Chromium standard electrode potential

The standard Gibbs energies and the standard electrode potentials for the reactions (22) and (23) were calculated according to the thermodynamic data available in the literature.10 The cathodic reactions include the reduction of chromium Cr(VI) to Cr(III) and hydrogen evolution, as presented in the following equations ... 

Theoretically, by poising the potential of an electrochemical cell at a value which is sufficient to reduce chromium(III) but not aluminium(III), chromium could be removed preferentially from solution. As chromium is a common contaminant of bauxitic alloys (the main feedstock for aluminium industry) electrochemistry may provide a means of selectively removing chromium from aluminium products. However, this process may be impractically slow. Much depends on the relative concentrations of aluminium and chromium, temperature, pH and cell design. Nevertheless, standard electrode potentials can be used as a preliminary evaluation of the feasibility of electrochemical methods for clean-up. 

Flengas and coworkers [2,3] have measured the equilibrium potentials of chromium in a molten equimolecular mixture of sodium and potassium chlorides at various concentrations of its ions, based on the ratio to a silver reference electrode. They determined [2,3] the standard electrode potentials E cr(ii)/Cn E°ci(niycr and the redox potential E°cr(m)/cr(U)-... 

The application of inorganic electroactive compounds in aqueous RFBs have been the subject of the vast majority of the literature to date. Table 2 summarizes the standard electrode potentials of common redox couples while Table 3 highlights prominent cell chemistries based on combinations of these redox couples. Of these chemistries, iron-chromium (ICB) [33], polysulfide-bromide (PSB) [14], and aU-vanadium (VRB) [31] systems have yielded industry-level demonstrations (order of 100 kW-10 MW). Below, these RFB chemistries are introduced in some detail with key advantages, disadvantages, and challenges highlighted. 

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To... 

Use data from the text to construct a standard electrode potential diagram relating the following chromium species in acidic solution. 

While the laws governing electrode potentials in non-aqueous media are basically the same as for potentials in aqueous solutions, the standardization in this case is not so simple. Two approaches can be adopted either a suitable standard electrode can be selected for each medium (e.g. the hydrogen electrode for the protic medium, the bis-diphenyl chromium(II)/ bis-diphenyl chromium(I) redox electrode for a wide range of organic... 

A suitable extrathermodynamic approach is based on structural considerations. The oldest assumption of this type was based on the properties of the rubidium(I) ion, which has a large radius but low deformability. V. A. Pleskov assumed that its solvation energy is the same in all solvents, so that the Galvani potential difference for the rubidium electrode (cf. Eq. 3.1.21) is a constant independent of the solvent. A further assumption was the independence of the standard Galvani potential of the ferricinium-ferrocene redox system (H. Strehlow) or the bis-diphenyl chromium(II)-bis-diphenyl chromium(I) redox system (A. Rusina and G. Gritzner) of the medium. 

What should be the standard cell potential of a cell consructed by using nickel (Ni) and chromium (Cr) electrodes ... 

Based on the standard reduction potentials given above, if a silver electrode and a chromium electrode are connected in a voltaic cell, which electrode will undergo oxidation and which will undergo reduction Explain how you can tell. 

Practice Problem B A galvanic cell with Eceii = 0.30 V can be constructed using an iron electrode in a 1.0 A/Fe(N03)2 solution, and either a tin electrode in a 1.0 A/ Sn(N03)2 solution, or a chromium electrode in a 1.0 MCr(N03)3 solution—even though Sn /Sn and Cr /Cr have different standard reduction potentials. Explain and give the overall balanced reaction for each cell. 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

Tab. 2 Potentials of zero charge ( pzc) for pc-Au electrode in contact with selected nonaqueous solvent-electrolyte systems. The pzc for pc-Au IH2O is equal to 0.20 V versus aqueous standard hydrogen electrode (SHE) and 0.85 V versus bis(biphenyl)chromium(l)/(0) redox couple (BBCr) [4]...

An increasing interest in nonaqueous media, which began in the seventies of the XXth century, also resulted in the extended studies on the properties of the relevant Hg[solvent interface. Most of the papers discussing the dependence of Epzc of Hg on the solvent used were published in the eighties [2]. The pzc values for selected solvents, including water for comparison, are collected in Table 1. They are expressed versus standard hydrogen electrode (SHE) and, in order to eliminate the unknown liquid junction potential, also versus the bis(biphenyl)chromium(l)/(0) standard potential, that was assumed to be solvent independent. 

The standard potentials of these two electrode reactions are significantly lower than that of the oxygen electrode. The transpassive dissolution of chromium therefore can occur at potentials well below those needed for oxygen evolution. As an illustration, Figure 6.36 shows anodic polarization curves for the transpassive... 

Physical methods of analysis normally involve a measurement of a physical parameter other than mass or volume. For example, a water sample suspected of being polluted with hexavalent chromium can be injected into an inductively coupled plasma atomic emission spectrometer (ICP/AES) and the intensity of light given off by the very hot chromium atoms emitted by the sample measured to give the chromium concentration. Or fluoride in a water sample can be determined by measuring the potential versus a reference electrode of a fluoride ion-selective electrode immersed in the sample and comparing that value with the potential measured in a standard F" solution to give the value of [F"]. 

Very recently, Harrington and co-workers (55) have used controlled-potential coulometry at 0.0 V vs. Hg/HgSO reference electrode to simultaneously prepare and standardize solutions of chromium (III) in 1 M sulphuric acid. A precision of better than 0.1 per cent was claimed. 

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