Paragraph 4 REAGENTS

4.1

Nitric acid (HNO₃ ρ = 1.40 g/ml).

4.2

Urea [(NH₂)₂ C O].

Ammonium biflouride (NH₄HF₂) solution (10% w/v)

4.3

Keep the solution in a plastic container.

Ammonium hydroxide solution (1 + 1)

4.4

Mix 1 volume of ammonia (NH₄OH, ρ = 0.9 g/ml) with 1 volume of water.

Sodium thiosulfate standard solution

4.5

Dissolve 7.812 g of sodium thiosulfate pentahydrate (Na₂S₂O₃.5H₂O) with water in a litre volumetric flask. This solution must be prepared so that 1 ml = 2 mg Cu. For stabilization, add several drops of chloroform. The solution must be kept in a glass container and protected from direct light.

4.6

Potassium iodide (KI).

Potassium thiocyanate (KSCN) solution (25% w/v)

4.7

Keep this solution in a plastic flask.

Starch solution (about 0.5%)

4.8

Place 2.5 g of starch in a 600 ml beaker. Add about 500 ml of water. Boil while stirring. Cool to ambient temperature. The solution has a short preservation period. Its preservation can be extended by adding about 10 mg of mercury iodide.