PRINCIPLE

1. Sulfate ion reacts with barium ion, forming a turbid suspension of insoluble barium sulfate. The scatter of light caused by this suspension is an indication of the sulfate concentration in the sample. Measurements are carried out on a spectrophotometer using a wavelength of 550 nm.

EQUIPMENT

1. Spectrophotometer (315 to 999 nm range)
2. Polystyrene Disposable Cuvets
3. Disposable glass test tubes (20 mm x 150 mm)
4. Calibrated 5 g scoop

REAGENTS

Ammonium Chloride	NH4Cl
Calcium Hydroxide	Ca(OH)2
Barium Acetate	Ba(C2H3O2)2
Acetic Acid	CH3COOH
Hydrochloric Acid	HCl

STANDARDS

1. Stock Solution:
   1. 100 ppm SO₄: Use certified, NIST traceable standard.
2. Instrument calibration standards:
   1. Pipet the designated amounts of 100 ppm SO₄ stock solution into 100 mL. volumetric flasks. Add 2.674 g ammonium chloride. Dissolve and dilute to volume with deionized water and mix well.

Standard #	Stock Solution mL	Final Concentration ppm
1	3	3
2	6	6
3	15	15
4	30	30
5	45	45
6	60	60

WORKING SOLUTIONS

1. Precipitating Solution:
   1. Weigh 255 g barium acetate into a 1 liter volumetric flask containing 500 mL. deionized water. Swirl to dissolve.
   2. Add 100 mL. acetic acid.
   3. Dilute to volume with deionized water and mix well.
2. Seed Solution:
   1. Add 210 mL. of 100 ppm SO₄ stock solution to a 500 mL. volumetric flask.
   2. Add 2 mL. hydrochloric acid.
   3. Add 13.87 g ammonium chloride.
   4. Dilute to volume with deionized water and mix well.

EXTRACTING SOLUTION

1. 0.5 N Ammonium Chloride:
   1. Weigh 26.74 g ammonium chloride into a 1 liter volumetric flask containing 700 mL. deionized water.
   2. Add 0.06 g calcium hydroxide.
   3. Dissolve, dilute to volume with deionized water and mix well.

SOIL EXTRACTION

1. Scoop 5 g soil into a 40 mL. cylindricale bottle.
2. Add 20 mL. extracting solution.
3. Shake for 15 minutes on a shaker box at high speed (250 oscillations per minute).
4. Filter using Whatman #42 paper.

PROCEDURE

1. Pipet 5 mL. of soil extract into glass test tubes (20 mm x 150 mm).
2. Add 1 mL. seed solution.
3. Add 1 mL. precipitating solution and mix by slow inversion. This step is critical. Each sample should be mixed in exactly the same manner.
4. Pour the extracts into the polystyrene cuvets and allow to stand for 10 minutes.
5. Analyze the extracts on the spectrophotometer at 550 nm.
6. Treat the 6 calibration standards in the same manner.
7. Prepare one extraction blank for each set of samples.

CALCULATIONS

1. Construct a graph of SO₄ concentration vs absorbance.
2. Determine ppm SO₄ in the extract from the graph.
3. (ppm SO4 in extract - ppm SO4 in blank) X 8 = lbs SO4/acre.

NOTES

1. All absorbance readings should be completed within 20 minutes of the seed solution addition.
2. Because our routine soil extract contains sulfuric acid, special precautions must be taken to prevent sample contamination.
   1. All glassware should be thoroughly cleaned and rinsed with deionized water before use.
   2. Filter paper should be folded just prior to filtration. Do not use any prefolded filter paper.

REFERENCES

1. Massuomi, A. and A. H. Cornfield. 1963. A Rapid Method of Determining Sulfate in Water Extracts of Soils. Analyst. 88:321-322.
2. Chaudry, I. A. and A. H. Cornfield. 1966. The Determination of Total Sulfur in Soil and Plant Material. Analyst. 91:528-530.