23 октября, 2015 
Pokraskin In a 1-liter, five-necked flask, fitted with a stirrer, preferably of the paddle type, operating through a mercury seal in the center neck, a dropping funnel, a thermometer, a gas inlet tube, and a gas outlet tube, 72 grams (0.5 mole) of dry Д-naphthol is treated with 280 grams of hot (100°C.) completely dry nitrobenzene. (The simplest method for drying nitrobenzene is to distill and discard the first fraction which contains all of the water.) The resulting completely clear solution is allowed to cool with stirring, part of the naphthol crystallizing out. The mixture is cooled in ice, and 35 cc. (62 grams) sulfuric acid-free chlorosulfonic acid is dropped in slowly, while the temperature is kept below 5°. The hydrogen chloride which is formed is passed through a calcium chloride tube into a suitable absorption apparatus. The ^-naphthol goes into solution rapidly, and toward the end of the reaction part of the naph — tholsulfonic acid precipitates. When all of the chlorosulfonic acid has been added, the remaining dissolved HC1 is swept out by means of a stream of dry air.
Chlorosulfonic acid takes up water readily, decomposing into sulfuric and hydrochloric acids. Therefore, it must be kept in containers having absolutely tight ground glass stoppers. Ordinary glass stoppers are not usually sufficiently tight for this purpose. If the chlorosulfonic acid is not known to be of the highest quality, it should be distilled before use, carrying out the distillation in the complete absence of moisture.
The nitrobenzene solution, together with the suspended solid material, is shaken with a mixture of 300 cc. water and 100 grams of ice, then twice with 100-cc. portions of cold water. The combined aqueous extracts are filtered through a moistened paper and then saturated with 200 grams of salt, whereupon sodium 2-naphthol-l-sulfonate is precipitated in colorless plates. Stirring is continued until all of the salt is dissolved, and the mixture is allowed to stand overnight, then filtered. The product is pressed out and washed twice with saturated salt solution. The filter cake is used directly for the preparation of 2-naphthyl- amine-l-sulfonic acid after tests have shown that it does not fluoresce in soda solution and does not yield a water soluble, orange red dye with diazotized aniline and thus does not contain isomeric naphtholsul — fonic acids.
If it is desired to isolate the sodium sulfonate in a pure state, the filter cake is dissolved in a small amount of cold water, the solution is extracted with several portions of benzene to remove nitrobenzene, then filtered and the product again precipitated with salt, filtered off, washed with saturated salt solution, pressed out, and dried. This yields a material which is pure except for the presence of salt, which can be removed if desired by crystallization from alcohol.
The product is analyzed by boiling a weighed sample under reflux for 2 hours with 10 parts of 20 per cent sulfuric acid. The sulfo group is split off by this treatment. The resulting 0-naphthol is dissolved by adding dilute sodium hydroxide and titrated with 0.1 N diazobenzene solution.
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