(a) Direct Estimation.

The method consists in titrating the amine in very dilute solution with hydrochloric acid and sodium nitrite, the resultant diazonium solution being then coupled up with an exactly determined amount of a phenol, usually Schaffer salt, the diazotization being thus controlled. In the cases of H-acid, Amido-R-salt, etc., one portion is diazotized whilst another sample is coupled up with aniline or other component. Sometimes it is possible to estimate two substances in the presence of one another if one reacts much more rapidly than the other. For instance, with a little practice it is quite possible to estimate G-salt and R-salt side by side with a fair degree of accuracy, as R-salt couples very rapidly and gives a red dye with aniline, whilst G-salt couples afterwards and forms a yellow dye. In addition to these methods there are a number of special methods which make it possible to estimate the various components in a mixture. Such methods have been noticed when discussing mixtures of R- and G-salts (cf. p. 233).

Certain other diazo components are used occasionally in place of benzene diazonium chloride; thus, many works prefer to use m-xylidine in place of aniline, but there is very little point in so doing, as its solution is less stable than the aniline solution. Again, in some cases p-aminoacetanilide is made use of as it couples up rather more energetically and is quite reasonably stable (vide Chromotrope acid); ortho- and para-nitranilines are rarely used.

5 Gms. sodium carbonate are required for each gm. nitrite or, if the coupling is carried out in acetic acid solution, at least 15 gms. sodium acetate are added, or twice this amount in the case of the nitranilines. Should the substances contain sulphonic groups, still more sodium carbonate or acetate will be required. The temperature for the coupling should not at any time exceed 50, and the solution should be very dilute (about 1 %).

The excess of diazonium salt is determined by spotting on filter-paper, easily soluble dyes being first salted out. As a suitable reagent for amines or easily-coupling phenols such as resorcinol, a solution of R-salt or H-acid is used. Some factories make use of a freshly prepared solution of hydrocyanic acid which gives a yellow coloration. The presence of excess of the phenol or amine which is being determined is ascertained by spotting on filter-paper with the diazonium solution. The loss so caused is so small that it may be neglected.

(b) Indirect Determination.

In the case of certain amines it is not possible to estimate them by direct diazotization owing either to their forming diazoamino compounds or diazonium salts which blacken starch-iodide paper in the same way as free nitrous acid. Amines of this type, e.g. nitranilines, chloranilines, etc., must therefore be estimated indirectly. Thus, 1/100 mol. of the amine in question is dissolved either in concentrated or slightly diluted hydrochloric acid, poured into water and ice and diazotized with a fair excess of sodium nitrite. The clear diazonium solution is poured into a measuring flask, made up to a known volume, and is then run in from a burette or measuring cylinder to a strongly alkaline β-naphthol solution of known strength until, on testing on filter-paper, no naphthol can be detected with diazonium solution on the edge of the spot. Usually the quantities are so chosen that the number of c.cs. used divided into 100 gives the percentage of amine sought for.

Example: 3.45 gms. p-nitraniline (=2.5/100 mol.) are dissolved in 10 c.cs. hydrochloric acid of 30 % strength, and 10 c.cs. water. The clear solution is poured on to 50 gms. ice and 50 c.cs. water, and is then treated with a solution of 2 gms. 100 % sodium nitrite in the form of a 20 % solution. Owing to the slight excess of nitrous acid (about 0.2 gm. NaNo2), a clear solution is produced, which is made up to 250 c.cs. in a graduated flask. 100 C.cs. of this solution are measured out into a cylinder, and are then added by degrees with stirring to a solution of 1.44 gms. 100 % β-naphthol dissolved in 2 c.cs. of 30 % NaOH to which 20 gms. sodium carbonate in 300 c.cs. ice water have been added. By means of spotting tests the point is determined at which β-naphthol ceases to be indicated with diazonium solution on the rim of the spot. The number of c.cs. of nitraniline solution used divided into 100 (100/c.cs.) gives the percentage required. If the nitraniline is 100 % pure, then exactly 100 c.cs. will be required. Usually 101-102 c.cs. are needed.