Paragraph 4 REAGENTS

4.1

Potassium sulfate.

4.2

Iron powder, reduced with hydrogen (the prescribed quantity of iron must be able to reduce at least 50 mg of nitric nitrogen).

4.3

Potassium thiocyanate.

4.4

Potassium nitrate.

4.5

Ammonium sulfate.

4.6

Urea.

4.7

Sulfuric acid solution: dilute an appropriate volume of sulfuric acid (ρ = 1.84 g/ml) with an equal volume of water.

4.8

Sulfuric acid, 0.1 M solution.

4.9

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

4.10

Sodium or potassium hydroxide, 0.2 M solution, free from carbonates.

4.11

Stannous chloride solution:

Dissolve 120 g of stannous chloride (SnCl₂.2H₂O) in 400 ml of concentrated hydrochloric acid (ρ = 1.18 g/ml) and make up to 1 litre with water. The solution must be perfectly 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 of 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 be alkaline to litmus paper.
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 of SnCl₂.2H₂O.
At least 80% of the total tin present in the solution thus prepared must be in bivalent form. For the titration, at least 35 ml of 0.1 M iodine solution must therefore be used.

4.12

Sulfuric acid, concentrated (ρ = 1.84 g/ml).

4.13

Hydrochloric acid solution: dilute an appropriate volume of hydrochloric acid (ρ = 1.18 g/ml) with an equal volume of water.

4.14

Glacial acetic acid.

4.15

Sulfuric acid solution, approximately 30% (W/V) H₂SO₄.

4.16

Ferrous sulfate, crystalline, FeSO₄.7H₂O.

4.17

Sulfuric acid, 0.05 M solution.

4.18

Octan-l-ol.

4.19

Potassium carbonate, saturated solution.

4.20

Sodium or potassium hydroxide, 0.1 M solution, free from carbonate.

4.21

Barium hydroxide, saturated solution.

4.22

Sodium carbonate solution, 10 g per 100 ml.

4.23

Hydrochloric acid, 2 M solution.

4.24

Hydrochloric acid, 0.1 M solution.

4.25

Urease solution:

Suspend 0.5 g of active urease in 100 ml of distilled water.
Using 0.1 M hydrochloric acid (4.24), adjust the pH to 5.4, measured by pH meter.

4.26

Xanthydrol solution, 5 g per 100 ml in ethanol or methanol (4.31) (do not use products giving a high proportion of insoluble matter). The solution may be kept for three months in a well-stoppered bottle, away from the light.

4.27

Copper oxide (CuO): 0.3 to 0.4 g per determination or an equivalent quantity of copper sulfate pentahydrate (0.95 to 1.25 g) per determination.

4.28

Anti-bump granules washed in hydrochloric acid and ignited.

4.29

Indicator solutions:

4.29.1

Mixed indicator solution:

Solution A: dissolve 1 g methyl red in 37 ml 0.1 M sodium hydroxide solution and make up to one litre with water.
Solution B: dissolve 1 g methylene blue in water and make up to 1 litre. Mix 1 volume of solution A with 2 volumes of solution 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.

4.29.2

Methyl red indicator solution:

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.

4.30

Indicator papers:

Litmus, bromothymol blue (or other papers sensitive in the range pH 6 to 8).

4.31

Ethanol or methanol: solution 95%.