In the previous pages we have used for brevity the expression "indigo indigo-white" where we have intended one or another of

These will be discussed separately, and it will be found that henceforth they must be sharply distinguished.

PREPARATION OF THE SULPHONATES OF INDIGO.1

Commercial samples of synthetic and natural (Bengal) indigo which we tested were found to contain impurities as described by Perkins and Bloxam (1907) and Perkins (1907). Purification by sublimation, first used by Crum (1823) was employed by Dumas (1834) in his study of the constitution of indigo and by von Sommaruga (1879) in determining the molecular weight. Bloxam (1905) especially recommends it, but, since we were to depend largely upon recrystallization of the sulphonates, a preliminary purification of indigo was carried out by the possibly less satisfactory method of extraction. The finely powdered, crude indigo was extracted successively with hot ethanol, cold acetic acid, and, again, with hot ethanol, in both of which solvents indigo red and indigo brown are somewhat soluble. The final ethanol extract was no longer reddish, but had a blue tinge. The material was then dried, reground, and washed with ten times its weight of 10 per cent hydrochloric acid solution first hot and then cold-to extract the so-called indigo gluten and mineral material. It was finally washed with hot water, dried first at 60° C., and finally at 110° C., reground, and bottled. By nitrogen analysis a synthetic indigo was found to have been raised from 94 to 98 per cent purity by this method. By the same criterion, Bloxam's (1905) purest preparation was about 98 per cent indigo. Such was the stock from which most of our sulphonates were prepared.

Bloxam (1906) has detailed the preparation of the sulphonates, and we have followed his descriptions, in the main.

The sulphonates are formed by the action of sulphuric acid. Dependent upon temperature, time of action, and the amount and

Many authors have distinguished the chief constituent of crude indigo by the term "indigotin" or "Indigotine." However, this term has been loosely used or made synonymous with "indigo" by otherWise careful writers. That constituent of crude indigo which is regarded as the important constituent and as a chemical individual has been referred to as indigo from the beginning of chemistry. We shall retain this term.

strength of sulphuric acid, there are formed successively the mono, di-, tri-, and tetra-sulphonic acids. Room temperature and 10 to 20 parts of ordinary sulphuric acid to 1 of indigo are conditions yielding the monosulphonic acid. Warming under the above conditions. leads to the disulphonic acid, indigo carmine. Moderate warming with sulphuric acid containing 15 per cent excess sulphur trioxide gives the trisulphonic acid; while the tetra sulphonic acid is obtained at high temperatures and with fuming sulphuric acid containing 20 per cent excess sulphur trioxide.

As the degree of sulphonation increases, the practically insoluble indigo is converted into products of progressively higher solubility. Consequently, in the crystallization of the salts the tendency is for the contaminant to be one or more of the lower sulphonates. The aim in sulphonation should be, therefore, to carry the reaction as completely as possible to the stage desired without oversulphonation.

Potassium indigo monosulphonate was prepared as follows: One part indigo and 20 parts concentrated sulphuric acid were thoroughly mixed and allowed to stand one hour at room temperature. The mixture was frequently stirred and the progress of the reaction tested by transferring a drop of the mixture into water. When a violet solution in hot water was obtained, the whole mass was mixed with enough water to dilute the sulphonic acid approximately ten times. The diluted mixture was allowed to stand several hours and the mono sulphonic acid settled out. The precipitate was filtered with suction on a Büchner funnel and was washed with a little water. The filtrate, which may have contained some disulphonic acid, was discarded.

The reddish precipitate was dissolved in boiling water (approximately 300 cubic centimeters of water for each gram of original indigo used) and filtered. To the hot filtrate, powdered potassium carbonate was added, a little at a time, with stirring, until a precipitate began to occur. Then about 100 cubic centimeters of a 50 per cent solution of potassium acetate were added and the whole was cooled first in water and then in ice to 5°. A precipitate of potassium indigo monosulphonate formed. This salt was not definitely crystalline. It was purified by dissolving in boiling water and precipitating with the addition of a few cubic centimeters of the potassium acetate solution while chilling in ice and salt. This process was repeated. Then followed two crystallizations from water without addition of potassium acetate. Finally the sulphonate was filtered on hardened paper and washed with a little water and with ethanol to remove traces of potassium acetate. The compound was heated in an oven at 60° for two hours and at 80° for one hour, to free it from ethanol. It was then heated at 100° for a short time until it separated from the hardened filter paper.

Two sets of the potassium monosulphonate were made; preparation I was on a small preliminary scale; preparation 5 on a large scale. The analyses of the samples are given in Table I.

Since preparation 5 was made from an unpurified synthetic indigo, it was thought possible that it might contain some indigo red monosulphonate. Accordingly it was ground with ethanol in which indigo red sulphonates are soluble. Preparation 5 was slightly soluble in 95 per cent ethanol but with a blue color. After treatment with ethanol the insoluble part was recrystallized from water and labeled 5 A.

Potassium indigo disulphonate was prepared as follows: One part indigo and 20 parts concentrated sulphuric acid were heated on a water bath one and one-half to two hours with frequent stirring. The product was poured into water, 200 parts, and the mixture stirred and filtered hot. After cooling to about 15° to allow any monosulphonic acid present to settle out, the solution was filtered again. Practically no insoluble matter was present. The filtered solution was then treated with solid potassium carbonate, a little at a time, with stirring, until the potassium salt of the dye showed evidence of precipitating out. About 100 c. c. of a filtered 50 per cent solution of potassium acetate were then added, and the mixture was brought to boiling and filtered hot. Cooling first in water and then to 5° in ice for an hour gave a precipitate of blue needles, more or less contaminated with potassium sulphate. The precipitate was filtered on a Büchner funnel. If the filtrate happened to be a deep blue, a little of the potassium acetate solution was added and the mixture warmed with stirring and chilled as before until more disulphonate began to crystallize out. This second precipitate was filtered on the same Büchner funnel, washed with potassium acetate to wash out sulphate and with ethanol to wash out the acetate. The precipitate was purified by taking up in a minimum amount of boiling water, filtering hot, cooling in ice, and filtering

the precipitate. This process of solution and precipitation was repeated two or three times. The potassium indigo disulphonate crystallized in bunches of needles. The dry salt is purplish blue.

