Calcium-EDTA complexation

In patients on EDTA therapy, calcium cannot be determined by the Indirect colorimetric or fluorometrlc methods based on the chelation of a calcium - EDTA complex. However, In calcium determinations by atomic-absorption spectroscopy, the complexlng agent Is destroyed In the flame and the direct concentration of calcium can be determined. 

Fujishiro, Y. et al.. Coating of hydroxyapatite on titanium plates using thermal dissociation of calcium-EDTA complex in phosphate solutions under hydrothermal conditions, 7. Colloid Interf. Sci., 173, 119, 1995. 

C. Replacement or substitution titration. Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium. The metal cation M + to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs ... 

An interesting application is the titration of calcium. In the direct titration of calcium ions, solochrome black gives a poor end point if magnesium is present, it is displaced from its EDTA complex by calcium and an improved end point results (compare Section 10.51). 

Notes. (1) The usefulness of the HHSNNA indicator for the titration of calcium depends upon the fact that the pH of the solution is sufficiently high to ensure the quantitative precipitation of the magnesium as hydroxide and that calcium forms a more stable complex with EDTA than does magnesium. The EDTA does not react with magnesium [present as Mg(OH)2] until all the free calcium and the calcium-indicator complex have been complexed by the EDTA. If the indicator is added before the potassium hydroxide, a satisfactory end-point is not obtained because magnesium salts form a lake with the indicator as the pH increases and the magnesium indicator-lake is co-precipitated with the magnesium hydroxide. 

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. 

Nickel may be determined in the presence of a large excess of iron(III) in weakly acidic solution by adding EDTA and triethanolamine the intense brown precipitate dissolves upon the addition of aqueous sodium hydroxide to yield a colourless solution. The iron(III) is present as the triethanolamine complex and only the nickel is complexed by the EDTA. The excess of EDTA is back-titrated with standard calcium chloride solution in the presence of thymolphthalexone indicator. The colour change is from colourless or very pale blue to an intense blue. The nickel-EDTA complex has a faint blue colour the solution should contain less than 35 mg of nickel per 100 mL. 

Calcium can be determined as the oxalate by precipitation from homogeneous solution by cation release from the EDTA complex in the presence of oxalate ion.28... 

As with (105), ligand (106, dota) also forms strong complexes with a range of both non-transition and transition metal ions (Stetter Frank, 1976 Delgado da Silva, 1982 Spirlet, Rebizant, Desreux Loncin, 1984) which are often more stable than the corresponding edta complexes. In particular, the calcium complex shows extremely high stability and very stable complexes are also formed with the trivalent lanthanides (Desreux, 1980 Spirlet, Rebizant, Desreux Loncin, 1984). 

If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected, a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or where a suitable indicator cannot be found, back titration procedures are used. A measured excess of EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution. Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will displace the zinc or magnesium. 

In the original work [12], the reagent flow-rate was varied. An example of such a titration is the determination of calcium by titration with EDTA using the Cd ISE and the Cd-EDTA complex as an electrometric indicator [105]... 

Although changes in pM can readily be followed by physical means (e.g. potentiometrically), following colour changes associated with the formation and dissociation of metal coordination complexes visually or spectrophotometrically is a more versatile and convenient procedure. The serendipitous discovery of so-called metallochromic indicators made by G. Schwarzenbach (1945) led immediately to the introduction of murexide (50) as an indicator in calcium titrations and initiated the search for indicators for other metal-EDTA systems. It will be realized that the chosen metal indicator must be considerably less stable than the metal-EDTA complex, but not so weak as to dissociate appreciably in the vicinity of the end-point when the concentration of free metal... 

EDTA is used in the separation of the rare earth elements. EDTA complexes have a stability that varies slightly from the rare earth elements. This slight variation allows EDTA to effectively separate rare-earth ions. EDTA is used as an anti-coagulant in blood. It prevents coagulation by sequestering the calcium ions required for clotting251. 

To determine the amount of calcium in a water sample, e.g. the titration with EDTA (ethylenediaminetetraacetate, C2H4N2(CH2COOH)4) can be used. First of all, NaOH is added to the sample to obtain a pH value of at least 12. Then, a color indicator is admixed and titration with EDTA performed until the color changes. In doing so, all Ca is converted to a Ca-EDTA complex and detected in this form. 

The ion-exchange equilibrium of calcium ions is determined by the ratio of the calcium ion species in the solution. Only positive calcium ions (calcium aqua complexes) can participate in the ion-exchange reaction the negative CaEDTA2- and CaHEDTA complexes do not. The ratio of CaEDTA2 and CaHEDTA- depends on the EDTA concentration and pH. When the concentration of EDTA is much smaller than the concentration of calcium ions, the quantity of EDTA complexes is negligible at any pH values. EDTA has an effect when its concentration is more... 

To summarize, calcium ions in the solution can be present as hydrated Ca2+ions, and CaHEDTA and CaEDTA2 complexes, of which only the positive calcium aqua complexes (Ca2+) participate in the ion-exchange reaction. In the process, calcium ions dissolve from montmorillonite to the solution, and mostly hydrogen ions get into the interlayer space of montmorillonite. In addition, sodium ions are also present in the system (EDTA is added as disodium salt H4EDTA is hardly soluble in water), which also affects the ion-exchange process. The ratio of cCa aCa can be plotted as a function of the concentration of Ca2+ (Figure 2.11). 

FIGURE 2.11 The ratio of cCa aCa can be plotted as a function of the concentration of Ca2+ on montmorillonite (the exchange isotherm of a calcium-hydrogen-sodium ion exchange) with and without the EDTA complex-forming agent. (Reprinted from Konya and Nagy 1998, with permission from Elsevier.)... 

The sorption of calcium ions is as expected at low pH values, the calcium ions, present as Ca2+, sorb, as in the absence of a complex-forming agent the quantity of the sorbed calcium increases when the pH increases. At pH values where negative Ca-EDTA complexes are already formed and the concentration of hydrated Ca2+ ions decreases, the sorption of calcium on the surface of montmorillonite also decreases. 

Next consider in more detail a specific example of the use of the mercury-EDTA electrode in the titration of calcium with standard EDTA. In such a titration a small amount of a solution containing mercury-EDTA complex HgY" is added to the calcium solution, and the indicating electrode is a mercury-coated gold wire. The following equilibria are involved ... 

Numerous tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Ethylenediamine tetra-acetic acid (EDTA) can be used for determination of 40 elements by direct titration using metal-ion indicators for endpoint detection. Direct titration procedures are limited to metal ions that react rapidly with EDTA. Back titration procedures are useful for the analysis of cations that form very stable EDTA complexes and for which a satisfactory indicator is not available. EDTA is also used for determining water hardness the total concentration of calcium and magnesium expressed in terms of the calcium carbonate equivalent. 

Curve A in Figure 17-6 is a plot of data for the titration in Example 17-4. Curve B is the titration curve for a solution of magnesium ion under identical conditions. The formation constant for the EDTA complex of magnesium is smaller than that of the calcium complex, which results in a smaller change in the p-function in the equivalence-point region. 

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