Chromium standard reduction potentials

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

Table 2. Standard Reduction Potentials for Chromium Species ...

Latimer diagrams were invented by W. M. Latimer and consist of lines of text of the various oxidation states of an element arranged in descending order from left to right, with the appropriate standard reduction potentials (in volts) placed between each pair of states. The diagram for chromium in acid solution is written as ... 

Althongh the transition metals are less electropositive (or more electronegative) than the alkali and alkaline earth metals, their standard reduction potentials suggest that all of them except copper should react with strong acids such as hydrochloric acid to produce hydrogen gas. Flowever, most transition metals are inert toward acids or react slowly with them becanse of a protective layer of oxide. A case in point is chromium ... 

Why does chromium seem to be less reactive than its standard reduction potential suggests ... 

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. 

Thermal deamination of tris(ethylenediamine)chromium(III) complexes is a standard preparative method for cis- and trans-diacidobis(ethylenediamine) complexes421,422 and the thermal behaviour of the starting materials has been related to their crystal structures.423 The cyano complex cis-[Cr(CN)2(en)2]C104 in DMSO undergoes stepwise reduction III— 11 — I at the DME. The standard redox potential for the Cr /Cr11 couple is -1.586 V (versus SCE). 

Similarly, from the Tables 1.17 and 1.18 we can see, for example, that permanganate ions (in acid medium) can oxidize chloride, bromide, iodide, iron(II), and hexacyanoferrate(II) ions, also that iron(III) ions may oxidize arsenite or iodide ions but never chromium(III) or chloride ions etc. It must be emphasized that the standard potentials are to be used only as a rough guide the direction of a reaction will depend on the actual values of oxidation-reduction potentials. These, if the concentrations of the species are known, can be calculated easily by means of the Nernst equation. 

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 ... 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

The corrosion products, that are produced in corrosion of higher chromium-alloyed steels, contain chromium salts that lower pH levels considerably in the process of hydrolysis. To investigate the potential influence of this more pronounced pH level reduction, steels with different chromium contents were tested [66]. Table 75 lists the alloying contents of the steels used in order of increasing chromium content. These are four standard commercial types and four test melts. Bending samples were used as described in DIN 50915 [64]. The sample material was adjusted to different hardness values by means of simulated heat treatments. 

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