Formation Constants of Complex Ions

The equilibrium constant for the formation of a complex ion is called a formation constant (or stability constant) and given the symbol Kf. A typical example is 

The large Kf value means that the forward reaction goes virtually to completion. Addition of ammonia to a solution of AgN03 will convert nearly all the Ag+ ions to Ag(NH3)2+ (see Example 15.6). 

This means that in 0.10 M NH3 there are 170,000 Ag(NH3)2+ complex ions for every Ag+ ion. 

From Example 15.6 we see that the reaction for the conversion of Ag(NH3)2+ to Ag(S203)23 has a large equilibrium constant. Looking at it in a slightly different way, we can say that of these two complexes of Ag+, the Ag(S203)23 ion is the more stable. 

Addition of Cl- ions to the yellow complex Co(NH3)63+ converts it to the more stable complex Co(NH3)5Q2+, which has an intense purple color (Table 15.3). 

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Table 15.4 lists formation constants of complex ions. In each case, Kt applies to the formation of the complex by a reaction of the type just cited. Notice that for most complex ions listed, Kf is a large number, 10s or greater. This means that equilibrium considerations strongly favor complex formation. Consider, for example, the system... 

Equations for the evaluation of formation constants of complexed ion species in cross-linked and linear polyelectrolyte systems. J. A. Marinsky, Ion Exch. Solvent Extr., 1973,4, 227-243 (18). 

D. DeFord and D. N. Hume, The Determination of Consecutive Formation Constants of Complex Ions from Polarographie Data, J. Am. Chem. Sot 73 5321 (1951). 

DEF/HUM] DeFord, D. D., Hume, D. N., The determination of consecutive formation constants of complex ions from polarographic data, J. Am. Chem. Soc., 73, (1951), 5321-5322. Cited on page 431. 

Voltammetric methods also provide a convenient approach to establish the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier sections, the standard electrode potential, the dissociation constants of weak acids and bases, solubility products, and the formation constants of complex ions can be evaluated from polarographic half-wave potentials, if the electrode process is reversible. Furthermore, studies of half-wave potentials as a function of ligand concentration provide the means to determine the formula of a metal complex. 

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