4.—4.1 Concentrated nitric acid (d = 1.40 g/ml).
4.2 Molybdate reagent:
4.2.1Preparation of the reagent based on sodium molybdate:
Solution A: dissolve 70 g sodium molybdate dihydrate in 100 ml water.
Solution B: dissolve 60 g citric acid monohydrate in 100 ml water and add 85 ml concentrated nitric acid (4.1).
Solution C: stir solution A into solution B to obtain solution C.
Solution D: to 50 ml water add 35 ml concentrated nitric acid (4.1), add 5 ml freshly distilled quinoline. Add this solution to solution C, mix thoroughly and leave standing overnight in the dark. Make up to 500 ml with water, mix again and filter through a sintered glass funnel (5.3).
4.2.2Preparation of the reagent based on ammonium molybdate:
Solution A: dissolve 100 g ammonium molybdate in 300 ml water, heating gently and stirring from time to time.
Solution B: dissolve 120 g citric acid monohydrate in 200 ml water and add 170 ml of concentrated nitric acid (4.1).
Solution C: add 10 ml freshly distilled quinoline to 70 ml of concentrated nitric acid (4.1).
Solution D: slowly pour, stirring well, solution A into solution B. After thoroughly mixing, add solution C to this mixture and make up to 1 litre with water. Leave standing for two days in a dark place and filter through a sintered glass funnel (5.3).
The reagents 4.2.1 and 4.2.2 can be used in the same way; both must be kept in the dark in stoppered polyethylene bottles.
