Periodate

Periodate pəˈraɪ.ədeɪt is an anion composed of iodine and oxygen. It is one of a number of oxyanions of iodine and is the highest in the series, with iodine existing in oxidation state +7. Unlike other perhalogenates, such as perchlorate, it can exist in two forms: metaperiodate IO4− and orthoperiodate IO65−. In this regard it is comparable to the tellurate ion from the adjacent group. It can combine with a number of counter ions to form periodates, which may also be regarded as the salts of periodic acid. Periodates were discovered by Heinrich Gustav Magnus and C. F. Ammermüller; who first synthesised periodic acid in 1833. Classically, periodate was most commonly produced in the form of sodium hydrogen periodate (). This is commercially available, but can also be produced by the oxidation of iodates with chlorine and sodium hydroxide. Or, similarly, from iodides by oxidation with bromine and sodium hydroxide: \overset{sodium\ iodate}{NaIO3} + Cl2{} + 4 NaOH -> Na3H2IO6{} + 2NaCl{} + H2O NaI + 4 Br2 + 10 NaOH -> Na3H2IO6 + 8 NaBr + 4 H2O Modern industrial scale production involves the electrochemical oxidation of iodates, on a lead dioxide () anode, with the following standard electrode potential: H5IO6 + H+ + 2e- -> IO3- + 3 H2O E° = 1.6 V Metaperiodates are typically prepared by the dehydration of sodium hydrogen periodate with nitric acid, or by dehydrating orthoperiodic acid by heating it to 100 °C under vacuum. Na3H2IO6 + 2 HNO3 -> NaIO4 + 2 NaNO3 + 2 H2O H5IO6 -> HIO4 + 2 H2O They can also be generated directly from iodates by treatment with other strong oxidizing agents such as hypochlorites: NaIO3{} + \overset{sodium\ hypochlorite}{NaOCl} -> NaIO4{} + NaCl Periodate can exist in a variety of forms in aqueous media, with pH being a controlling factor. Orthoperiodate has a number of acid dissociation constants. H5IO6 H4IO6- + H+ pK_a = 3.29 H4IO6- H3IO6^2- + H+ pK_a = 8.31 H3IO6^2- H2IO6^3- + H+ pK_a = 11.60 The ortho- and metaperiodate forms also exist in equilibrium.