Example Oxidation States of Manganese

Manganese can exist in an unusually large number of oxidation states in 1.0 mol aqueous acid, as summarized in Eq. 15.24, and these afford a colorful illustration of how E° data can be manipulated. Some examples follow. 

Identification of species likely to disproportionate. According to thermodynamics, the half-reaction 

The Mn04 /Mn02 couple, however, is much less oxidizing in 1.0 mol L aqueous alkali ([H+] = 1 x 10 mol L ) than in aqueous acid  

The Mn +/Mn + couple is ostensibly pH independent, but we must bear in mind that manganese(III), in particular, will hydrolyze unless the acidity is very high, and both Mn(OH)3 and Mn(OH)2 will come out of solution in alkaline media. Small degrees of hydrolysis in solution have little impact on E°, but precipitation of hydroxides (and all metal hydroxides except those of the alkali metals and Ca, Sr, Ba, and Ra are poorly soluble in water) affects E° profoundly. Thus, in alkaline media, the electrode potentials (often called E by geologists, wherever [H+j is not the standard value) of Mn + and Mn + are controlled by the solubility products (RTsp) of Mn(OH)2 and Mn(OH)3, respectively. In practice, Mn(OH)3 tends to dehydrate to MnO(OH), so we consider the Mn +(aq)/Mn(s) couple  

Effects of complexation. Just as Kgp of an insoluble phase can control the concentration of free metal ion and hence E, so control by complexing the metal ion can alter E°. For example, E° for the half-reaction 

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