Chromium redox reaction, acidity

The early research of RFB systems was mainly carried out in the United States and Japan. NASA built the first 1 kW true RFB system with an Fe/Cr redox couple in the 1970s [11]. In this system, an aqueous solution of ferric-ferrous is employed as the positive reactant redox couple, and the negative reactant is a solution of chromos-chromic couple, with hydrochloric acid as a supporting electrolyte in most cases. Because of the poor kinetics of the chromium redox reaction, a serious deterioration of RFBs was observed after a long period of time moreover, a relatively low open circuit potential was also obtained. These drawbacks limited its practical application. In the following years, several RFB systems were evaluated, but none of them was developed on a commercial scale until the bromine/polysulphide RFB and vanadium system was invented [4]. In this section, in addition to these two systems, we will also introduce new progress in tme RFB systems. 

The following redox reaction between persulfate ions and chromium ions is carried out in aqueous acidic solution ... 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

The most common oxidation states of chromium in inorganic componnds are 11, III, and VI. By comparison, there are few stable compounds of tetravalent or pentavalent chrominm these oxidation states are largely represented by reactive intermediates in redox reactions. 7r-Acid ligands like CO (see Tt -A del Ligand) stabilize chromium in very low formal oxidation states (down to -IV ). Table 3 lists oxidations states and coordination environments for representative chromium compounds. 

Let s balance the redox reaction between dichromate ion and iodide ion to form chromium(III) ion and solid iodine, which occurs in acidic solution (Figure 21.2). The skeleton ionic reaction shows only the oxidized and reduced species ... 

Most oxidation reactions with chromium(VI) are carried out under acidic conditions The redox potential for chromium(VI) under acidic (pH = 0) conditions (Eq. 1) is + 1.33 V, whereas... 

Inorganic anions such as iodide, iodate, nitrate, and chlorate have been determined on the basis of redox reactions. Iodides have been determined by their reaction with chromium(vi) in an acidic medium. The unreacted Cr is then extracted into MIBK from a 3 M HCl solution. The iodide concentration is quantitatively related to the increase of the Cr atomic signal in aqueous solution or the decrease of the Cr signal in the organic phase. Selenium(iv) is reduced by iodide to elemental selenium. The atomic absorption signal of selenium is then measured in the selenium precipitate. 

For speciation studies it is very important that the analyses be performed as soon as possible after collection. If samples have to be stored, the risk of Cr(VI) reduction, especially in the presence of organic material under acidic conditions, has to be considered. Cr(VI) losses in the range of 15-20% have been observed during the first day after sampling. In contrast, Cr(III) is remarkably susceptible to oxidation in alkaline medium. Only in neutral solutions are the different chromium species comparatively resistant to redox reactions, partly because... 

Apart from acid-base titrations various addition-, substitution- and redox-reactions have been found to be of analytical interest. Iodine numbers of fats and essential oils may be determined i and bromine may be used to titrate organic compounds which can form bromoderivatives i. For the titration of phenol with bromine the addition of sodium acetate has been recommended. Redox reagents are in acetic acid chromium(VI) oxide, sodium permanganate, bromine, titanium(III) chloride and chromium(II) saltsi32,i33 xhe titrations are usually carried out in perchloric acid solutions and in an inert atmosphere but traces of water are tolerable. 

Alkali ferricyanides oxidize acid solutions of diethylaniline and other aromatic amines, as well as certain monoazo dyestuffs a change of color results and ferrocyanide is formed. This reversible redox reaction, proceeds very slowly and incompletely. However, if the ferrocyanide ions are removed as insoluble white zinc ferrocyanide, the oxidation proceeds rapidly. This acceleration is a result of raising the oxidation potential of ferricyanide ions through removal of ferrocyanide ions. The white zinc ferrocyanide is deeply tinted by adsorption of the colored quinoidal oxidation products of the amines, and thus affords a sensitive test for zinc. The test is especially useful in the presence of chromium and aluminum. It can also be used in other instances provided no other cations are present, which form colored precipitates with potassium ferrocyanide (Co+, Ni+, Fe+, Mn+ , Cu+ ). Traces of iron, which in practice are always to be reckoned with, do not interfere with the zinc test. Anions which oxidize the amines must be absent, e.g., permanganate, chromate, vanadate, persulfate, iodate. 

In iron-chromium redox flow cells, carbon materials are commonly used as the (inert) electrodes (carbon fiber cloth, felt, etc.). Johnson and Reid (1985) suggested depositing traces of lead and gold (p,g/cm ) on the carbon material of the electrode in the chromium redox system (the chromium electrode ) in order to accelerate the electrode reaction. The solution for the positive half-cell usually contains a certain concentration of hydrochloric acid (HCl) in addition to FeCls and FeCl2. 

Chromium at oxidation state -i-III is under the form of chromic ions Cr + Chromic hydroxide Cr(OH)3 already precipitates at weakly acidic pH [Als(Cr(OH)3)= 10 °]. The predominance diagram /pH of the different species of chromium is given in Fig. 20.3. The couple used in chromimetry is Cr207 /Cr +. Hence, it is practiced only for pH <4. The half-redox reaction is... 

Kabir-ud-Din, Hartani, K., Khan, Z., Micellar catalysis on the redox reaction of glycolic acid with chromium(VI). Int. J. Chem. Kinet. 2001, ii(6), 377-386. 

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