contact ion pair. When the water molecule(s) of the second hydration shell are replaced by ligands, the complex is said to be an outer-sphere complex, or solvent-shared ion pair. The formation of solvent-shared or contact ion pairs is particularly relevant to the determination of solvation numbers of aqua ions by methods that require the use of concentrated solutions of salts, as ion pairing is concentration-dependent. Consider, for example, the formation of the complex [MgCl]+ in solutions of MgCl2. The formation constant K of the complex is about 1 but varies with ionic strength. The concentration of the rather weak complex increases from about 0.1% for a 10mM solution to about 70% for a 1M solution (1M = 1 mol dm−3). Electrochemistry The standard electrode potential for the half-cell equilibrium Mz+ + z M(s) has been measured for all metals except for the heaviest trans-uranium elements. {| class=wikitable style=text-align:center |+Standard electrode potentials /V for couples Mz+/M(s) |- | H0 |- | Li−3.040|| Be2+ −1.85 |- | Na −2.71|| Mg2+ −2.372|| || || || Al3+−1.66 |- | K −2.931|| Ca2+ −2.868|| Sc3+−2.90 |...||Zn2+ −0.751||Ga3+ −0.53||Ge2+ +0.1 |- | Rb −2.98|| Sr2+ −2.899|| Y3+−2.37 |...||Cd2+ −0.403||In3+ −0.342||Sn2+ −0.136||Sb3+ +0.15 |- |Cs −3.026|| Ba2+ −2.912||Lu3+ −2.25 |...||Hg2+ −0.854||Tl3+ +0.73||Pb2+ −0.126||Bi3+ +0.16||Po4+ +0.76 |- | Fr−2.9 || Ra2+−2.8||Lr3+ −1.96 |- | |- | || ||La3+−2.52 ||Ce3+ −2.32 ||Pr3+ −2.34||Nd3+ −2.32|| Pm3+−2.30 ||Sm3+ −2.28 |Eu3+ −1.98||Gd3+ −2.27||Tb3+ −2.27 |Dy3+ −2.32||Ho3+ −2.37||Er3+ −2.33 |Tm3+ −2.30||Yb3+ −2.23 |- | || || Ac3+ −2.18||Th4+ −1.83||Pa4+ −1.46||U4+ −1.51||Np4+ −1.33||Pu4+ −1.80 |Am3+ −2.06||Cm3+ −2.07||Bk3+ −2.03 |Cf3+ −2.01||Es3+ −1.99||Fm3+ −1.97||Md3+ −1.65||No3+ −1.20 |} {| class=wikitable style=text-align:center |+Standard electrode potentials /V for 1st. row transition metal ions !Couple!!Ti!!V!!Cr!!Mn!!Fe!!Co!!Ni!!Cu |- |M2+ / M||−1.63||−1.18||−0.91||−1.18||−0.473||−0.28||−0.228||+0.345 |- |M3+ / M||−1.37||−0.87||−0.74||−0.28||−0.06||+0.41 |} {| class=wikitable style=text-align:center |+Miscellaneous standard electrode potentials /V !Ag+ / Ag!!Pd2+ / Pd!!Pt2+ / Pt!!Zr4+ / Zr!!Hf4+ / Hf!!Au3+ / Au!!Ce4+ / Ce |- | +0.799||+0.915||+1.18||−1.53||−1.70|| +1.50||−1.32 |} As the standard electrode potential is more negative the aqua ion is more difficult to reduce. For example, comparing the potentials for zinc (-0.75 V) with those of iron (Fe(II) -0.47 V, Fe(III) -0.06 V) it is seen that iron ions are more easily reduced than zinc ions. This is the basis for using zinc to provide anodic protection for large structures made of iron or to protect small structures by galvanization. References Bibliography Further reading H. L. Friedman, F. Franks, Aqueous Solutions of Simple Electrolytes, Springer; reprint