of Pr6O11 in boiling acetic acid. The reactivity of praseodymium conforms to periodic trends, as it is one of the first and thus one of the largest lanthanides. At 1000 °C, many praseodymium oxides with composition PrO2−x exist as disordered, nonstoichiometric phases with 0 < x < 0.25, but at 400–700 °C the oxide defects are instead ordered, creating phases of the general formula PrnO2n−2 with n = 4, 7, 9, 10, 11, 12, and ∞. These phases PrOy are sometimes labelled α and β′ (nonstoichiometric), β (y = 1.833), δ (1.818), ε (1.8), ζ (1.778), ι (1.714), θ, and σ. Praseodymium is an electropositive element and reacts slowly with cold water and quite quickly with hot water to form praseodymium(III) hydroxide: 2 Pr (s) + 6 H2O (l) → 2 Pr(OH)3 (aq) + 3 H2 (g) Praseodymium metal reacts with all the stable halogens to form trihalides: 2 Pr (s) + 3 F2 (g) → 2 PrF3 (s) [green] 2 Pr (s) + 3 Cl2 (g) → 2 PrCl3 (s) [green] 2 Pr (s) + 3 Br2 (g) → 2 PrBr3 (s) [green] 2 Pr (s) + 3 I2 (g) → 2 PrI3 (s) The tetrafluoride, PrF4, is also known, and is produced by reacting a mixture of sodium fluoride and praseodymium(III) fluoride with fluorine gas, producing Na2PrF6, following which sodium fluoride is removed from the reaction mixture with liquid hydrogen fluoride. Additionally, praseodymium forms a bronze diiodide; like the diiodides of lanthanum, cerium, and gadolinium, it is a praseodymium(III) electride compound. Praseodymium dissolves readily in dilute sulfuric acid to form solutions containing the chartreuse Pr3+ ions, which exist as [Pr(H2O)9]3+ complexes: 2 Pr (s) + 3 H2SO4 (aq) → 2 Pr3+ (aq) + 3 (aq) + 3 H2 (g) Dissolving praseodymium(IV) compounds in water does not result in solutions containing the yellow Pr4+ ions; because of the high positive standard reduction potential of the Pr4+/Pr3+ couple at +3.2 V, these ions are unstable in aqueous solution, oxidising water and being reduced to Pr3+. The value for the Pr3+/Pr couple is −2.35 V. However, in highly basic aqueous media, Pr4+ ions can be generated by oxidation with ozone. Although praseodymium(V) in the bulk state is unknown, the existence of praseodymium in its +5 oxidation state (with the stable electron configuration of the preceding noble gas xenon) under noble-gas matrix isolation conditions was reported in 2016. The species assigned to