Complexes of Ferrous Iron With Tannic Acid

Solutions of tannic (digallic) acid at concentrations of 5 or 50 parts per million reduce dissolved ferric iron to the ferrous state when the pH is less than 4. In solutions with a pH of 4 or more, a black material containing ferric iron and tannic acid is precipitated. In a solution containing 500 parts per million of tannic acid, a ferrous complex forms at a pH of more than 5. This complex is oxidized at a slow rate, and some ferrous iron remains in solution after a month of storage in contact with air. Uncomplexed ferrous iron is oxidized and precipitated from solution in a few hours or less at pH levels of more than 5. The approximate value of the first dissociation constant for tannic acid is between 1X 10~5 and 1X 1G~6 . The stability constant for the ferrous complex is comparable in magnitude to that of the pink bipyridine complex used in the determination of iron.