Methemoglobinaemia is a form of abnormal hemoglobin where the iron centre has been oxidised from the ferrous +2 oxidation state (the normal form, which on binding with oxygen changes to the ferric state) to the ferric +3 state. This causes a leftward shift in the oxygen hemoglobin dissociation curve, as any residual heme with oxygenated ferrous iron (+2 state) is unable to unload its bound oxygen into tissues (because 3+ iron impairs hemoglobins cooperativity), thereby increasing its affinity with oxygen. However, methemoglobin has increased affinity for cyanide, and is therefore useful in the treatment of cyanide poisoning. In cases of accidental ingestion, administration of a nitrite (such as amyl nitrite) can be used to deliberately oxidise hemoglobin and raise methemoglobin levels, restoring the functioning of cytochrome oxidase. The nitrite also acts as a vasodilator, promoting the cellular supply of oxygen, and the addition of an iron salt provides for competitive binding of the free cyanide as the biochemically inert hexacyanoferrate(III) ion, [Fe(CN)]. An alternative approach involves administering thiosulfate, thereby converting cyanide to thiocyanate, SCN, which is excreted via' the kidneys.  Methemoglobin is also formed in small quantities when the dissociation of oxyhemoglobin results in the formation of methemoglobin and superoxide, O, instead of the usual products. Superoxide is a free radical and causes biochemical damage, but is neutralised by the action of the enzyme superoxide dismutase.