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The Aqueous Chemistry of the Elements$
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George K. Schweitzer and Lester L. Pesterfield

Print publication date: 2010

Print ISBN-13: 9780195393354

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195393354.001.0001

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The Fe–Co–Ni Group

The Fe–Co–Ni Group

(p.351) 15 The Fe–Co–Ni Group
The Aqueous Chemistry of the Elements

George K. Schweitzer

Lester L. Pesterfield

Oxford University Press

The three elements to be treated in this chapter (Fe, Co, Ni) are the sixth, seventh, and eighth members of the first transition series. The first five members (Sc, Ti, V, Cr, Mn) have been treated in previous chapters (Chapters 12, 13, and 14). The ten elements of this first transition series (Sc through Zn) are characterized by electron activity in the 4s–3d levels. All elements in the 3d transition series are metals, and many of their compounds tend to be colored as a result of unpaired electrons. Most of the elements have a strong tendency to form complex ions due to participation of the d electrons in bonding. Unlike the previous three elements (V, Cr, Mn), these three do not show a variety of oxidation states. The higher oxidation states are almost absent in compounds, Fe showing principally the II and III, Co the II and III, and Ni only the II. The III states are less stable than the II states unless they are stabilized by complex formation. The resemblance of these three elements is notable, they being more like each other than they are to the elements below them. a. E–pH diagram. The E–pH diagram in Figure 15.1 shows Fe in oxidation states of 0, II, and III. This diagram, which involves iron at 10−1.0 M is oversimplified in several ways. The Fe(II) and Fe(III) cations are more properly designated as Fe(HOH)6+2 and Fe(HOH)6+3, reflecting the coordination number of 6. The region just to the left of the Fe+2/Fe(OH)2 line involves Fe(OH)+, and the region just to the left of the Fe+3/FeO(OH) line involves numerous hydroxo complexes such as Fe(HOH)5OH+2 and Fe(HOH)4(OH)2+. At very high pH values, there is a tendency for Fe(OH)2 to be converted to HFeO2−, and at very high E values, the powerful oxidant, red-purple FeO4−2 can be produced. When these latter five species are introduced, Figure 15.2 results. This, too, is probably simplified since it is known that other species are present, such as Fe2(OH)4+4 and Fe3(OH)4+5. The species FeO4−2 can only be prepared in a very strong base.

Keywords:   hydroxides, occurrence, oxides, redox reactions

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