The Fe(II) of all forms of haemoglobin, with the exception of sulphohaemoglobin, is oxidised by ferrocyanide to Fe(III) to methaemoglobin, which in turn reacts with ionised cyanide (CN-) to form cyanmethaemoglobin, a very stable derivative that absorbs at 540 nm. The intensity of the colour formed is proportional to the concentration of total haemoglobin in the sample.
