Paragraph 4 REAGENTS

4.1

Sulfuric acid (ρ = 1.84 g/ml).

4.2

Powdered iron reduced in hydrogen.

4.3

Potassium sulfate, finely pulverised.

4.4

Sulfuric acid, 0.05 M solution. } for variant (a) (see page 21)

4.5

Sodium or potassium hydroxide, 0.1 M solution, carbonate free. } for variant (a) (see page 21)

4.6

Sulfuric acid, 0.1 M solution. for variant (b) } for variant (b) (see page 21)

4.7

Sodium or potassium hydroxide, 0.2 M solution, carbonate free. } for variant (b) (see page 21)

4.8

Sulfuric acid, 0.25 M solution. } for variant (c) (see page 21)

4.9

Sodium or potassium hydroxide, 0.5 M solution, carbonate free. } for variant (c) (see page 21)

4.10

Indicator solutions:

Mixed indicator:

4.10.1

Solution A: dissolve 1 g methyl red in 37 ml 0.1 M sodium hydroxide solution and make up to 1 litre with water.

Solution B: dissolve 1 g methylene blue in water and make up to 1 litre. Mix 1 volume of A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops) of this indicator solution.

Methyl red indicator:

4.10.2

Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.

Solution of stannous chloride:

4.11

Dissolve 120 g of stannous chloride (SnCl₂.2H₂.O) in 400 ml concentrated hydrochloric acid (ρ = 1.18 g/ml) and make up to 1 litre with water. The solution must be completely clear and prepared immediately before use.

It is essential to check the reducing power of the stannous chloride. Dissolve 0.5 g of stannous chloride in 2 ml concentrated hydrochloric acid (ρ = 1.18 g/ml) and make up to 50 ml with water. Then add 5 g of Rochelle salt (potassium sodium tartrate) and a sufficient quantity of sodium bicarbonate for the solution to show an alkaline reaction to a litmus paper test.
Titrate with 0.1 M iodine solution in the presence of a starch solution as an indicator.
1 ml of 0.1 M iodine solution corresponds to 0.01128 g SnCl₂.2H₂O.
At least 80% of the total tin present in the solution thus prepared must be in the bivalent form. For the titration at least 35 ml of 0.1 M iodine solution should be used.

4.12.

Sodium hydroxide solution, 30 g per 100 ml, ammonia free.

4.13

Standard nitrate-ammoniacal solution:

Weigh out 2.500 g of potassium nitrate and 10.160 g of ammonium sulfate into a 250 ml graduated flask. Dissolve in water and make up to 250 ml. 1 ml of this solution contains 0.010 g of nitrogen.

4.14

Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.