The following table provides E 1/2 values for selected reduction reactions. Values are from Dean, J. A. Analytical Chemistry Handbook , McGraw-Hill: New York, 1995. Element \(E_{1/2}\) (volts vs. SCE) Matrix \(\ce{Al^{3+}}(aq) + \ce{3 e-} \ce{<=>} \ce{Al}(s)\) –0.5 0.2 M acetate (pH 4.5–4.7) \(\ce{Cd^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Cd}(s)\) –0.6 0.1 M KCl 0.050 M H2SO4 1 M HNO3 \(\ce{Cr^{3+}}(aq) + \ce{3 e-} \ce{<=>} \ce{Cr}(s)\) –0.35 \((+3 \ce{->} +2)\) –1.70 \((+2 \ce{->} 0)\) 1 M NH4Cl plus 1 M NH3 1 M NH4+/NH3 buffer (pH 8–9) \(\ce{Co^{3+}}(aq)+ \ce{3 e-} \ce{<=>} \ce{Co}(s)\) –0.5 \((+3 \ce{->} +2)\) –1.3 \((+2 \ce{->} 0)\) 1 M NH4Cl plus 1 M NH3 \(\ce{Co^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Co}(s)\) –1.03 1 M KSCN \(\ce{Cu^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Cu}(s)\) 0.04 –0.22 0.1 M KSCN 0.1 M NH4ClO4 1 M Na2SO4 0.5 M potassium citrate (pH 7.5) \(\ce{Fe^{3+}}(aq) + \ce{3 e-} \ce{<=>} \ce{Fe}(s)\) –0.17 \((+3 \ce{->} +2)\) –1.52 \((+2 \ce{->} 0)\) 0.5 M sodium tartrate (pH 5.8) \(\ce{Fe^{3+}}(aq) + \ce{e-} \ce{<=>} \ce{Fe^{2+}}(aq)\) –0.27 0.2 M Na2C2O4 (pH < 7.9) \(\ce{Pb^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Pb}(s)\) –0.405 –0.435 1 M HNO3 1 M KCl \(\ce{Mn^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Mn}(s)\) –1.65 1 M NH4Cl plus 1 M NH3 \(\ce{Ni^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Ni}(s)\) –0.70 –1.09 1 M KSCN 1 M NH4Cl plus 1 M NH3 \(\ce{Zn^{2+}}(aq) + \ce{2 e-} \ce{<=>} \ce{Zn}(s)\) –0.995 –1.33 0.1 M KCl 1 M NH4Cl plus 1 M NH3