Frost Diagrams for Actinides

Latimer & Frost Diagrams for elements in acid & alkaline (aq) indicate

• actinides are quite electropositive
• Pa - Pu show significant redox chemistry

  e.g. all 4 oxidation states of Pu can co-exist in appropriate conditions in (aq)

• stability of high oxidation states peaks at U (Np)
• An³⁺ is the maximum oxidation state for (Cf)Es - Lr
• No²⁺(aq) is especially stable ~ most stable state for No in (aq)
• redox potentials show strong dependence on pH (data for Ac - Cm)
  • high oxidation states are more stable in basic conditions
  • even at low pH hydrolysis occurs Æ formation of polymeric ions

    when hydrolysis leads to precipitation measurement of potentials is difficult!

    e.g. Pa⁵⁺ hydrolyses easily; potentials that indicate it to be the most stable oxidation state are recorded in presence of F^- or C₂O₄^2-

  • tendency to disproportionation is particularly dependent on pH

    e.g. at high pH 3Pu⁴⁺ + 2H₂O PuO₂²⁺ + 2Pu³⁺ + 4H⁺

• early actinides have a tendency to form complexes
  • ~ complex formation influences reduction potentials

    e.g. Am⁴⁺(aq) only exists when complexed by fluoride (15 M NH₄F(aq))

• radiation-induced solvent decomposition produces H^• and OH^• radicals

  which lead to reduction of higher oxidation states e.g. Pu^V/VI, Am^IV/VI

Stability of Actinide ions in aqueous solution:

Ion	Colour	Stability	Preparation
Md2+		easy to oxidize, but stable to water	Zn or Cr2+ on Md3+
No2+		stable	normal oxdn state in acid
Ac3+	colourless	stable	normal oxdn state in acid
U3+	claret	evolves H2 on standing; easily oxidized by air	Na or Zn/Hg on UO22+
Np3+	blue-purple	stable to water; easily oxidized by air	Zn/Hg or H2(Pt) reduction
Pu3+	blue-violet	stable to water & air; readily oxidized	SO2 or NH2OH reduction
Am3+	pink	stable; difficult to oxidize	I, SO2 , etc... on higher states
Cm3+	pale yellow	stable; chemical oxidation not possible	normal oxdn state in acid
Bk3+	green	stable; can be oxidized to Bk4+	normal oxdn state in acid
Cf3+	green	stable	normal oxdn state in acid
Es3+		stable	normal oxdn state in acid
Fm3+		stable	normal oxdn state in acid
Md3+		stable, but easily reduced to Md2+	normal oxdn state in acid
No3+		easily reduced to No2+	CeIV or BrO3 on No2+
Lr3+		stable	normal oxdn state in acid
Th4+	colourless	stable	normal oxdn state in acid
Pa4+	colourless	stable to water; easily oxidized	Zn/Hg on PaV(aq)
U4+	green	stable to water; easily oxidized by air to UO22+	Zn/Hg on UO22+
Np4+	yellow-green	stable to water; slowly oxidized by air to NpO2+	SO2 on NpO2+ in H2SO4
Pu4+	tan-brown	stable in 6M acid, disproportionates at higher pH	SO2 or NO2 on PuO22+
Am4+	pink?	only stable as fluoride complex; easily reduced	Am(OH)4 in 15 M NH4F
Cm4+	pale yellow	only as fluoride complex; stable only 1 hr at 25°C	CmF4 in 15 M CsF
Bk4+	yellow	marginally stable; easily reduced
Pa5+	colourless	stable; readily hydrolyzed	normal oxdn state in acid
UO2+		unstable to disproportionation (least at pH 2-4)	a transient species
NpO2+	green	stable; disproportionates at high acidity	hot HNO3 on Np4+
PuO2+	pink-purple	tends to disproportionate (least at low pH)	NH2OH on PuO22+
AmO2+	yellow	disproportionates in acid; reduced by its a-decay	ClO or cold S2O82-on Am3+
UO22+	yellow	stable; difficult to reduce	oxidation by air in HNO3
NpO22+	pink-red	stable; easy to reduce	oxidation by CeIV, MnO4, BrO3 , etc.
PuO22+	orange-pink	stable; easy to reduce; reduced by its a -decay	oxidation by CeIV, MnO4, BrO3 , etc.
AmO22+	rum-brown	easy to reduce; rapidly reduced by its a -decay	oxidation by CeIV, MnO4, BrO3 , etc.
NpO53-	deep green	only in alkaline solution	O3 or S2O82- on NpO22+ + alkali
PuO53-	deep green	only in alkaline solution; oxidizes water	O3 or S2O82- on PuO22+ + alkali

 Colour omission Þ concentrated enough solution to tell colour has not been obtained

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