Sulfates peroxodisulfate, oxidation

To remove the sulfate introduced during the peroxodisulfate oxidation, the crude product is dissolved in a minimum amount of 1 M hydrochloric acid ( 160 ml.) at 40°C., and barium chloride (3 g.) dissolved in a minimum amount of hot water is added. The solution is allowed to stand for 20-30 minutes, rewarmed to 50°C., and the barium sulfate filtered off on a medium-porosity sintered-glass filter. Concentrated hydrochloric acid (20 ml.) is added to the filtrate, which is cooled until precipitation is complete. The solid is recrystallized by dissolving it in a minimum amount of 0.1 M hydrochloric acid... 

The presence of Ir02 nanoparticles on the BDD surface causes a considerable decrease in the overpotential for oxygen evolution, in the inhibition of sulfate to peroxodisulfate oxidation, and in the modification of the mechanism of organic oxidation. 

An expanding development is the use of peroxodisulfates as oxidants in organic chemistry (80,81). These reactions are initiated by heat, light, gamma rays, or transition-metal ions. The primary oxidising species is usually the sulfate ion radical, P hskip -3pt peroxodisulfate anion... 

Hydrogen peroxide was first made in 1818 by J. L. Thenard who acidified barium peroxide (p. 121) and then removed excess H2O by evaporation under reduced pressure. Later the compound was prepared by hydrolysis of peroxodisulfates obtained by electrolytic oxidation of acidified sulfate solutions at high current densities ... 

Peroxodisulfuric acid, H2S2O8, is a colourless solid mp 65° (with decomposition). The acid is soluble in water in all proportions and its most important salts, (NH4)2S208 and K2S2O8, are also freely soluble. These salts are, in fact, easier to prepare than the acid and both are made on an industrial scale by anodic oxidation of the corresponding sulfates under carefully controlled conditions (high current density, T < 30°, bright Pt electrodes, protected cathode). The structure of the peroxo-disulfate ion [now preferably called hexaoxo-/r-peroxodisulfate(2-)]0 l is OaSOOSOa " with... 

In the electrochemical processes, an aqueous solution of sulfuric acid (550 to 570 g/L) (Degussa-WeiKenstein Process) or of sulfuric acid (260 g/L) and ammonium sulfate (210 to 220 g/L) (Lowenstein-Riedel Process) is electrochemically oxidized at the anode to peroxodisulfuric acid or ammonium peroxodisulfate respectively and reduced at the cathode producing hydrogen. 

Aqueous solutions of peroxodisulfate anions are manufactured by electrolysis only, that is, by anodic oxidation of sulfate or HSO ... 

Pt anodes. The S20 ions are formed from the oxidation of 804 and HSO4 species at very high potentials (Table 1, Eqs.7 and 8). In 2000, Michaud et al demonstrated that BDD anodes are suitable to produce peroxodisulfate [4]. Serrano et al discussed the mechanism of formation of peroxodisulfate [5]. Only HSO4 and molecular H2SO4 react with OH radicals to form sulfate radicals, per the following reactions (Eqs. 3-5) ... 

Transition metal ions also exhibit homogeneous catalysis, where they and the reactant(s) are in the same physical state usually all are in solution. For example, iron(iii) ions act as a homogeneous catalyst for the reaction between peroxodisulfate and iodide ions to form sulfate ions and iodine molecules (Chapter 16). Iron catalyses the reaction by interconverting between its two common oxidation states, iron(ii) and iron(iii). This facilitates the electron transfer processes that occur. Many metal-containing enzymes, especially those in the electron transport chain, act in a similar way inside cells. 

From another standpoint, a positive oxidation number must not exceed the number of electrons in the external shell of the element. If it is negative, it must not exceed the number of electrons necessary to saturate the external shell. For example, for the redox couple peroxodisulfate—sulfate (S20g /S04 ) assigning the value —II to oxygen leads to the oxidation number h-VII for the two sulfur atoms of the ion peroxodisulfate. This result is wrong because the sulfur atom possesses six electrons only in its external shell. Actually, the peroxodisulfate ion contains a peroxo bridge in... 

During the course of an oxidation with the peroxodisulfate ion, each oxygen atom of the peroxo bridge captures one electron while the 0-0 bond is breaking and, finally, two sulfate ions are formed. The half-redox reaction can be written as... 

Polystyrenesulfonic acid (PSS) is the most prominent polymeric coimterion for PEDOT, and the complex of PEEXDTPSS is described in detail in Chapter 9. As shown in the early work of Jonas and Krafft on PEDOT dispersions, PEDOTPSS is, strictly speaking, a complex with two types of coimterions. The complex contains sulfate ions as obtained from the oxidation agents iron(III) sulfate and potassium peroxodisulfate as well as polystyrenesulfonic acid. However, the sulfate ion can easily be removed using, for example, ion exchange resins and a true PEDOTPSS complex is prepared. ... 

At high H2SO4 concentration (> 2.0 M) the main anodic reaction is the electrochemical oxidation of sulfate to peroxodisulfate (eq. 20.7). Small amounts of monopersulfate (eq. 20.10) and H2O2 (eq. 20.11) are also formed by the chemical decomposition of peroxodisulfate. 

Figures 21.8a and 21.8b depict two alternative methods for wastewater treatment that use BDD anodes for the oxidation of biorefractory organic species. In the first treatment (Fig. 21.8a), peroxodisulfate is produced with a high current efficiency from a concentrated sulfate solution. The oxidant (peroxodisulfate) produced is subsequently mixed with the heated wastewater in order to achieve its activation, i.e., the production of hydroxyl radicals. The AOP occurs efficiently in the bulk of the wastewater, and thus the mass transfer limitation is avoided.

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