17.3 Gms. (1/10 mol.) 100 % sulphanilic acid are dissolved in 100 c.cs. of water with the aid of 5.5 gms. soda.1 25 C.cs. hydrochloric acid are added and the whole diazotized with 35 c.cs. 20 % sodium nitrite solution with good stirring. The diazotization takes about 10 minutes, and the temperature may be allowed to reach 150.

Diazo compounds which contain a sulphonic group are usually sparingly soluble and come down as their internal anhydrides in the form of white or yellow crystalline precipitates. As, in addition, many amino sulphonic acids are also sparingly soluble, it is necessary to diazotize them indirectly. For this purpose the sodium salt of the sulphonic acid is mixed with the necessary quantity of the sodium nitrite and the mixture is then poured into the acid.

Further difficulties are caused by the fact that certain amines couple with themselves, for instance, Cleve acid, and for these an excess of about 5 % sodium nitrite is necessary.