Available sulphur

Estimation of Available Sulphur

(Williams & Stein bergs, 1959)(NaCl extractable-S)

Principle

Inorganic sulphate -S(SO4 - S) and a portion of organically held SO4 - S are mobilized by special heat treatments and extraction with 1.0 per cent sodium chloride. Sulphate-S is converted into BaSO4 suspension using BaCI2 crystals and a special conditioning reagent. The resulting turbidity is determined by spectrophotometer

Apparatus required

1.  100 ml shaking bottle.

2.  Reciprocating shaker

3.Measuring cylinders of 25 ml  and 10 ml

4. 25 ml volumetric flask

5.  Pipettes of 5 ml and 1 ml

6.  Spectrophotometer I photoelectric colorimeter

Reagents required                                                                      

1.      1.0% Sodium Chloride (NaCl): Dissolve 10 g of NaCl in 700 ml of distilled water and  make up to 1 litre with distilled water.

2.      Barium chloride dihydrate: BaCI2 . 2H20 crystals, 20 to 30 mesh.

3.      Conditioning reagent:  Dissolve 75 g NaCI in 275 ml of distilled water in a 500 ml volumetric flask, stirring with magnetic stirring bar, add 30 ml of conc. HCL, 100 ml of absolute ethanol and 50 ml of glycerol. Rinse glycerol into flask. Continue stirring until NaCI dissolves. Remove stirring bar and make up to volume with distilled water.

4.      Standard sulphate Solution: Dissolve 0.5434 g  K2SO4 in 1 litre of distilled water. This contains 100 mg S/ml. Dilute 10 times to obtain 10 ppm solution. Transfer  0, 5, 10, 15 and 20 ml (containing 0, 50,100,150, and 200 µg SO4-S) to Erlenmeyer flask of 150 ml bringing the volume with distilled water to 30 ml.

Procedure

·         Weigh five gram soil into a silica basin and add 20 ml distilled water

·         Place the basin on a boiling water bath and evaporate to dryness

·         Then, heat it in a hot-air oven at 1020 C for 60 minutes

·         After cooling transfer the soil to a 50 ml centrifuge tube and extract it with 33 ml of NaCI

·         Pipette out 25 ml of aliquot into a silica basin and evaporate to dryness with 2 ml of 3 % H2O2. This is done to remove interfering organic matter.

·         Heat the basin in a hot-air oven at 1020 C for 60 minutes and ensure the removal of excess H2O2.

·         After cooling, take up the residue in 25 ml water, transfer to a centrifuge tube and centrifuge to remove suspended matter. Determine the sulphur content by taking a suitable aliquot.

·         Pipette out 10 or 20 ml of extract in a 150 ml Erlenmeyer flask and add 20 or 10 ml of H2O thus bringing the volume to 30 ml.

·         Add 2.5 ml of stabilizing solution and 0.2 to 0.3 g of BaCI2 crystals by a spatula to the  standards and extract

·         Shake the flasks for 1 minute each at constant rate

·         After 1 to 3 minutes, measure the turbidity in a colorimeter using a blue filter

·         Turbidity can be more accurately measured at 340 nm on a  spectrophotometer

·         The turbidity remains constant during 3-10 minutes.

Calculation

SO4 - S (mg kg-1)      = S (mgL-1) in the aliquot  x  vol. of extract (ml)

                                                      Volume of aliquot (ml)  x oven dry soil (g)