Vitamin B12 oxidation. Test tubes showing three stages in the oxidation of cobaltamine (vitamin B12). The colours show the change in the oxidation state of the cobalt found in vitamin B12. Cobalt is unique in that it changes between 3 oxidation states in biological reactions: +1, +2 and +3. The corresponding colours seen here are blue, yellow, and black. Vitamins are trace nutrients that form an essential part of the diet. Only small amounts are needed because they are biological catalysts and do not get used up during reactions. Vitamin B12 is used in digestion and metabolism.