P-Toluidine salts, of sulphonic acids

Dissolve 1 g. of the sulphonic acid or its sodium salt in the minimum volume of boiling water and add a saturated aqueous solution of 1 g. of p-toluidine hydrochloride. Cool, filter off the precipitate of the p-toluidine salt, and recrystallise it from hot water or from dilute ethanol. 

The yellow crust is then finely ground and extracted with 95% alcohol. This dissolves the toluidine, thiotoluidine, and the dehydrothiotoluidine, leaving the insoluble primuline base (p. 389). The extract is evaporated to dryness and heated to 250°, which removes the toluidine and part of the thiotoluidine. The mixture is then sulphonated with 25% oleum and poured on to ice, filtered, and well washed with water until the washings give only a faint acid reaction. The toluidine and thiotoluidine sulphonic acids pass into solution. The residue is dissolved in 50 gms. of 20% ammonia solution and 800 c.cs. water, and heated to 80°, other 100 c.cs. of water being then added. The solution is filtered hot, if necessary, and the ammonium salt of dehydrothiotoluidine sulphonic acid separates out in the course of 2 days. Primuline can be obtained from the mother liquor by saturating with common salt at the boiling point. 

These are prepared by the interaction of the sulphonic acid or its sodium salt with p-toluidine hydrochloride in aqueous solution ... 

Dissolve 1.0 g of the alkali metal salt of the sulphonic acid in the minimum volume of hot water and add 0.5 g of p-toluidine and 2 ml of concentrated hydrochloric acid. If a solid separates or the p-toluidine does not dissolve completely, add more hot water and a few drops of concentrated hydrochloric acid until a clear solution is obtained at the boiling point Cool the solution if crystallisation does not occur immediately, scratch the walls of the test tube to induce crystallisation. Collect the product by suction filtration, and recrystallise it from hot water containing a drop of concentrated hydrochloric acid, or from dilute ethanol. 

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