Stability constants of EDTA complexes

The stability of a complex is characterised by the stability constant (or formation constant) K  

Some values for the stability constants (expressed as logX) of metal-EDTA complexes are collected in Table 2.4 these apply to a medium of ionic strength 7 = 0.1 at 20 °C. 

In equation (q) only the fully ionised form of EDTA, i.e. the ion Y4 , has been taken into account, but at low pH values the species HY3, H2Y2, H3 Y and even undissociated H4Y may well be present in other words, only a part of the EDTA uncombined with metal may be present as Y4. Further, in equation (q) the metal ion M + is assumed to be uncomplexed, i.e. in aqueous solution it is simply present as the hydrated ion. If, however, the solution also contains substances other than EDTA which can complex with the metal ion, then the whole of this ion uncombined with EDTA may no longer be present as the simple hydrated ion. Thus, in practice, the stability of metal-EDTA complexes may be altered (a) by variation in pH and (b) by the presence of other complexing agents. The stability constant of the EDTA complex will then be different from the value recorded for a specified pH in pure aqueous solution the value recorded for the new conditions is termed the apparent or conditional stability constant. It is clearly necessary to examine the effect of these two factors in some detail. 

The factor at can be calculated from the known dissociation constants of EDTA, and since the proportions of the various ionic species derived from EDTA will be dependent upon the pH of the solution, a will also vary with pH a plot of log a against pH shows a variation of logoc = 18 at pH = 1 to loga = 0 at pH = 12 such a curve is very useful for dealing with calculations of apparent stability constants. Thus, for example, from Table 2.4, log K of the EDTA complex of the Pb2+ ion is 18.0 and from a graph of log a against pH, it is found that at a pH of 5.0, log a = 7. Hence from equation (30), at a pH of 5.0 the lead-EDTA complex has an apparent stability constant given by  

These results imply that at the specified pH the magnesium complex is appreciably dissociated, whereas the lead complex is stable, and clearly titration of an Mg( II) solution with EDTA at this pH will be unsatisfactory, but titration of the lead solution under the same conditions will be quite feasible. In practice, for a metal ion to be titrated with EDTA at a stipulated pH the value of log K H should be greater than 8 when a metallochromic indicator is used. 

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G. Anderegg, Critical Survey of Stability Constants of EDTA complexes , Pergamon, Oxford, 1977. 

The formation constants of EDTA complexes are gathered in Table 11.34. Based on their stability, the EDTA complexes of the most common metal ions may be roughly divided into three groups ... 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

EDTA is a very unselective reagent because it complexes with numerous doubly, triply and quadruply charged cations. When a solution containing two cations which complex with EDTA is titrated without the addition of a complex-forming indicator, and if a titration error of 0.1 per cent is permissible, then the ratio of the stability constants of the EDTA complexes of the two metals M and N must be such that KM/KN 106 if N is not to interfere with the titration of M. Strictly, of course, the constants KM and KN considered in the above expression should be the apparent stability constants of the complexes. If complex-forming indicators are used, then for a similar titration error KM/KN z 108. 

In a similar manner, in a solution containing the species Hg2+, HgY2-, MY,n 4)+ and M"+, where Y is the complexing agent EDTA and M"+ is a metallic ion which forms complexes with it, the concentration of the mercury ion is controlled by the stability constants of the complex ions MYhigh stability constant), and the concentration of the metal ions M"+. Hence, a mercury electrode placed in this solution will acquire a potential which is determined by the concentration of the ion M"+. 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

EDTA, the abbreviation for ethyldiaminotetraethyl acid, usually is identified as H4Y. When dissolved at different pH values it exists in different forms. In strong acid solutions (pH < 1), it exists primarily as HgY. In the pH range of 2.67-6.16 it exists mainly in the form. At a pH > 10.26, it exists mainly in the Y form. The pH value employed for the separation of RE elements with EDTA as the chelate displacer is the key to a successful operation. The RE elements can be separated by using EDTA because the stability constants of the complexes they form are different. These stability constants increase with the increasing atomic number of the RE elements. In the steady-state zones obtained for the RE elements in the course of their separation, the pH value in each element separation zone is sizably different. The more stable the complex, the lower is the pH. Conversely, the less stable the complex, the higher is the pH. The pH values associated with some of the RE elements in their respective steady-state zones are listed in Table 8. 

Compared to later elements in their respective transition series, scandium, yttrium and lanthanum have rather poorly developed coordination chemistries and form weaker coordinate bonds, lanthanum generally being even less inclined to form strong coordinate bonds than scandium. This is reflected in the stability constants of a number of relevant 1 1 metal-edta complexes ... 

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

There is an appreciable difference between the stability constants of the CDTA complexes of barium (log K = 7.99) and calcium (log K = 12.50), with the result that calcium may be titrated with CDTA in the presence of barium the stability constants of the EDTA complexes of these two metals are too close together to permit independent titration of calcium in the present of barium. 

The transport of EDTA into a bacterial strain capable of its degradation has been examined (Witschel et al. 1997). Inhibition was observed with DCCD (ATPase inhibitor), nigericin (dissipates ApH), but not valinomycin (dissipates Av /), and was dependent on the stability constant of metal-EDTA complexes. 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

Speciation of Pb(II) in Glatt river. The concentrations given for CO2, Pb(II), Cu(II) and [Ca2+] as well as for the pollutants EDTA and NTA are representative of concentrations encountered in this river, The speciation is calculated from the surface complex formation constants determined with the particles of the river and the stability constants of the hydroxo-, carbonate-, NTA- and EDTA-complexes.The presence of [Ca2+] and [Cu2+] is considered. 

Table 9.1, provides the values of the logarithms of stability constants (K) of EDTA-complexes of certain metals normally occurring in pharmaceutical substances ... 

There have been many studies of the complex formation of plutonium with ethylene-diaminetetraacetic acid (EDTA) and DTPA. Over the range pH 1.0 to 3.0 Pu(III) formed 1 1 complex with EDTA with a stability constant of 1.3 x 1018 (22). At pH 3.3 Pu(IV) reacted with EDTA to form two complexes with Pu EDTA ratios of 1 1 and 1 2 with stability constants of 4.5 x 1012 and 1.6 x 1024, respectively. 

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