but has a low methodological error. Ion chromatography may be affected by interference from phosphates and speed of sample throughput, but a suitable method is given by Combs et al. (1998). The most widely used method is by turbidimetry, where sulphate is precipitated as a white suspension of barium sulphate by the addition of barium chloride solution to the soil extract. The absorption of light is often measured using a nephelometer, spectrophotometer, or colorimeter at 480 nm, however, measurements at lower wavelengths increase sensitivity, but may incur possible curvature of the calibration graph.

Reagents (extraction).

• Activated charcoal, purified - place about 50 g Darco G-6 activated carbon in a wide-neck screw-cap container, add sufficient calcium phosphate extractant to completely wet it, then cap the bottle and shake for 5 min. Filter slowly with suction through a Buchner funnel, then wash three times successively with deionized water. Test the final leachate with a solution of barium chloride (approximately 1.4% m/v in 0.3 M HCl). If turbidity indicates the presence of sulphate, return the charcoal to a beaker, thoroughly mix with deionized water (boil for 15 min if necessary to get a clear test), refilter, wash and test for S as above. When satisfactory, dry overnight at 105°C and store in a tightly capped bottle.

• Calcium phosphate extractant, 500 mg l-1 of phosphorus - dissolve 20.3 g calcium phosphate [Ca(H2PO4).2H2O] in water and make up to 10 l.

Procedure (extraction). Weigh 10 g air-dry soil sieved to ^2 mm (10 mesh) into a 50-ml conical flask. Add 25 ml of calcium phosphate extractant (50 ml for peat or compost) and shake on a reciprocating shaker (at approximately 200-275 oscillations of 25 mm per minute) for 30 min. If the presence of sol uble organic matter is suspected, use a scoop to add 0.15 g purified activated charcoal (carbon), and shake for a further 3 min to enable a subsequent clear filtrate. Filter through a 125-mm Whatman No. 40 (or 42) filter paper, rejecting the first few millilitres. Carry out a blank determination.

Reagents (determination).

• Barium chloride crystals - sieve the powdered BaCl2.2H2O crystals, retaining the 520-860 pm (30-20 mesh) fraction. Warning: barium chloride is poisonous; wear PPE when handling.

• Seed solution, 20 pg ml-1 of SO4-S - dissolve 0.1087 g of K2SO4 in 500 ml water in a 2-l beaker, and add 500 ml of hydrochloric acid (approximately 36% w/v HCl). Carefully place a Teflon-coated magnetic stirrer-bar into the beaker, place on to a magnetic stirrer and switch on. Add 2 g of acacia powder (see above discussion) slowly while stirring so as to avoid formation of any lumps. Transfer to a bottle and store in a refrigerator. Safety note: Acacia powder will cause severe irritation to the eyes, and also irritates the skin and the digestive and respiratory tracts; it is also a mutagen. Wear appropriate PPE in handling acacia powder.

• Sulphate stock standard solution, 500 pg ml-1 of SO4-S - dissolve 2.717 g potassium sulphate (K2SO4), previously dried at 105°C for 1 h and cooled in a desiccator, in calcium phosphate extractant, then transfer to a 1-l volumetric flask with washings and make up to the mark with extractant.

• Sulphate working standard solutions, 0-12 pg ml-1 of SO4-S - pipette 1, 2, 4, 6, 8 and 12 ml of the 500 pg ml-1 sulphate stock standard solution into 500-ml volumetric flasks, make up to the mark with calcium phosphate extractant and mix. This will give solutions containing 1, 2, 4, 6, 8 and 12 pg ml-1 of sulphate-S.

Procedure (determination). If charcoal has been used in the sample extraction stage, then 25-ml aliquots of the working standards should be shaken with a 0.15 g scoop of purified charcoal for 3 min, and filtered through a 125 mm Whatman No. 40 (or 42) filter paper, rejecting the first few millilitres. Pipette 10-ml aliquots of standards, blank and sample extracts into a 50-ml conical flask containing a magnetic stirrer bar, add 1 ml of the 'seed solution', swirl to mix. Next add a 0.3-g scoop of BaCl2.2H2O crystals and stir magnetically for 1 min. Within the next 8 min, read the absorbance at 420 nm on a colorimeter or spectrophotometer fitted with a 40 mm optical cell. The background absorbance resulting from fine clay particles passing through the filter paper should also be measured on the soil extract plus 'seed solution', but without addition of BaCl2 crystals.

Calculation. Read the pg ml-1 SO4-S for all the solutions from the standard graph. Add the values for the blank and background absorbance, and subtract the sum from the value of the sample solution to give a corrected value. Since 10 g soil provided 25 ml extract, multiply the corrected value of pg ml-1 SO4-S in the extract by 2.5 to give the pg g-1 SO4-S (= mg kg-1 SO4-S) in the air-

dry soil. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

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