The Fe (II) of all forms of hemoglobin, with the exception of sulfohemoglobin, is oxidized to the Fe(III) of methemoglobin which, in turn, reacts with ionized cyanide (CN-) to form cyanmethemoglobin, a stable derivate which absorbs at 540 nm. The intensity of the color produced is proportional to the concentration of total hemoglobin in the sample.