Metachrome mordant method

An exhaustive investigation of the acid mordant dyes was made by Stevens, Rowe, and Speakman J.S.D.C., 1943, 59, 165) with a view to ascertaining why some responded to the metachrome-mordant method and others did not. In the first place they examined the behaviour of wool towards the mordant in the absence of the dye. It was found that, after boiling or 6 hours, 99-7 per cent of the chromium in the dyebath had been adsorbed by tlie wool and about 42 per cent of the total ammonia had been wolved, leaving the liquor with a p of 5 02. They suggested that the first... 

By far the most widely used chroming agent is sodium dichromate, although the potassium salt has occasionally been preferred. The dichromate may be applied before the dye (mordant method), simultaneously with the dyeing process (metachrome method) or as an aftertreatment (afterchrome method), but only the afterchrome process remains of practical significance. The theoretical aspects of chroming have been reviewed [72,73] and the mechanism of the reactions may be represented as follows ... 

In the one-bath chroming method (metachrome process), complex formation must be preceded by dye diffusion. For this reason, the release of CrVI must be delayed. The dye liquor is prepared with metachrome mordant (a mixture of sodium chromate and ammonium sulfate) and sodium sulfate. After a prerun at 40-50 °C, the system is heated slowly and dyed at boiling temperature for 45-90 min. Shortly before dyeing is completed, the bath is exhausted by the addition of acetic acid. 

Mordant Dyes. MetaUizable azo dyes are appHed to wool by the method used for acid dyes and then treated with metal salts such as sodium chromate [7775-11-5] sodium dichromate [10588-01-9] and chromium fluoride [1488-42-5] to form the metal complex in situ. This treatment usually produces a bathochromic shift ia shade, decreases the solubUity of the coloring matter, and yields dyeiags with improved fastness properties. The chromium salts can be appHed to the substrate before dyeiag (chrome-mordant or chrome-bottom method), together with the dye ia a single bath procedure (metachrome process), or as a treatment after dyeiag (afterchrome process). 

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