22 октября, 2015 
Pokraskin To 250 grams of concentrated sulfuric acid (66° Вё) is added, with stirring, 144 grams (1.0 mole) of finely powdered, pure /ї-naphthol. The temperature rises to about 30°C. The mixture is heated for 8 hours in a boiling water bath, and then poured while still warm into 1 liter water. The diluted solution is held at 95-100° with stirring for about 1 hour. If necessary, water is added so that the total volume does not become less than 1 liter. An additional 500 cc. water and 100 grams of anhydrous sodium sulfate (or the corresponding amount of the hydrated salt) are then added, and the solution is neutralized at about 90° by the addition of chalk (about 190 grams is required). The chalk must be added very carefully in small portions, particularly toward the end of the neutralization, in order to prevent foaming over. The mixture is finally boiled and filtered hot to remove the calcium sulfate which is washed carefully with hot water until a portion of the washings, treated with soda, gives only a weak coloration with diazotized aniline. The combined filtrate and washings are heated to boiling and freed from residual calcium compounds by the addition of enough anhydrous soda (about 30 grams) to make the solution barely alkaline to phenolphthalein. (Complete removal of the calcium is necessary only if it is important that the product dissolve to form a clear solution even in the presence of soda.) At this point, a filtered test portion of the filtrate should give no further precipitation when more soda is added. The precipitated calcium carbonate is filtered off and washed with hot water, and the filtrate is made just acid with concentrated hydrochloric acid (about 40 cc.) and evaporated to 1 liter. The concentrated filtrate is cooled with stirring, during which the volume decreases still more by evaporation; after cooling, the volume should be 800 cc.
The mixture is allowed to stand overnight, and the resulting thick
paste is filtered by suction. In this filtration, the solid must be pressed out thoroughly, using a pestle or the flat surface of a glass stopper, to prevent the formation of cracks or perforations in the filter cake and to ensure as complete removal of the mother liquor as possible. When no more drops of filtrate are formed, the vacuum is broken and 50 cc. cold water is poured into the funnel. Again, care is taken that no canals are formed through which water can pass without effective washing, and that the wash water works its way uniformly through the precipitate. Vacuum is reapplied, slowly at first and then as fully as possible, and the solid is pressed out thoroughly again. Washing with 50-cc. portions of cold water is repeated twice more. If the operations are carried out properly, the final precipitate contains such a small quantity of liquid that pressing in a screw press can be omitted unless large quantites are being worked with. The product, after drying in a steam heated oven, consists of 150-155 grams of technically pure sodium 2-naphthol — 6-sulfonate, 1 gram of which uses 36.5 to 37 cc. 0.1 N diazo solution.
The yield is about 56 per cent of the theoretical amount, and the product is usable for most purposes without further treatment. The last traces of disulfonic acids can be removed, if desired, by recrystallization from 3 to 4 parts of hot water.
The combined filtrate and wash water from the monosulfonate are evaporated to a volume of 300 cc., and 40 grams of salt is added to the hot concentrated solution. The mixture is allowed to stand overnight, and the resulting precipitate is filtered off and washed three times with 20-cc. portions of a half-saturated salt solution (mixture of equal parts of saturated salt solution and water), following the same filtering procedure used with the first precipitate. The product, after being dried in a steam heated oven, consists of 50 to 55 grams of crude R salt, of which 1 gram corresponds to 23 to 26 cc. 0.1 N diazonium solution. Thus, the yield is 12.5 to 13 per cent of the theoretical amount. The purification of this product is discussed later.
The filtrate from the R salt is heated to boiling and 60 grams of potassium chloride is added. The solution is cooled and allowed to stand overnight. The copious precipitate is filtered off and washed, as described above, with 10 per cent potassium chloride solution. The product, dried in a steam heated oven, weighs 40 to 45 grams, and consists chiefly of G salt. One gram uses 23 to 25 cc. 0.1 N p-nitrodiazo — benzene solution. The yield is about 10 per cent of the theoretical amount.
The last mother liquor still contains about 12 per cent of the starting material in the form of sulfonic acids which can be titrated with p-nitrodiazobenzene solution. Besides residual Schaeffer salt, R salt, and G salt, other isomers are present which have not been investigated.
