20F1 Sulfur – 4 M HCl extraction (S_HCl), ICPAES finish

Considerable retained acidity may be stored in ASS in the form of jarosite and similar relatively insoluble Fe and Al hydroxy sulfate compounds. Their acidity and S are not recovered in the 1 M KCl suspensions of TAA (Method 20B1) and S_KCl (Method 20D1); however, they are soluble in 4 M HCl as are other sources of SO₄^2–. The difference in the S extracted by 4 M HCl and by 1M KCl [S_NAS (net acid soluble S) = S_HCl– S_KCl] provides an estimate of the insoluble (jarositic) S content of soils (McElnea and Ahern 2004c). Sulfur is commonly determined by ICPAES. On highly organic soil samples, however, 4 M HCl may extract appreciable organic S. Therefore, unless a SO₄^2– specific technique is used, such as IC (Method 20F2), the S_NAS result will be inflated.

The method involves the extraction of dry soil with 4 M HCl at a ratio of 1:40. The extract is centrifuged and the clear supernatant is analysed for S. Results are reported as % S (ideally SO₄-S) on an oven-dry basis. The 4 M HCl treatment will extract soluble and adsorbed SO₄^2–, as well as SO₄^2– from gypsum, jarosite or other insoluble SO₄2–salts. Acid volatile sulfides will be dissolved and lost to H₂S after 4 M HCl extraction, if not already lost in the sampling, transportation or drying process. Pyrite-S will largely remain unaffected by this acid extraction process (Morse and Cornwell 1987).

Reagents

4 M HCl

Add 400 mL of 31.5–33% w/v (or equivalent) HCl to an equal volume of deionised water, mix well, cool to room temperature, then make to 1.0 L with deionised water.

Sulfur Primary Standard

1 L contains 5000 mg of S.

Dry anhydrous sodium sulfate (Na₂SO₄) at 105°C for 4 h, cool in a desiccator, then accurately weigh 22.1491 g, dissolve, then make volume to 1.0 L in a volumetric flask. Add 2 drops of chloroform (CHCl₃) to suppress biological activity and store solution in borosilicate glass, preferably in the dark.

Sulfur Working Standards

Accurately add 10.0, 25.0, 50.0 and 100.0 mL Sulfur Primary Standard (5000 mg/L) to 1.0 L volumetric flasks, each containing 400 mL of 31.5–33% w/v HCl (or equivalent) and make to volume with deionised water. These S Working Standards have solution concentrations of 50, 125, 250 and 500 mg S/L. Soil equivalent concentrations for a 1:40 soil/solution ratio are 0.20, 0.50, 1.00 and 2.00% S. For volumes >10 mL, use a 50 mL burette.

Procedure

Weigh 2 g ± 0.01 g of finely ground soil (<0.5 mm, previously dried rapidly at 85°C) into a labelled 250 mL polyethylene bottle. Include a blank sample with each batch of samples. Dispense 80 mL of 4 M HCl into each bottle to make a 1:40 soil suspension, noting that soils high in carbonates can react vigorously with HCl and generate CO₂ gas. Wait until any effervescence subsides before stoppering the bottle. Shake (ideally end-over-end) for 16 h. Remove from tumbler, allow samples to stand for ≈30 min, centrifuge for 15 min or filter through thick, medium-speed, high-retention filter paper to obtain particulate-free extracts.

Determine S_HCl using ICPAES, guided by details provided in Method 10B3, using an appropriate suite of S Working Standards.

Report S_HCl (ICPAES) as % S on an oven-dry (85°C) basis.