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The use of hypochlorite, formed in situ by anodic oxidation of sodium chloride solution, for bleaching indigo‐dyed denim has been studied at room temperature and at 50 °C. A direct relationship between the charge flow applied and the bleaching effect was observed. This enables consistent oxidative bleaching to be achieved by controlling the electrochemical process. The use of mixed anolytes containing small concentrations of bromide ions enhanced the bleaching effect considerably, even after lowering the temperature to room temperature. The molar concentration of bromide ions required was much lower than that of chloride in the anolyte.

On-site production of hypochlorite for denim bleach was investigated using undivided unipolar electrolysers. The current efficiency of the electrolysis process was studied at laboratory scale with up to 20 A cell current and at the technical scale using a 1200 A electrolyser. NaCl solutions in the concentration range 0.236-0.944 M NaCl (13.8-55.2 g l )1 ) were used. Anodic current densities up to 400 A m )2 were used. Hypochlorite solutions with active chlorine concentration c(Cl)=0.10-0.12 M were obtained with current efficiency of 58-70%. For denim bleach processes a concentration of active chlorine up to c(Cl)=0.12 M is required. This corresponds to approximately 30 ml l )1 of commercial hypochlorite solution. Electrical energy consumption calculated per kg of active chlorine formed ranges from 5.7 to 8.6 kWh kg )1 . By coupling the electrolyser to a commercial 30 kg capacity drum washing machine the electrochemically prepared oxidant was successfully used in bleach processes. The technique overcomes problems of insufficient reproducibility of bleach, chemical costs and released wasted water due to the use of commercial NaOCl solution.

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