250 Gms. of commercial sulphanilic acid are dissolved in sufficient sodium carbonate to give a strongly alkaline solution, which is boiled until all aniline has disappeared. The volume is about 1 litre. The solution is now filtered and made strongly acid by means of hydrochloric acid. After standing 12 hours the product is filtered off, washed with a little water, and the crystals dissolved to a neutral solution in 400 c.cs. water and a sufficient quantity of soda (about 60 gms.) The hot solution is cooled down to o° with continuous stirring and the sodium sulphanilate is filtered off. If a small centrifuge is available, the mother-liquor is "whizzed " off. The crystals are dissolved in 500 c.cs. of distilled water, the solution filtered, and then acidified with pure concentrated hydrochloric acid. The liquid is kept well stirred during the addition in order to ensure the formation of small crystals; next day the precipitate is filtered off and is then washed with a little distilled water until the sodium chloride is removed. The purified crystals are again recrystallized from boiling distilled water and are then dried in an air oven at 1200 until of constant weight. The product is kept in a bottle having a well-fitting ground stopper. The sulphanilic acid so obtained is practically white and contains less than 0.01 % of impurities. Exactly 173 gms. are dissolved in 100 c.cs. pure ammonia (20 % Nh3), and it is then made up to 1 litre at 17.5°. Such a solution will remain unchanged in the dark for many months, but should be carefully recontrolled at intervals of 3 months.

This standard solution serves for the preparation of Normal sodium nitrite solution.