A reservoir assembly can stably hold a bicarbonate solution. The reservoir assembly includes a gas-permeable plastic container holding a bicarbonate solution and a gas-impermeable casing for encasing the container. An effective way of detecting pinholes in the casing of the reservoir assembly is also provided. An oxygen absorber and an oxygen-detecting agent are placed in the space between the container and the casing. The oxygen absorber is of the type that does not affect the pH or the bicarbonate ion concentration of the solution and mainly contains a crosslinked polymer having carbon-carbon unsaturated bonds.
