Calcium complexes, stability constants

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. 

Table 6. Free calcium concentrations in equilibrium with common complexing agents. A low free calcium concentration implies effective complexation, whether the complex formed is soluble or insoluble. The data were derived from either stability constants (soluble complexes) or solubility products (insoluble complexes).

Stability constants for calcium complexes of a selection of hydroxycarboxylate ligands are listed in Table VII (239,246,272-274). For tartrate, malate, and citrate stabilities decrease in the expected order Ca2+> Ba2+> Ra2+ (231,275). The stability constant for the complex of pyruvate (logiOifi 0.8 (273)) is similar to that for acetate calcium complexes of a-ketoglutarate and of oxaloacetate are somewhat more stable (logio-Ki = 1.3, 1.6 respectively (273)). The sequence logio-Ki = 3.0, 1.4, 1.1, 0.6 for the dicarboxylate ligands oxalate, malonate, succinate,... 

The temperature dependence of stability constants has been documented for a few calcium complexes, giving estimates for... 

Most of this section will be devoted to summarizing information relating to the stability constants reported for complexes of this group of Ca2+-binding ligands. However, we shall precede this main part with a short mention of a few relevant structures. Other properties of calcium phosphates and phosphonates will be mentioned in Sections VIII.B.4 and VIII.D below. An overall view of complexes of nucleosides, nucleotides, and nucleic acids is available (670). 

Fig. 2. Overview of stability constants (logio-Ka, on the molar scale) for formation of calcium complexes in aqueous solution, at (or close to) 298 K and in ionic strengths in the region of 0.1-0.15 M.

In contrast to the ionic complexes of sodium, potassium, calcium, magnesium, barium, and cadmium, the ease with which transition metal complexes are formed (high constant of complex formation) can partly be attributed to the suitably sized atomic radii of the corresponding metals. Incorporated into the space provided by the comparatively rigid phthalocyanine ring, these metals fit best. An unfavorable volume ratio between the space within the phthalocyanine ring and the inserted metal, as is the case with the manganese complex, results in a low complex stability. 

By extrapolation of the changes in the chemical shifts in the NMR spectrum of epi-inositol, the stability constant of the calcium complex has been calculated to be ca. 3M"1—i.e., about 70% of the cyclitol is present as a cationic complex in 1M calcium chloride solution. However, the extrapolation cannot be accurately carried out because there are secondary changes in chemical shifts caused by weaker complexing at two oxygen atoms. 

Table 5 pif Values and Stability Constants of Calcium Complexes with Ligands of Type (8) at i = 0.1 and 20 C... 

Tables 2.12 and 2.13 list the logarithm of the stability constants for the complexes of these chelating agents with various metal ions. Note that with the exception of Chel-138, calcium and magnesium form rather stable complexes with these chelating agents Fe3+ forms the most stable chelate of any metal listed. Generally, ferric iron is followed by Cu2+, Zn2+, Mn2+, Fe2+, Ca2+, and Mg2+. The weak acid properties of these chelating agents must be considered in any evaluation of their behavior. Because they are weak acids, the hydrogen ion tends to compete with the metal ions for association with the active groups.

As seen in Table 2.10, the stability constants of the tartarate and citrate complexes of lead and calcium ions are much smaller than those of EDTA and DTPA complexes. The calculations show that the dominant species of cations are Pb2+ and Ca2+ at pH < 4, and so the same ion exchange can be expected as would happen without complex-forming agents. At pH > 4, the effect of citric acid is significantly higher than expected from stability constants. The structure of citrate... 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

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