15 able to exist in significant concentrations simultaneously in all four of its lower oxidation states. The high reduction potentials associated with the hiaher oxidation states of americium prevent its similar behavior, especially if in the presence of oxidizable species (as in seawater); americium is expected to exhibit a marked preference for the +3 state. Unlike the higher oxidation states of plutonium, Pu (III), like the trivalent rare earths, show little tendency to form complex jons. Trivalent plutonium, which is stable in acidic solutions, begins to hydrolize near pH 7, and at only slightly higher pH, oxidizes to Pu(OH)4 (Andelman and Rozzell, 1968). Plutonium (IV) is noted for formation of very stable hydrolysis products which can grow to molecular weiahts of 19'?, basic solution. Such large polymetric species are favored in increasingly Pu (IV) hydroxides are the most stable (hence abundant) hydrolytic products of plutonium in the aqueous state in the pH range of natural water. The charge carried by colloidal? Pu (IV) has been discussed by Phodes (1957a, 1957b), Andelman and Rozzell (op.cit.) and has been further discussed by Price (1973) and Keller (op.cit.). These workers suggest that in vitro colloidal Pu (IV) is positively charged below pH 7-8 and negatively charaed at higher ph‘'s. Plutonium V greatly disproportionates above pH = 6, (Yeiler op.cit.), is thought to exist only as the oxyion Puo+ in aqueous solution (Andelman and Rozzel} fop. cit.}) and correspondingly shows little tendency to form chelates (Keller, op. cit.). Plutonium VI may behave some- wat similarly to Pu (III) but has greater tendency to form complexes, is rore stable in basic solution and, with such divalent anions as carbonate, it wey exist as an anionic molecule. The chemical properties of Pu (VI) and U ‘1. are cormonly reported to be quite similar.