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Dyes and Dyeing
223. Preparation of an Azo Dye: Methyl Orange (Section 534). - Dissolve 10 grams of sulphanilic acid and 3 grams of anhydrous sodium carbonate in 150 cc. of water. To the solution first add 4 grams of sodium nitrite dissolved in 20 cc. of water, and then a mixture of 7 cc. of concentrated hydrochloric acid and 10 cc. of water. (Eq.) Dissolve 6 cc. of dimethylaniline in a mixture of 20 cc. of water and 10 cc. of concentrated hydrochloric acid. Cool the solution under running water, and pour it slowly and with stirring into the diazotized sulphanilic acid. (Eq.) Add a dilute solution of sodium hydroxide until the solution is just alkaline; this can be seen by the change in color. Add to the mixture 50 grams of salt dissolved in 150 cc. of water. In order to facilitate the separation of the dye, heat the mixture to boiling until gas-bubbles disappear. Allow the solution to cool to room temperature, and filter it with a large funnel, without the use of suction. When the liquid no longer passes through the filter-paper, transfer the precipitated dye, which is mixed with much water, to a funnel connected with a filter-bottle and pump, and remove the last part of the solvent by suction. It is well to proceed in the way described, and not to attempt to filter by suction at first; in this case the paper soon becomes clogged with the dye and filtration proceeds very slowly.
The salt may be purified by recrystallization from hot water (about 150 cc.). Filter the boiling solution in the way described in §10, page 6. The yield should be about 12 grams.
Dissolve a little methyl orange in water and add alternately to the solution dilute hydrochloric acid and dilute sodium hydroxide as long as a change in color is observed. (Eqs.)
Dissolve about 0.2 gram of methyl orange in about 10 cc. of dilute hydrochloric acid, add about 1 gram of zinc dust and boil the solution as long as any change takes place. Methyl orange, like other azo dyes, is reduced to colorless compounds by hydrogen in acid solution at the N:N bond:
(CH3)2NC6H4N : NC6H4SO3H + 4H = (CH3)2NC6H4NH2 + H2NC6H4SO3H
224. Preparation of a Triphenylmethane Dye: Malachite Green (Section 541). - Melt cautiously over a flame in a 6-inch evaporating dish 7 grams of fused zinc chloride, as long as bubbles are given off. Remove the flame and as the zinc chloride cools rotate the dish slowly so that when cold the chloride covers an area from 3 to 4 inches in diameter. Put into the dish 13 grams of dimethylaniline and 5 grams of freshly distilled benzaldehyde. Cover the dish with a watch-glass and heat on the steam-bath for 4 hours. Stir the contents frequently with a glass rod, which should be left in the mixture. Transfer the reaction-product to a flask; the part that can not be poured out of the evaporating dish can be treated repeatedly with hot water and thus removed. Distil with steam as long as any oil passes over. Discard the distillate. Pour the water out of the flask and dissolve the material sticking to the side of the flask in the smallest amount of boiling alcohol. Filter the solution and set it aside to crystallize over night. The leuco base which separates may form at first as an oil, but on standing changes into masses of small crystals. Filter off the solid, dry and weigh it. The yield of leuco base, C6H5CH[C6H4N(CH3)2]2, should be from 7 to 10 grams.
To convert the leuco base into the color base it is oxidized by lead dioxide. The dye is finally obtained as the double chloride of the color base and zinc chloride. Prepare the lead dioxide as follows: Dissolve 8 grams of lead acetate in 50 cc. of water. Mix thoroughly 20 grams of fresh bleaching powder with 300 cc. of water and filter. Heat the solution of lead acetate, and add the solution of bleaching powder slowly, keeping the solution just below boiling. When all the lead has been precipitated and the solid has changed to a deep brown color, stop heating and let the precipitate settle. Pour off the liquid and wash the solid three times by decantation, using about 500 cc. of water each time.
Dissolve 5 grams of the leuco base in a mixture of 5 cc. of concentrated hydrochloric acid and 10 cc. of water. Pour this into a large beaker containing 400 cc. of water and 100 grams of ice. Stir, and add slowly, taking about 5 minutes for the operation, the lead dioxide from which as much water as possible has been removed by decantation. Next add a solution of 5 grams of sodium sulphate in 25 cc. of water; this is to precipitate the lead which has gone into solution as chloride. Filter, and to the filtrate add a solution of 5 grams of zinc chloride in 10 cc. of water. To precipitate the dye add 50 grams of powdered salt, and stir until the latter has dissolved. Filter with a large funnel, and finally transfer the dye, which is mixed with much water, to a funnel connected with a pump, and remove most of the solution by suction. Wash once with a solution of salt. Transfer the dye to a porous plate. When quite dry, weigh the compound. Write equations, using graphic formulas, for all the reactions involved in the preparation. The yield should be 7 grams.
