Indigo pure

Indigo pure is a stable dye, which gives a blackish blue smoke, but when the burning temperature of the smoke composition is too high, it is decomposed to generate yellow grey smoke. 

Procedure. Weigh out accurately from a weighing bottle about 0.2 g of the pure sodium carbonate into a 250 mL conical flask (Note 1), dissolve it in 50-75 mL of water, and add 2 drops of methyl orange indicator (Note 2) or preferably of methyl orange-indigo carmine indicator (Section 10.9), which gives a very much more satisfactory end point (Note 3). Rinse a clean burette three times with 5 mL portions of the acid fill the burette to a point 2-3 cm above the zero mark and open the stopcock momentarily, in order to fill the jet with liquid. Examine the jet to see that no air bubbles are enclosed. If there are, more liquid must be run out until the jet is completely filled. Re-fill, if necessary, to bring the level above the zero mark then slowly run out the liquid until the level is between the 0.0 and 0.5 mL marks. Read the position of the meniscus to 0.01 mL (Section 3.12). 

A simple qualitative test of the purity of the indigo obtained can be carried out as follows A little of the material is heated for some time (with shaking) in a test tube with pyridine and some drops of the liquid are then poured on to a filter paper. If the indigotin is pure the pyridine is not coloured, whereas impurities which may be formed when working on a small scale confer on it a more or less dirty brown colour, as shown by a spot test. If it is desired to purify the whole of the indigo with pyridine, the dye is collected at the pump after boiling with the liquid, washed with pure hot pyridine, boiled once more with hydrochloric acid, collected at the pump, washed with hot water, and dried.2... 

Colouring of a Flame by Lithium, Sodium, and Potassium Salts. Introduce a platinum or nichrome wire into the colourless flame of a burner. If the flame becomes coloured, wash the wire with pure hydrochloric acid and roast it in the burner flame until the latter becomes colourless. Wet a platinum wire in a potassium chloride solution and introduce it into the colourless flame of the burner. What do you observe Perform similar experiments with solutions of sodium and lithium chlorides. Observe the colouring of the flame by the potassium salts through an indigo prism. 

At ordinary temperatures, pure ozone is a pale blue gas (d — 2.1415 g/L at 0°C and 101.3 kPa (1 aim)) that can be condensed to an indigo blue liquid, which freezes to a deep blue-violet solid. The solubility of gaseous ozone at atmospheric pressure and 0°C is 1.1 g/I, H20. Gaseous ozone can be adsorbed by porous solid substrates such as silica gel and is often used in this form in organic synthesis. 

If, for example, the colour obtained with the indigo under examination coincides with colour No. 13 of the scale, 20 c.c. of the indigo solution correspond with 13 c.c. of the pure indigotin solution, its dyeing value being thus 65% of that of pure indigotin. 

Powdered artificial indigo usually consists of almost pure indigotin (about 98%) in the paste its concentration varies (usually 20%). 

Indigo carmine commonly consists of indigotindisulphonic acid or its sodium salt, which is sold as a moist paste or in blue balls or lozenges with reddish reflection. Indigotinmonosulphonic acid (iindigo purple) is little used. Pure indigo carmine is completely soluble in water, from which it is reprecipitated by addition of sodium chloride. Commercial carmines of low quality leave a more or less abundant greenish residue insoluble in water. 

Artificial Organic Dyes.—Indigo carmine may be adulterated with artificial organic dyes, especially aniline blue. If silk is dyed with an acidified solution of the carmine and then washed and boiled with water, the fibre will become colourless if the carmine is pure, but remains blue if aniline blue is present. 

Also by oxidising the carmine solution with permanganate for the determination of the mdigotin, many artificial organic dyes may be detected thus, pure indigo carmine solution remains yellow, whereas it may be bluish, violet, grey or reddish in presence of artificial organic dyes. 

The fermentation mix can also be used directly for dyeing. The textile is impregnated with the fermenting mash and then blued by oxidation in the air. This type of dyeing was carried out with woad (Isatis tinctoria ). Pure indigo dye was not extracted from woad because of its low content of indican (15). 

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