The crude R salt obtained in the above procedure always contains some Schaeffer «alt. The amount of the impurity should not be more than 5 per cent if the work has been done carefully. The crude R salt is usable as such for many purposes, but, if desired, a purified material can be obtained in the following way.
50 grams of the crude product is dissolved in 250 cc. boiling water and 25 grams of salt is added. The solution is then filtered to remove a slight turbidity and placed in the ice chest overnight. The resulting precipitate is filtered off with suction and washed with three portions, totaling 50 cc., of a mixture of 1 part of saturated salt solution and 3 parts of water. In this way, 65 to 70 per cent of the crude product is obtained as pure R salt if the crude product did not contain more than 5 per cent of the other sulfonic acids. In other cases, the purification must be repeated.
The filtrate is evaporated to half its original volume (i. e., to 125 cc.). On cooling, about 25 per cent of the original crude material crystallizes out. This second fraction is mainly Schaeffer salt which can be obtained in a pure state by recrystallizing from 4 parts of water. The mother liquor from the second fraction gives a precipitated mixture of Schaeffer and R salts on addition of salt. This material can be separated into its constituents by repeating the operations described.
The fraction obtained by precipitation with potassium chloride is G salt which contains 5 per cent, at the most, of R salt as an impurity. In the laboratory, this crude G salt can be purified by recrystallization from water. The method used in the plant is based on the fact that G salt couples with diazonium compounds much less readily that either R salt or Schaeffer salt. A test is made to determine the content of easily coupling sulfonic acids by titrating a sample with diazobenzene or diazoxylene in dilute solution. The crude G salt is then treated with the correct amount, as indicated by the titration, of a slowly reacting diazo compound, usually diazoxylene. Under these conditions, only the impurities couple, leaving a solution of pure R salt after the dye which is formed is removed. The G salt solution can be used directly in the preparation of azo dyes. The dye from the impurities is sold under the name Ponceau.
The products can be tested for purity by determining their fluorescence and their behavior with diazotized p-aminoacetanilide (acet-p-phenylenediamine). Pure Schaeffer salt in a neutral solution exhibits no preceptible fluorescence; its alkaline solutions have a violet-blue fluorescence of low intensity. R salt and G salt both show greenish blue fluorescence, weak in neutral solution, but very intense in alkaline solution. A slight contamination of Schaeffer salt by either R or G salt is sufficient to mask the fluorescence of the Schaeffer salt. When the products are colorless, the presence of small amounts of impurities in Schaeffer salt can be detected in this way, and by comparison with known mixtures the amount of contamination can be estimated. When the crude products are strongly colored, however, the sensitivity and accuracy of this test are greatly reduced.
Schaeffer salt reacts with diazotized p-aminoacetanilide to give an easily soluble orange red dye which is precipitated only by the addition of a large amount of salt. The dye from the same diazo compound and R salt, on the other hand, is almost completely precipitated, by very small amounts of salt, in the form of lustrous bronzy crystals which appear bluish red by transmitted light; the filtrate has only a light bluish red color. The presence of Schaeffer salt is shown by a stronger, more yellowish color in the filtrate. In dilute solution, slightly alkaline with carbonate, G salt does not couple, or couples very slowly, with diazotized p-aminoacetanilide. Hence, in testing a sample of G salt, an immediate formation of a deep red coloration shows the presence of an impurity, usually Schaeffer or R salt. The presence of G salt in products consisting mainly of the other sulfonic acids is shown by the intense G salt fluorescence in the filtrate after the dye, formed from the product and diazotized p-aminoacetanilide, has been salted out and filtered off. This filtrate shows no further coupling reaction with diazotized p-aminoacetanilide, but does react to form dye with more energetic diazo compounds, e. g., that from p-nitroaniline.
The titration of Schaeffer salt and R salt is done most satisfactorily with diazotized p-aminoacetanilide in slightly alkaline solution (carbonate), that of G salt with diazotized p-nitroaniline in the presence of bicarbonate. In the case of mixtures, the difference between the two titrations gives a measure of the amount of G salt present.
The analysis of mixtures of Schaeffer salt and R salt is described in the Analytical Section.
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