Preparations numbered 2, 6, and 17 were made by this method. Preparation 2 was a preliminary sample on a small scale; preparation 6 was made on a large scale, using an unpurified synthetic indigo; preparation 17 was made from purified indigo after the methods of making and purifying the compound were well understood, and was the best of the three. The results of analysis are given in Table I. Preparation No. 20 was made in a somewhat different way from the other disulphonates, so the method is given in detail. This preparation was intended to be a trisulphonate, but by an error the fuming sulphuric acid was added in half the quantity intended, giving about 10 per cent excess sulphur trioxide instead of 15 per cent. The preparation was made as follows: Sixty grams of the purified synthetic indigo were mixed with 210 grams of concentrated sulphuric acid and 195 grams of sulphuric acid containing 23 per cent excess sulphur trioxide. The stirred mixture was warmed between 40° and 50° for three hours, with the lower temperature predominating, and was then poured slowly into 6,000 c. c. of water. solution was cooled and filtered from a little insoluble matter. filtrate was treated with potassium carbonate and then with a 50 per cent solution of potassium acetate until heavy precipitation began. It was then boiled, chilled in ice, and filtered, giving precipitate 1. The filtrate from precipitate 1 was treated with more potassium acetate and the mixture boiled, chilled, filtered, and the precipitate 1-a combined with precipitate 1.

The

The

The combined precipitate and paper were placed in 4,000 c. c. of boiling distilled water, stirred and filtered. The filtered solution was then chilled in ice for one hour. Good precipitation occurred, suggestive of the disulphonate in crystalline appearance and in the color of the solution in water. The filtrate from precipitate 2, treated with 50 c. c. of potassium acetate and chilled, gave precipitate 3. Precipitates 2 and 3 were filtered separately and the filtrates discarded. The precipitates 2 and 3 combined were dissolved in 3,000 c. c. of boiling water, cooled, chilled, stirred up with 200 c. c. of 95 per cent ethanol to settle the precipitate and to free it from any potassium acetate that might be present. The filtered precipitate was precipitate 4.

Precipitate 4 was dissolved in 2,000 c. c. of boiling water, cooled, chilled in ice two hours, filtered, and washed with 100 c. c. of 50 per cent ethanol, dried by suction one hour at 60°, ground and bottled as preparation 20. It contained 8.82 per cent moisture, determined by starting the oven at 60° and keeping the sample at 120-122° for

three and one-half hours. On the basis of dry weight, preparation 20 gave the analyses shown in Table I.

Potassium indigo trisulphonate was prepared as follows: Fifty grams of purified indigo were treated with 500 c. c. of sulphuric acid containing 15 per cent excess sulphur trioxide. The mixture on the water bath was kept between 45° and 55° for three and one-half hours, with frequent stirring. The mixture was poured into water, made up to 5.000 c. c., and filtered. The filtrate was partly neutralized with solid potassium carbonate, a little at a time, with stirring, and then treated with a 50 per cent solution of potassium acetate until a heavy precipitate formed in the still acid mixture. The mixture was then brought to boiling to put the precipitate into solution and was filtered hot. The filtrate, cooled in water and then in ice for an hour, gave a heavy precipitate of dye and inorganic potassium salts. The precipitate was filtered off, and the filtrate again chilled. A second precipitate occurred, which was filtered with the first on Büchner funnels.

This material was dissolved in boiling water and the solution filtered from some insoluble material. The crystals obtained by cooling were filtered and dissolved in 2,000 c. c. of boiling water. When chilled, this solution gave precipitate 3. This, after filtration, was dissolved in 1,000 c. c. boiling water. On cooling, the precipitate which formed was stirred with 200 c. c. 90 per cent ethanol to extract acetate. The mixture was filtered on hardened paper and washed with 100 c. c. 90 per cent ethanol. It was partially dried with suction, pressed between several hardened filter papers, dried first at 60° one hour, then at 70°, and finally ground, and bottled. This was preparation 23, which, after drying at 120°, was found to contain 13.11 per cent moisture. The analyses are shown in Table I.

Other preparations of potassium trisulphonate, preparations numbered 3 and 7, were not as satisfactory as number 23. Sample No. 3 was made on a small scale with indigo treated at 50° with sulphuric acid containing 20 per cent excess sulphur trioxide and proved to be largely tetrasulphonate, after the preliminary measurements as of a trisulphonate had been reported (Sullivan and Clark 1921). Sample No. 7 erred on the other side in containing more or less disulphonate.

Potassium indigo tetrasulphonate was prepared as follows: Forty grams of purified indigo were stirred into a paste with 75 grams of concentrated sulphuric acid and 250 grams of sulphuric acid containing 50 per cent excess sulphur trioxide. The mixture was heated at 110° for 40 minutes, with frequent stirring. The mixture was allowed to cool to 50° and was then poured into 5 liters of water. While still warm the purple solution was filtered. The solution was treated with solid potassium carbonate, with stirring, until there was