this invention relates to equipment and methods for detecting the presence of , measuring the amount of , and / or monitoring the level of , one or more selected components in a liquid mixture , employing an electrode sensing system . we have discovered that a class of mediating compounds has extremely useful properties for mediating the transfer of charge between enzyme - catalyzed reactions and electrode surfaces in electrode sensing systems . specifically , the specification discloses as electrode sensor mediators , organometallic compounds composed of at least two organic rings , each of which has at least two double bonds in a conjugated relationship ; a metal atom is in electron - sharing contact with those rings . an enzyme capable of catalyzing a reaction at a rate representative of the selected compound concentration is in contact with an assay mixture , and the mediator compound transfers charge between the enzyme and the conductive surface of the electrode at a rate representative of the enzyme catalyzed reaction rate .