Two polytetrafluoroethylene (PTFE) “pucks” are fitted onto the ends of each half cell and are sealed against the glass with an O-ring. The opening in the pucks determines the surface area of the separator that is exposed to solution. The standard cell kit uses pucks with a 1 cm² opening. A second O-ring fits in a groove on the outside of each puck, and the cell kit frame results in those O-rings being perfectly centered, sealing on the membrane/separator, as the assembly is tightened.