REAGENT
4.—4.1 2% citric acid solution (20 g per litre), prepared from citrci acid and monohydrate.
Note: Verify the concentration of this citric acid solution by titrating 10 ml of the latter with a sodium hydroxide standard solution 0.1 N, using phenolpthalein as an indicator. If the solution is correct, the titre should be 28.55 ml.