Vanadium potential diagram

Reduction of yellow [V02] in acidic solution yields successively blue [VO] +, green and violet The potential diagram in Figure 21.8 shows that all oxidation states of vanadium in aqueous solution are stable with respect to disproportionation. 

FIGURE 3 Oxidation state-potential diagrams for vanadium and chromium [1. OM H+(aq)]. 

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

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

Figure 1 Potential versus pH diagram for the vanadium-water system at 25 °C. The dashed lines indicate the domains of relative predominance of the dissolved forms of the metal, but the various dissolved forms for each oxidation state are not explicit. The solid lines correspond to saturated solutions with a total vanadium concentration of 0,51 gdm-3. The long dashed lines correspond to oxidation and reduction of water (for E° values of 1.23 and 0.00 V respectively) (adapted from E. Deitombe, N. Zoubov and M. Pourbaix, in Atlas d Equilibres Electrochimiques , ed. M. Pourbaix,...

Fig. 1. Reduction potential E (referenced to the standard hydrogen electrode) versus pH for various species of vanadium. Boundary lines correspond to E, pH values where the species in adjacent regions are present in equal concentrations. The short dashed lines indicate uncertainty in the location of the boundary. The upper and lower long dashed lines correspond to the upper and lower limits of stability of water. Standard reduction potentials are given by the intersections of horizontal lines with the abscissa pH = 0. The half reactions are 02 + 4H+ + 4e = 2H20, E° = 1.23V V02+ + 2H+ + e = V02+ + H20, E° = 1.0V V02+ + 2H+ + e = V3+ + H,0, E° = 0.36V 2H+ + 2e = H2, E° = 0.0V and V3+ + e = V2+, E° = -0.25V. V2+ is therefore a strong reductant. Air oxidation of V02+ presumably proceeds by the reaction 4V02t + 02 + 2H20 = 4VOJ + 4H+, E° = 0.23V which is favored at higher pH. Not all known species are represented on this diagram. Reproduced with permission from Ref. 30...

Figure 1 Diagram of potential (vs. SHE) vs. pH for vanadium species present in dilute solutions. The upper and lower dashed lines represent 1 atm 02 and 1 atm H2, respectively (reproduced with permission from Baes and Mesmer 3 1976,Wiley. This material is used by permission of Wiley).

S19.1 The Latimer diagram for vanadium in basic solution is provided in Resource Section 3. The reduction potential for O2/H2O couple in alkaline solution is ... 

Baes-Messmer diagram for the aqueous vanadium system. Standard redox potentials (in V) vs the normal hydrogen electrode (NHE) are given. The two parallel dashed lines running from the upper left to the lower right indicate the range of stability of water. Reproduced from C. F. Baes and R. E. Messmer, The Hydrolysis of Cations, pp. 197-210. Copyright (1976), with permission from John Wiley Sons, Ltd. 

Figures 3 and 4 show diagrams for the transition metals vanadium and chromium and for the nonmetal nitrogen, all in l.OM aqueous acid solution. The slopes of lines joining redox couples represent the potentials for the halfreactions in question, a positive slope representing a reduction potential and a negative slope an oxidation potential. An intermediate state above tie lines joining higher and lower states will be unstable to disproportionation [e.g., Cr(V) and NO2] while species below such tie lines are stable [e.g., Cr(III) and N2]. The diagrams are a convenient way to postulate the feasibility of various possible one-and two-electron pathways in multistep redox processes involving a particular element.

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