Two different determinations are always made. First of all the amount of nitrite required is measured and this figure is termed the "Nitrite figure." Then the amount of diazonium solution needed is determined, this figure being known as the "Coupling figure" If both figures agree we know that the melt has been done correctly, but if the nitrite figure is too high, we may conclude that the melt has been too short, whilst if it is less than the coupling figure the temperature of the melt was too high. A properly prepared aminonaphthol sulphonic acid should give nitrite and coupling figures which agree to within less than 1 per cent.

It is hardly necessary to add that all these estimations, as in all cases of quantitative analysis, should be done in duplicate.

Aminonaphthol disulphonic acid 1:8:3:6: (H-acid).

(a) Nitrite figure (calculated upon the acid sodium salt, mol. 341): 3.41 gms. H-acid are dissolved in 5 c.cs. of 10 % sodium carbonate solution, diluted to 250 c.cs., precipitated with 25 c.cs. concentrated hydrochloric acid and diazotized at 50 with normal nitrite. The H-acid should give a fine yellow diazo compound which separates in beautiful crystals on salting out. The number of c.cs. used multiplied by 10 gives the percentage.

(b) Coupling figure: 3.41 gms. H-acid are dissolved in 50 c.cs. of 10 % sodium carbonate solution, which are then diluted to 300 c.cs.; and N/10 diazobenzene solution is then added until a slight excess is present. To test this a little heap of salt is placed upon a piece of filter-paper, and to it is added a few drops of the red solution. After waiting five minutes the colourless rim is touched with the diazotized aniline solution. If H-acid is still present a red rim is at once produced. If diazo solution is in excess a drop of H-acid solution will also give a red rim. The last portions of H-acid often separate out but slowly from the dye so that towards the end it is necessary to wait for a quarter of an hour. At the very end there is always a more or less strong after-coupling. The purer the H-acid the fainter this after-effect. The nitrite figure for a good H-acid is about 0.3 % higher than the coupling figure. The number of c.cs. of aniline solution used gives the percentage composition.

All the aminonaphthol disulphonic acids and monosulphonic acids are determined in this way. The diazonium solution is placed in a 100 c.c. measuring cylinder, and the percentage read off directly. Many works use ice-jacketed burettes which are very neat, but somewhat complicated. For stirring, a stirring rod is used, the end of which is bent round in a big loop. The coupling is carried out in a clean porcelain dish.

Estimation of Naphthol Sulphonic Acids, Disulphonic Acids, and of Dihydroxy Naphthalene Mono- and Di-Sulphonic Acids.

Example: Neville and Winther's Acid (=Naphthol sulphonic acid

1:4). M.w. 224.

The acid is coupled with N/10 aniline solution exactly as described for H-acid, the dye being salted out in the dish towards the end of the reaction so that it becomes easy to determine the remainder of the acid. Using 2.24 gms. of the acid the number of c.cs. of aniline solution used up gives directly the percentage of H-acid. The coupling is effected at 0°.

Schaffer-salt, R-salt, and other naphthol sulphonic acids are also estimated in the same way. Sultones, however, must first be hydrolysed by treatment with a little hot caustic soda.

Dihydroxynaphthalene mono- and di-sulphonic acids couple up so rapidly, even as regards the second coupling, that the reaction is effected in acetic acid solution in presence of sodium acetate, by means of aniline or the more energetic p-aminoacetanilide. In many cases the coupling takes several hours, e.g. with dihydroxynaphthalene disulphonic acid 1:8:3:6 (chromotrope acid), and, in addition, the resultant colouring matter separates out from the unchanged chromotrope acid only slowly owing to its great solubility, so that great care must be taken.

It is sometimes possible to estimate various sulphonic acids in presence of one another, but the results obtained are rarely accurate.

For instance, Schaffer salt (sodium naphthol sulphonate 2:6) can be determined fairly accurately in presence of R-salt (sodium naphthol disulphonate 2:3:6) in the following manner: first of all the total amount of material capable of being coupled up is estimated with "aniline solution." Another portion is then dissolved in as small a quantity of water as possible, and to it is then added twenty times its volume of 96 % alcohol. The R-salt is precipitated and the residue can be analysed for its Schaffer content whilst the disulphonic acid can be estimated in the extract. It is necessary to shake up the precipitate with the alcohol (after mixing) for half an hour as otherwise too much Schaffer salt is occluded.

An alternative method consists in first estimating the total by means of aniline solution and then removing the Schaffer salt from a second portion by means of formaldehyde. For example, 5 gms. of the mixture are dissolved in 100 c.cs. water, 5 c.cs. pure 30 % hydrochloric acid are added and 2.5 c.cs. 40 % formaldehyde, and the mixture heated on the water-bath for an hour, after which the disulphonic acid is determined. The difference between the two figures gives the content of mono-sulphonic acid.

Yet a third method may be noted, the Iodine method, which depends upon the following fact. Iodine reacts with R-salt, and also with Schaffer salt, preferably in the presence of sodium bicarbonate. The mixture is first titrated directly with N/10 iodine solution, using an excess of this, and then titrating back. Another sample is then separated by means of alcohol, as noted above, and the extract titrated again. This method is believed by the Elberfeld works to be the best, as the coupling method gives too high results, and so far as can be ascertained this contention appears to be correct.