An apparatus that can be used for producing chlorine dioxide is provided. The apparatus comprises a single fluid proportioning device, which comprises three or more fluid transferring devices, a conduit to the inlet of each fluid transferring device, a conduit to the outlet of each fluid transferring device, a water inlet to the device, and a water outlet from the device in which the fluid transferring device is proportionally actuated by the flow of water through the device. Also provided is a process that can be used to produce chlorine dioxide. The process comprises flowing water through the fluid proportioning device to create a downstream water and to actuate the fluid transferring devices; drawing three or more precursor compounds each from a separate source and flowing each compound separately through one of the fluid transferring devices; and injecting the precursor compounds into the downstream water.