NOVELTY - The electrolyser comprises a stack of a first and a second electrolytic cells (10), and a stack of cell comprising single ion exchange membranes (11). Each electrolytic cell comprises an anode, a cathode, and an ion exchange membrane positioned between the anode and the cathode. The ion exchange membrane of the first and second electrolytic cells is separated by a bipolar electrode (4) constituting the anode of the first electrolytic cell on the one hand and the cathode of the second electrolytic cell on the other hand. The bipolar electrode comprises a bipolar plate.    USE - The electrolyser is useful in an electrolytic ensemble for the production of chemical substance such as hydrogen, oxygen, chlorine, hypochlorous acid or sodium hydroxide by electrolysis of pure water or water containing salt, base and/or acid such as sodium chloride, hydrogen sulfate, potassium hydroxide or sodium hydroxide (claimed).    ADVANTAGE - The electrolyser is capable of economically producing the chemical substance with high purity.    DETAILED DESCRIPTION - The electrolyser comprises a stack of a first and a second electrolytic cells (10), and a stack of cell comprising single ion exchange membranes (11). Each electrolytic cell comprises an anode, a cathode, and an ion exchange membrane positioned between the anode and the cathode. The ion exchange membrane of the first and second electrolytic cells is separated by a bipolar electrode (4) constituting the anode of the first electrolytic cell on the one hand and the cathode of the second electrolytic cell on the other hand. The bipolar electrode comprises a bipolar plate, which is associated with a grid and a porous plate (12). The grid and the porous plate are integrated with each other before their mounting in the electrolyser. The ion exchange membrane has a thickness of 100-500 mu m. The single ion exchange membranes are provided between the anode and the cathode, and include a non-selective membrane and a selective membrane such as Nafion (RTM: Perfluorinated membrane). An intermediate chamber is positioned between the single ion exchange membranes. INDEPENDENT CLAIMS are included for:    (1) an electrolytic ensemble;    (2) a process for preparing a hypochlorous acid; and    (3) an electrolyser cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an electrolytic cell.    Bipolar electrode (4)    Electrolytic cells (10)    Ion exchange membrane (11)    Porous plate (12)    Frame. (13)