Stability constants other carboxylates

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Formation constants for the species [M(OAc) ] "" (M = Zn or Cd n = 1—3) have been determined in addition, zinc forms a tetra-acetato-complex. Stability constants for complexation of Zn and Cd by other carboxylic and hydroxycarboxylic acids " have been reported. 

Munze (19) has used a Born-type equation to calculate stability constants of In(III) and An(III) complexes of carboxylates as well as other ligands which agreed well with experimental values. His approach was modified by allowing the dielectric constant to be a parameter (the "effective" dielectric constant, De) in an analysis of fluoride complexation by M(II), M(III) and M(IV) cations (20). A value of De = 57 was found satisfactory to calculate trivalent metal fluoride stability constants which agreed with experimental values for Ln(III), An(III) and group IIIB cations (except Al(III). Subsequently, the equation was used... 

This stability constant deals only with the species of EDTA where all carboxylic acid groups are deprotonated. The pKa values for these groups are pKai 2.0, pKa2 2.67, pKaj 6.16, and pKa410.3, so the fully deprotonated EDTA dominates only in very alkaline solutions with pH above 12. In solutions of lower pH one has to calculate [Y ] at the actual pH and use this concentration in calculating a stability constant. Similarly, if the metal M participates its other complexes, one has to calculate the [M"+] actually available for EDTA complexation. The stability constants incorporating the effects of pH and other complex reaction are called apparent stability constants. The absolute stability constant for a number of cations has been determined and is tabulated in Table 6.6. 

Lis et al. [70] have shown recently by using AI/AI2O3 electrodes that a 1 2 chelate of Eu(lll) and coumarin-3-carboxylate is electrochemiluminescent in aqueous solution. The same group has also smdied some other weak complexes that show ECL but cannot be used as labels due to their too small stability constants [71]. 

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