Complex ions instability constants

The hydroxo complex ions formed in this way have instability constants, just as ammine or other complexes do. These instability constants are somewhat special, in that one of the products of the equilibrium is the insoluble amphoteric hydroxide. Thus, for aluminum hydroxide,... 

The [A1(OH)3(s)] is omitted, as usual, because it is a solid. Because the solids are part of the equilibrium, the instability constants of these hydroxyl complex ions can be applied only to solutions that are saturated with respect to the solid. 

The constant has a value of 10 x 10" 21 at room temperature. By inspection of this expression it must be evident that if cyanide ions are present in excess, the silver ion concentration in the solution must be very small. The lower the value of the instability constant, the more stable is the complex and vice versa. A selected list of instability constants, (all of which have importance in qualitative inorganic analysis) is shown in Table I.IS. 

It is interesting to compare these values and to predict what happens if, to a solution which contains the complex ion, a reagent is added which, under normal circumstances, would form a precipitate with the central ion. It is obvious that the higher the value of the instability constant, the higher the concentration of free central ion (metal ion) in the solution, and therefore the more probable it is that the product of ion concentrations in the solution will exceed the value of the solubility product of the precipitate and hence the precipitate... 

Because of the low values of the instability constants, the complexes are practically undissociated, thus the concentration of both complex ions are 0-5 mol 1. The concentration of cyanide ions being 10 1 mol 1, the concentrations of the free metal ions can be expressed from (i) and (ii) as... 

The constant Ki (or, ) is called the instability constant of the complex ion it is apparent that the greater its value the greater the tendency of the complex to dissociate into simple ions, and hence the smaller its stability. The reciprocal of the instability constant is sometimes encountered it is referred to as the stability constant of the complex ion. 

Determination of Instability Constant.—Two methods have been mainly used for determining the instability constants of complex ions one involves the measurement of the e.m.f. s of suitable cells, which will be described in Chap. VII, and the other depends on solubility studies. The latter may be illustrated by reference to the silver-ammonia (argent-ammine) complex ion. If the formula of the complex is Agm(NH8)n , the... 

By expressing the concentration, or activity, of the ions in the titrated solution, and hence the potential of an M electrode, in terms of c, the initial concentration of the solution, x, the amount of titrant added, and fc the instability constant of the complex ion, it is possible, utilizing the method of differentiation described in connection with precipitation titrations (page 258), to show that dE/dx is a maximum at the point corresponding to complete formation of the complex ion. Further, the value of dEldx at this point, and hence the sharpness of the inflection in the titration curve, can be shown to be greater the smaller the instability constant. 

Sillen and Martell s Stability constants of metal-ion complexes [1]. This compilation is sufficiently comprehensive to show data for a variety of ligands and generally includes either the proposed ligand or a very similar compound. There is also sufficient information to investigate selectivity for a diverse collection of metals, providing clues to selectivity. Other compendia of data are available, such as the CRC handbook and Yatsimirskii and Vasiliev s, Instability constants of complex compounds [2]. An especially useful source of data on complex compounds is Gmelin and Meyer [3]. 

Values of p/fh from Yatsimirksii. K. B. Vasil ev, V. P. instability Constants of Complex Compounds Pergamon Elmsford, NY. 1960, except for Bi, Hf, Lu. Pu, Sc, and Tl. which arc from Stability Constants of Metal-Ion Complexes Part II, Inorganic Ligands Bjenum, J. Schwarzenbach. 0. Sillen, L. G.. Eds. The Chemical Society London, 1958. For many elements there is considerable uncertainty in the hydrolysis constants not only as a result of experimental errors but also because some have not been corrected to infinite dilution. Z /r values were calculated from ionic radii in Table 4.4. 

TURyRUV] Tur yan, Y. 1., Ruvinskii, 0. E., Sulphate anion effect on kinetic and catalytic polarographic currents of nickel(ll) aquo ion and nickel(Il) complexes. Determination of the instability constant of nickel-sulphate complex, J. Electroanal. Chem. Interfacial Electrochem., 28, (1970), 381-390. Cited on pages 181, 182, 183, 187, 344. 

Moskvin AI, Khalturin GV, Gel man AD (1962) Determination of the composition and instability constants of citrate and tartrate complexes of americium(III) by the ion-exchange method. Radiokhim 4 162-166... 

A number of statements appear in the literature to the effect that, in dilute aqueous solution, silver ions do not combine with hydroxyl ions. . . whereas they do combine with ammonia. The solubility product for silver hydroxide and the instability constant for the ammonia complex, however, are respectively 2 X 10"" and... 

Menger et al. synthesized a Ci4H29-attached copper(II) complex 3 that possessed a remarkable catalytic activity in the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) and the nerve gas Soman (see Scheme 2) [21], When 3 was used in great excess (ca. 1.5 mM, which is more than the critical micelle concentration of 0.18 mM), the hydrolysis of DNP (0.04 mM) was more than 200 times faster than with an equivalent concentration of the nonmicellar homo-logue, the Cu2+-tetramethylethylenediamine complex 9, at 25°C and pH 6 (Scheme 4). The DNP half-life is calculated to be 17 sec with excess 1.5 mM 3 at 25°C and pH 6. The possible reasons for the rate acceleration with 3 were the enhanced electrophilicity of the micellized copper(II) ion or the acidity of the Cu2+-bound water and an intramolecular type of reaction due to the micellar formation. On the basis of the pH(6-8.3)-insensitive rates, Cu2+-OH species 3b (generated with pK3 < 6) was postulated to be an active catalytic species. In this study, the stability constants for 3 and 9 and the thermodynamic pvalue of the Cu2+-bound water for 3a —> 3b + H+ were not measured, probably because of complexity and/or instability of the metal compounds. Therefore, the question remains as to whether or not 3b is the only active species in the reaction solution. Despite the lack of a detailed reaction mechanism, 3 seems to be the best detoxifying reagent documented in the literature. 

In aqueous solution only low equilibrium concentrations of Cu+ (<10-2 M) can exist (see later) and the only simple compounds that are stable to water are the highly insoluble ones such as CuCl or CuCN. This instability toward water is due partly to the greater lattice and solvation energies and higher formation constants for complexes of the CuB ion, so that ionic Cu1 derivatives are unstable. Of course numerous Cu1 cationic or anionic complexes are stable in aqueous solution. 

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