Abstract
Aniline and N-methylaniline are common contaminants in commercially produced indigo. It is known, that commercially produced indigo contains up to 0.6% aniline and 0.4% N-methylaniline by weight and indigo dye shows a small mutagenic effect, most probably due to the presence of these contaminants. The present work describes a new and powerful analytical method to determine the concentration of these contaminants in indigo. This method is based on the transformation of water insoluble indigo into soluble leucoindigo and allows therefore the acidic extraction of the aromatic contaminants. This transformation step is essential, because the main part of these contaminants are strongly included in the indigo crystals. The amount of extracted aniline and N-methylaniline from the leucoindigo solution was quantified with high performance liquid chromatography (HPLC, combined with a photo diode array detector). A possible accumulation of the aromatic amines at the indigo crystal surface was investigated using FTIR and by adsorption studies. Therefore this method allows an accurate monitoring of these toxic by-products in the indigo dye, which is important for an economic and environmental assessment of the denim production.
Highlights
Aniline and N-methylaniline are common contaminants in commercially produced indigo
The washing of indigo with acidic aqueous solution does not dissolve the indigo and the acidic extract of solid indigo contains no aniline and N-methylaniline and leads to the conclusion, that these substances are incorporated in the indigo crystal during indigo formation
It seemed promising to perform the analysis with an extract of the leucoindigo solution, because this would eliminate the possibility of incomplete extraction due to an incorporation in the indigo crystal
Summary
Aniline and N-methylaniline are common contaminants in commercially produced indigo. It is known, that commercially produced indigo contains up to 0.6% aniline and 0.4% N-methylaniline by weight and indigo dye shows a small mutagenic effect, most probably due to the presence of these contaminants. The present work describes a new and powerful analytical method to determine the concentration of these contaminants in indigo This method is based on the transformation of water insoluble indigo into soluble leucoindigo and allows the acidic extraction of the aromatic contaminants. Indigo has a great economic importance for the dyeing of approximately 4 billion denim textiles per year For this purpose 70,000 t of indigo are used and an amount between 10 and 20 million m3 water is released with residual chemicals from the dyeing step and by products from the indigo synthesis, i.e. aniline and N-methylaniline[1]. By steam distillation and later replaced by continuous solvent e xtraction[3] These byproducts can be found in the waste water, whereby aniline and anthranilic acid are more biodegradable than N-methylaniline. The maximum concentration of aniline in the air at the workplace (MAK value) in Germany is fixed to 7.7 mg/m3
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