Лако-красочные материалы — производство

Examplet Nevile-Winther Acid (l-ISaphthol-4-sulfonic
Acid) (Molecular Weight 224)

Coupling is carried out with a 0.1 N diazotized aniline solution, just as prescribed for H acid, and at the end the dye is salted out in the re­action dish so that the remainder of the naphtholsulfonic acid is easily determined. Starting out with a 2.24-gram sample, the volume of diazo­tized aniline in cubic centimeters gives the per cent purity directly. Coupling should be carried out at 0°C.

Schaeffer salt, R salt, and other naphtholsulfonic acids are analyzed in exactly the same manner. Sultones, on the other hand, must first be split by hot sodium hydroxide.

Dihydroxynaphthalenemono — and disulfonic acids couple so rapidly, even the second time, that the coupling is carried out in acetic acid solu­tion containing sodium acetate, using diazotized p-aminoacetanilide. With many of the acids, the coupling requires several hours, as, for example, with chromotropic acid (l,8-dihydroxynaphthalene-3,6-di — sulfonic acid). In this case, also, the dye is highly soluble and separates slowly from the unreacted chromotropic acid so that considerable care must be exercised.

The determination of the individual sulfonic acids in a mixture can

be accomplished in some cases, although the results are rarely accurate. For example, mixtures containing Schaeffer salt (sodium 2-naphthol-6- sulfonate) and R salt (sodium 2-naphthol-3,6-disulfonic acid) can be analyzed in the following way. The total content of coupling substance is determined by titration with diazotized aniline solution. Another sample is dissolved in the minimum amount of water and treated with 20 parts of 96 per cent alcohol to precipitate the R salt. The mixture is shaken for about 30 minutes to free the Schaeffer salt enclosed in the precipitate, and then the precipitate and the filtrate are analyzed separately to determine R salt and Schaeffer salt, respectively.

A second method consists of first titrating the whole mixture with diazotized aniline, and then eliminating the Schaeffer salt in a second sample by means of formaldehyde. For example, 5 grams of the mixture is dissolved in 100 cc. water, and to the solution are added 5 cc. pure 30 per cent hydrochloric acid and 2.5 cc. 40 per cent formaldehyde. The mixture is heated for 1 hour on a water bath, and then reanalyzed, the difference between the two values representing the content of Schaef­fer salt.

A third method utilizes iodine titration. Both R salt and Schaeffer salt are iodinated by iodine, preferably in the presence of sodium bi­carbonate. A sample is titrated with 0.1 N iodine solution, adding an excess and back-titrating. A second sample is then separated into its constituents by alcohol, as described above, and a second determina­tion is made. Some dye chemists believe that this is the best method, since the coupling methods give values which are too high; this belief is probably correct.