Paragraph 4 REAGENTS

Dilute hydrochloric acid solution (HCl), about 6 M

4.1

See Method 25d (4.1).

Copper solution (70 mg/l) in 1.5 M hydrochloric acid

4.2

Dissolve 275 mg of copper sulfate (CuSO₄ 5H₂O) weighed to within 0.1 mg in 250 ml of the 6 M hydrochloric acid solution (4.1) in a 1000 ml volumetric flask. Make up to volume with water and mix thoroughly.

Ascorbic acid solution (50 g/l)

4.3

Dissolve 50 g of ascorbic acid (C₆H₈O₆) in water in a 1000 ml volumetric flask. Make up to volume with water, mix thoroughly and keep in a refrigerator.

n-butyl acetate

4.4

Ammonium thiocyanate solution, 0.2 M

4.5

Dissolve 15.224 g of NH₄SCN in water in a 1000 ml volumetric flask. Make up to volume with water; mix thoroughly and store in a dark-coloured bottle.

Stannous chloride solution (50 g/l) in 2 M hydrochloric acid

4.6

This solution must be perfectly clear and prepared immediately before use. Very pure stannous chloride must be used otherwise the solution will not be clear.

To prepare 100 ml of solution, dissolve 5 g of SnCl₂.2H₂O in 35 ml of 6 M HCl solution (4.1). Add 10 ml of the copper solution (4.2). Make up to volume with water and mix thoroughly.

Molybdenum calibration solutions

4.7

Molybdenum stock solution (500 mg/ml)

4.7.1

Dissolve 0.920 g of ammonium molybdate [(NH₄)₆Mo₇O₂₄.4H₂O]; weighed to within 0.1 mg in the 6 M hydrochloric acid (4.1) in a 1 litre volumetric flask. Make up to volume with that solution and mix thoroughly.

molybdenum intermediate solution (25 mg/ml)

4.7.2

Place 25 ml of the stock solution (4.7.1) in a 500 ml volumetric flask. Make up to volume with 6M hydrochloric acid (4.1) and mix thoroughly.

Molybdenum working solution (2.5 mg/ml)

4.7.3

Place 10 ml of the intermediate solution (4.7.2) in a 100 ml volumetric flask. Make up to volume with 6 M hydrochloric acid (4.1) and mix thoroughly.