225. Preparation of a Phthalein: Fluorescein (Sections 543, 545). - Melt cautiously over a flame in a 6-inch evaporating dish, 5 grams of anhydrous zinc chloride, as long as bubbles are given off. When the zinc chloride solidifies, add 10 grams of resorcin and 7 grams of phthalic anhydride. Cover the dish with a watch-glass, place it on a wire gauze, and heat very carefully over a flame about 2 cm. high, the top of which is 2 or 3 cm. below the gauze. (Eq.) The heating should be at such a rate that the mixture produces bubbles slowly. In about 15 minutes, when the bubbles cease to be formed and the mass becomes stiff, stop the heating, and add to the dish, after it has cooled somewhat, 100 cc. of water and 20 cc. of concentrated hydrochloric acid. Heat the mixture to boiling, and break up with a glass rod the mass on the bottom of the dish. In a few minutes pour off the liquid through a filter and grind the solid with a pestle. Add 100 cc. of water and 20 cc. of concentrated hydrochloric acid, and boil again. Finally filter and wash with water. Let the fluorescein, the insoluble residue, dry in the air. Weigh the product. The yield should be about 12 grams.
Fluorescein can be crystallized by dissolving it in hot alcohol or in hot glacial acetic acid, and adding water.
Dissolve a trace of fluorescein in a dilute solution of sodium hydroxide and examine the solution by reflected and by transmitted light.
226. Preparation of Eosin (Section 545). - In a small flask cover 5 grams of fluorescein with 20 cc. of alcohol. Add slowly in small portions, taking 10 minutes for the operation, 4 cc. of bromine. Shake the flask frequently. When about one-half of the bromine has been added most of the fluorescein passes into solution as the dibrom-substitution-product; as the addition continues the tetrabrom-substitution-product crystallizes out. (Eq.) Allow the mixture to stand for an hour; filter and wash twice with a small amount of cold alcohol. The yield is about 7 grams.
Dissolve a little eosin in sodium hydroxide and examine the solution by transmitted and by reflected light.
Salts of eosin are used for dyeing. Prepare some of the ammonium salt as follows: Place in a desiccator about 25 cc. of concentrated ammonia. Place about a gram of powdered eosin on a small watch-glass and support it on a triangle in the desiccator. Cover the latter. At the next exercise test the solubility of the substance on the watch-glass; it if has been completely converted into the ammonium salt it will dissolve without a residue in water.
Dissolve some of the ammonium salt in a little water, and then use the solution as ink, with a new pen.
Dissolve about 0.1 gram of the salt in 200 cc. of water and dye a piece of silk by heating it with the solution for about 10 minutes.
227. Dyeing with a Substantive Dye: Congo (Sections 528-530, 534). - The cotton to be used in this and the experiments which follow should be heated for 10 minutes in a boiling solution of 1 gram of sodium carbonate in 500 cc. of water. Prepare in this way seven pieces of white cotton cloth about 6 inches square. Wash the cotton three times with water and let it stand under water until needed.
Dissolve 0.1 gram of Congo and 0.1 gram of sodium carbonate in 500 cc. of water. Heat the solution to a temperature just below boiling and add pieces of white cotton cloth, woolen cloth, and silk about 6 inches square. Stir occasionally to obtain even dyeing. At the end of 10 minutes remove the fabrics and wash in hot water as long as the dye is removed. Cut the pieces in two, and place half of each material in a very dilute solution of hydrochloric acid. Let the samples stay in the air, until dry.
228. Dyeing with an Adjective Dye: Mordants: Malachite Green (Sections 530, 541). - Mordant two pieces of boiled out cotton cloth (see previous experiment) by allowing them to stand for 15 minutes in a cold solution of 0.2 gram of tannic acid in 200 cc. of water. Fix the tannic acid on one of the pieces by placing the cloth without washing into a solution of 0.2 gram of tartar emetic in 200 cc. of water, and allowing it to stand for 10 minutes.
Make a dye bath by dissolving 0.1 gram of malachite green in 500 cc. of water. Add to the bath a piece of boiled-out cotton which has not been mordanted, the piece of cotton mordanted with tannic acid from which most of the solution has been removed by pressure, the piece of cotton cloth on which the tannic acid was fixed by tartar emetic, a piece of wool, and a piece of silk. Dye for 15 minutes at a temperature just below boiling. Work the pieces over and over with a rod occasionally to obtain even dyeing.
Wash the dyed material in boiling water as long as color is removed. Dry. Compare the appearance of the three pieces of cotton cloth. Why is the piece which was treated with tannic acid and tartar emetic darker than the one mordanted with tannic acid alone?
229. Dyeing with an Ingrain Color: Primuline (Section 531). - Dye three pieces of boiled-out cotton cloth (see experiment 227) in a solution of 0.2 gram of primuline and 0.2 gram of sodium carbonate in 500 cc. of water, at a temperature just below boiling for 15 minutes. Wash the cloth twice in about 500 cc. of water.
Prepare a diazotizing bath by dissolving 10 cc. of concentrated hydrochloric acid in 500 cc. of cold water and then adding 0.2 gram of sodium nitrite. Place in the bath the three pieces of cloth which have been dyed with primuline, and allow them to stay 10 minutes. Stir occasionally. Prepare three baths in which the diazotized primuline is to be developed. Dissolve 0.1 gram b-naphthol in 1 cc. of a 10 per cent solution of sodium hydroxide, and add 100 cc. of water; dissolve 0.1 gram resorcinol in 2 cc. of the sodium hydroxide solution and add 100 cc. of water; dissolve 0.1 gram of phenol in 1 cc. of the sodium hydroxide solution and add 100 cc. of water.
Transfer the cloth from the diazo bath to a beaker containing about 500 cc. of water and stir. Put one piece of cloth in each of the developing solutions, and allow them to stay 5 minutes.
Wash the dyed cloth twice with water and dry.