A method for the purification of spent sulfuric acid and particularly the purification of spent acid, from titanium dioxide rutile manufacture through a chloride route, is provided. At least two light reflectivitytransmittance control plates are placed on a path through which the light passes such that light reflectivitytransmittance is varied depending on an angle between the generated light and the plates. The rotation direction portion may provide torque in different directions to the output mode portion. In the preferred embodiment, the transparent outer casing is removable to enable the user to easily access and modify the indicia.