Paragraph 4 REAGENTS

In a 1 litre graduated flask, dissolve 400 g of citric acid monohydrate in approximately 600 ml ammonia solution (ρ = 0.925 g/ml), containing 200 g NH₃ per litre; this may be prepared by diluting 760 ml ammonia solution (ρ = 0.880 g/ml) from a freshly opened bottle with water to 1 litre. The citric acid is added successively in quantities of 50 to 80 g maintaining the temperature below 50°C. Make up the volume to 1 litre with ammonia solution (ρ = 0.925 g/ml).

In a litre graduated flask, dissolve 432 g of diammonium citrate in 300 ml of water. Add 440 ml of ammonia solution (ρ = 0.925 g/ml) (see 4.1.1 above). Make up the volume to 1 litre with water.

Transfer a 10 ml sample of the citrate solution to a 250 ml flask. Make up the volume with distilled water. Determine the ammoniacal nitrogen content on 25 ml of this solution using Method 2. In these conditions the reagent is considered to be correct when the volume of 0.25 M sulfuric acid required is between 17.7 and 18.0 ml (1 ml 0.25 M H₂SO₄ = 0.008516 g NH₃). If the titre is too low add 4.25 ml of ammonia (ρ = 0.925 g/ml) per 0.1 ml below the 18 ml indicated above.