A laboratory cell equipped with multiple porous electrodes connected electrically in parallel and hydrostatically in series was used to reduce dispersed Colour Index (C.I.) Sulphur Black 1 to synthesise C.I. Leuco Sulphur Black 1. At higher cell current bipolar effects limit the maximum thickness of a porous cathode element. A mathematical model has been established to estimate the maximum thickness of a cathode element. The cell voltage can be calculated using a simple resistor network. The presented model calculations permit optimisation of a cell with multiple porous electrodes with regard to dimensions, cell costs and energy consumption. A multi-cathode cell, equipped with 10 cathodes for a total cell current of 10 A, was used to study production of highly concentrated C.I. Leuco Sulphur Black 1 solutions. Compared to chemical reduction, dyeing behaviour was found to be independent of reduction method used.