Complex instability constant

It should be apparent that the larger the equilibrium constants for these reactions, the more unstable the complexes. Consequently, the equilibrium constants for these reactions are called dissociation or instability constants. One must remember that the first step in the dissociation of ABm corresponds to the reverse of the last step in the formation of the complex. Thus, if we represent the instability constants as fc and a total of six steps are involved, the relationships between the fe, and K values are as follows. 

Nalidixic acid and sodium nalidixate form a strong colored complex with iron III. The maximum of absorbance of the complex is 410 nm and Beer s Law is obeyed from 10 to 250 yg of nalidixic acid per ml(33)5 and from 0.43 to 17.05 mg iron III per ml.(34) Nalidixic acid complexes through the oxo-group on C-4 and the carboxylic acid group on C-3. Three moles of nalidixic acid complex with one mole of iron III. The instability constant of the complex was calculated to be 2.11 x 10 8 by Dick and Murgu.(34)... 

K B. Yatsimirksii, V. P. Vasil ev, Instability Constants cf Complex Compounds, Pergamon, New York, 1960. 

Vasilev, V. P. Influence of ionic strength on the instability constants of complexes. Russ. J. Inorg. Chem., 1962, 7, 8, 924 927. 

Yatsimirksii KB, Vasil ev VP (1960) Instability constants of complex compounds. Pergamon, New York... 

The equilibrium constant for the formation of a metal complex is known as its stability constant. (Some authors, however, present the datum as its reciprocal, the instability constant of the complex, by analogy with the dissociation of a weak acid.) There are two kinds of stability constants stepwise (Ad, Ad, Ad,..., Kn) and overall (/ ). We will assume that there are six aqua ligands to be replaced by some other unidentate ligand Xx, in an aqueous solution of Mm+ ... 

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. 

As the complex becomes more stable, the formation constant becomes larger. The reciprocal of the formation constant is called the instability constant. 

Yatsimirski, K. B., Vasilev, V. P. Instability Constants of Complex Compounds. Oxford Pergamon Press 1960. 

Potentiometric, cryoscopic, and conductometric measurements of the reaction between Fe3+ and B40 have revealed the presence of FeB4Of (373), and in oxalate media, the cationic complexes FeB02+ and Fe(B02)2 were assumed to be present (370). The instability constant for FeBOi+ was estimated as 3.12 x 10 by spectrophotometric techniques (68). The appearance of IR absorption bands at 1260,1105, 860, and 815 cm 1 was attributed to the formation of an iron borate complex during oxidation of elemental iron in borate solutions (118). 

Instability constants for the formation of Al(BO )s, AlBO +, and Al(B02)i have been evaluated (374), although the reliability of the results is in question. Other metals that have been examined in borate solutions include lead (372), cobalt (270, 371), zinc, and cadmium (369), providing evidence for Pb(B02>3, PbB02, CoIBOa) , NilBO s, ZnlBO, and Cd(BO )S . Silver is believed to form a weak neutral complex AgB02 (177). 

Stability constant — is the - equilibrium constant of formation of a - complex. The reciprocal quantity is called instability constant. Extensive compilations of various kinds of stability constants are available [i—iv]. Stability constants are reported for a given ionic strength (or extrapolated to zero ionic strength), and pH. In many cases, stability constants can be determined with the help of electrochemical techniques, e.g., -> chronopotentiometry, -> potentiometry, -> polarogra-phy, and - voltammetry, provided that the systems exhibit -> electrochemical reversibility, and some other prerequisites are fulfilled [v]. See also -> conditional equilibrium constants. 

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... 

Yatsimirskii, K.B., and Vasilev, V.P. "Instability Constants of Complex Compounds. 214 p. Consultants Bureau, New York, 1960. 

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. 

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