Abstract: A rotating disc reactor for production of sodium hydrosulfite comprising a reactor main housing containing a plurality of elongated reaction chambers with at least partially arcuate interior periphery. Each reaction compartment contains a top gaseous layer, an intermediate buffer solution layer, and a bottom sodium amalgam layer. Positioned in each compartment is a rotatable shaft with a plurality of rotatable discs mounted thereon for reacting and mixing the three layers of reactants. The reactor includes means for distributing amalgam and means for distributing recirculated buffer solution into each reaction compartment. At least one of the side walls of the housing is connected to an end box having inlet and outlet means for circulating the amalgam and the sodium hydrosulfite product. The reactor further includes a sulfur dioxide inlet manifold having an arcuate interior which is offset from the reactor housing.