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There is one other use for aniline yellow which dates from the year of its discovery, when Dale and Caro found that by adding sodium nitrite to aniline hydrochloride and heating the mixture, a blue colouring-matter is produced.

The same result can, in fact, be arrived at by heating dry aniline hydrochloride, i.e. the hydrochloric acid salt of aniline, with methyl alcohol or wood-spirit in strong metallic boilers under great pressure.

By a similar operation, using ethyl alcohol, or spirit of wine, diethylaniline can be obtained, and by heating dry aniline hydrochloride with aniline under similar conditions a crystalline base called diphenylamine is also prepared.

The diazotisation takes place in the manner shown in the equation:— C6H5NH2 + HCl  +        HCl            +        NaNO2 = Aniline hydrochloride, Hydrochloric acid, Sodium nitrite, NaCl    +      C6H5N:NCl        +      H20 Salt,        Diazo-benzene chloride,          Water.

An example of this important process is that of nitrous acid on aniline hydrochloride shown in the following equation:— C6H5NH2 + HCl + HNO2 2H2O + C6H5N:NCl Hydrochloric acid Nitrous Water, Diazo-benzene aniline, acid, chloride.