Vatting with sodium dithionite

Indigo turns yellow when it is vatted with sodium dithionite and ammonia. When the vat is shaken with ethyl acetate, the supernatant ethyl acetate layer turns blue (33). 

If the violet dyeing becomes only somewhat paler when treated with 102 sulfuric acid, the specimen is washed out with water and vatted with sodium dithionite and ammonia. If the dyeing then turns yellow and is reoxidized in the air to a blue, this dyeing is one with alkanna (C.I. Natural Red 20) on alum mordant. 

When a brown natural dyeing turns yellow upon vatting with sodium dithionite and ammonia and the original shade returns upon reoxidation, the dyeing probably was produced with walnut shells (C.I. Natural Brown 7) or some other hydroxynaphthoquinone. 

The threads on the face of the carpet that have faded (because of their moderate lightfastness) become colorless when they are vatted with sodium dithionite and ammonia, and they turn magenta upon subsequent reoxidation in the air. This dyeing is, therefore, one made with orchil (C.I. Natural Red 28). This dye was known as French purple , and very often it was used in tapestries in the 16th-19th centuries despite its poor lightfastness. 

Hydroxyacetone (12.48), mentioned in section 12.8.1 in connection with sulphur dyes, is sulphur-free and biodegradable. This compound was originally proposed for use with vat dyes and continues to generate some interest. This agent can be used for the pad-steam application of vat dyes in the presence of high concentrations of sodium hydroxide (about 3.5-4.5 g/1). Hydroxyacetone does not cause over-reduction of indanthrone vat dyes but does give different shades with carbazole dyes, compared with sodium dithionite [218]. 

Vat dyes are insoluble in water and are applied to cellulosic fibres, usually with sodium dithionite under alkaline conditions, by a vatting process involving reduction to produce a... 

A higher quality a-modification with enhanced tinctorial strength and transparency is prepared from the leuco form of bromoisoviolanthrone. This intermediate in turn is manufactured by vatting crude bromoisoviolanthrone with sodium dithionite/aqueous sodium hydroxide. The product is separated and oxidized in an aqueous alkaline medium in the presence of surfactants. Application of shearing forces, preferably by means of a sand or pearl mill, and maintaining a temperature of 50°C produces improved pigment quality [26]. 

For the dyeing of cotton, indigo is reduced with sodium dithionite in alkaline solution. The dyer speaks of vatting . When this reaction is carried out in dye-... 

Vat dyes are usually reduced with sodium dithionite and solubilized under alkaline conditions, applied to the textile material and reoxidized (Rivlin, 1992). Unfixed vat dye enters the waste stream in the form of water-insoluble pigments and can be recovered with ultrafdtration (Elliot, 1996). The reduction step, however, can also be achieved by electrochemical means (Roessler and Rys, 2003). Electrochemical reduction with electrocatalytic hydrogenation or a redox mediator reduces the need for chemicals and thus pollution of effluents. 

Most vat dyes are based on the quinone stmcture and are solubilized by reduction with alkaline reducing agents such as sodium dithionite. Conversion back to the insoluble pigment is achieved by oxidation. The dyes are appHed by either exhaust or continuous dyeing techniques. In both cases the process is comprised of five stages preparation of the dispersion, reduction, dye exhaustion, oxidation, and soaping. 

By far the most important reducing system for the batchwise application of vat dyes is sodium dithionite (Na2S204) in a solution of sodium hydroxide. Obviously the theoretical concentrations required will depend on the number of keto groups in the dye molecule and on its relative molecular mass and concentration, but the reaction can be represented as in Scheme 12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline... 

The pigment form [16] is obtained from the leuco form, which in turn is prepared by oxidation with alkali fusion, followed by treatment with sodium hydro-gensulfite solution or sodium dithionite (vatting). 

Heating the leuco form in an aqueous alkaline medium in the presence of air, possibly also in the presence of a surfactant, affords the pigment form. Oxidation may also be achieved with sodium-m-nitrobenzenesulfonate. Another alternative is to precipitate the vat acid from the salt of the leuco form and to subsequently oxidize the product. It is also possible to mill the suspension of the leuco form together with aqueous sodium hydroxide/sodium dithionite in air, using a pearl mill. 

Sodium dithionite (hydro sulfite) is a proven reducing agent for sulfur vat dyes, and is used together with alkali. The color yield of some sulfur dyes is remarkably increased by the addition of hydrosulfite. However, as a result of possible overreduction or destruction of the dye chromophore, hydrosulfite and similar reducing agents can be used only with selected dyes. 

Before the availability of sodium dithionite (1871), the reduction of indigo was brought about by bacteria with reducing properties (fermentation vat). The vat of indigo has a brown-yellow colour. The cloth is dipped in the vat and then exposed to air to allow reoxidation to indigo which is precipitated and finely distributed onto the fibre. A consequence of this process is the low rubbing fastness of the dyes. It causes the faded appearance of indigo-dyed jeans and makes possible the manufacture of faded jeans . 

Anthraquinone, its derivatives, and a series of polycyclic quinones (Vat dyes) are reduced with an alkaline solution of sodium dithionite heated to 80 °C to dihydroxy derivatives. Solutions of alkaline salts of quinones are usually of a different color (for example, red for anthraquinone). After dropping the sample on filter paper the original color returns slowly in consequence of air oxidation. 

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