Method 512b Determination of extractable sulphur automated method

An automated method should improve reproducibility by maintaining constant conditions for the formation of the BaSO4 precipitate. A method suitable for the segmented-flow analysis of sulphate in soil and plant extracts using Skalar Analytical equipment has been proposed by Coutinho (1997). Soils are said to be extracted with double-distilled water according to the method in MAFF/ADAS (1986, pp. 215-216), however, that reference uses approximately 1.5 M HCl (10 g soil extracted with 70 ml water and 10 ml HCl, 36% m/m); and the absorbance is measured at 420 nm using a 50-mm cell path. It is suitable for soils up to 10 pg SO4-S ml-1 without a dilution step, and for soils up to 100 pg SO4-S ml-1 with automatic dilution of the sample. A method for soils low in sulphur (up to 1 pg SO4-S ml-1) using Technicon (Bran and Luebbe) AutoAnalyzer equipment was described by Bettany and Halstead (1972). The Turner Model 111 Fluorometer was modified to enable it to function as a neph-elometer, but presumably a colorimeter could be substituted with measurement at 480 nm using a long path (40-50 mm) optical flowcell. The method, with some amendments, is summarized below.


• Barium chloride reagent - add 10.0 g of polyvinyl alcohol, 40.0 g of barium chloride (BaCl2.2H2O, turbidimetric grade) and 120 ml of 1.0 M HCl to 1800 ml of water. Warning: barium chloride is poisonous; wear PPE when handling. Heat on a stirrer-heater unit until the solution clarifies, then allow to cool before adjusting to 2 l. Allow to stand for 2-3 days, then filter through glass wool prior to use.

• Calcium chloride extractant, 0.01 M - completely dissolve 1.47 g CaCl2.2H2O in water and make up to 1 l.

• EDTA buffer wash - dissolve 40 g tetrasodium EDTA, and 6.75 g ammonium chloride in approximately 800 ml water, then, in a fume cupboard, carefully add 57 ml ammonia solution (approximately 35% m/m NH3). Stir and make up to 1 l with water.

• Sodium peroxide solution, 2% m/v. (Note: sodium peroxide is highly corrosive, very irritant to skin and mucous membranes, and may ignite combustible materials; wear suitable PPE.)

• 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 water, then transfer to a 1-l volumetric flask with washings and make up to the mark with extractant.

• Sulphate intermediate standard solution, 50 pg ml-1 - pipette 10 ml of the sulphate stock standard into a 100-ml volumetric flask, make up to the mark with water and mix.

• Sulphate working standards, 0.25-1.5 pg ml-1 - pipette aliquots of 1, 2, 3, 4, 5 and 6 ml of the sulphate intermediate standard solution into 200-ml volumetric flasks, make up to the mark with water and mix. This gives a series of standard solutions of 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 pg SO4-S ml-1.

Procedure (extraction). Add 25 g air-dry soil, sieved to ^2 mm (10 mesh), into a 125-ml conical flask. Add 50 ml calcium chloride extractant, 0.01 M, and shake on a reciprocating shaker (at approximately 200-275 oscillations of 25 mm per minute) for 30 min. Filter through a Whatman No. 30 filter paper, rejecting the first few millilitres, and pipette 25 ml of the filtrate into a 50-ml beaker. Slowly add 2 ml sodium peroxide solution using a plastic pasteur pipette. Allow to stand for 5 min, after which the gelatinous precipitate of interfering cations and organic matter is filtered off using a Whatman No. 42 filter paper. Wash the precipitate and filter paper with water, adjust the filtrate to approximately pH 3 with 0.5 M HCl, transfer with rinsing to a 50-ml volumetric flask, make up to the mark with water and mix. Carry out a blank determination.

Procedure (determination). Samples, blank, and standards are poured into the sample cups. Industrial 8.5-ml autoanalyser cups (Part No. 127-0080-01) are available from Gradko or LIP (Equipment & Services) Ltd (see Chapter 1, 'Chemistry module'). Technicon Sampler IV would require the 10-ml cups, same Part No. from Bran + Luebbe. Every 10th cup should contain the EDTA buffer wash solution followed by a cup containing water; this is to prevent build-up of barium sulphate on the walls of the mixing coil and flowcell. Pump the reagents for about 30 min at the start to flex the pump tubes. Sample the highest standard several times and adjust the sensitivity of the colorime-ter/spectrophotometer and/or chart recorder to give a reading of about 90% full-scale. Adjust the zero setting and baseline reading to about 5% full-scale for aspiration of the wash solution. If the high standard reading is too low or high, alter the ratio of flow rates of the sample and barium chloride reagent as appropriate. If the readings from the soil samples are too high, take a smaller weight of sample for the extraction. The flow diagram is shown in Fig. 5.5.

Calculation. The 25 g soil was shaken with 50 ml calcium chloride extractant, and 25 ml of this extract was diluted to 50 ml. There is therefore a x4 dilution factor. Calculate the concentration of SO4-S in the blank and samples by comparison with the standard curve. Subtract the blank value from the sample values and multiply by 4 to give the pg SO4-S g-1 (= mg SO4-S kg-1) 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).

To waste

To waste

To sampler wash-<

To chart recorder

From sampler -4-wash (if required)

To sampler wash-<

To chart recorder

From sampler -4-wash (if required)

Sampler 20 h-1 ratio 1:3 sample:wash Pump

sample sample sample sample

BaCl2 reagent


0 085 M HCl flowcell waste

Fig. 5.5. Manifold for the automated determination of sulphate in soil extracts.

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