Formation constants with EDTA

Formation constants for EDTA complexes in Table 13-1 are large and tend to be larger for more positively charged metal ions. Note that K( is defined for reaction of the species Y" " with the metal ion. At low pH, most EDTA is in one of its protonated forms, not Y . 

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

TABLE 9.1 Formation Constants for Complexes of EDTA with Metal Cations at 25°C... 

EDTA salts are used for the treatment of heavy metal poisoning. Roosels and Vanderkeel142) were able to extract lead from urine in the presence of EDTA with dithizone by adding calcium to presumably release the lead from EDTA. In view of the fact that the formation constant of the lead-EDTA chelate is 20,000,000 times larger than that of the corresponding calcium chelate, it is doubtful that the calcium actually releases the EDTA from the lead. 

EDTA complexes of trivalent metals can be extracted successively with liquid anion exchangers such as Aliquat 336-S by careful pH control. Mixtures of lanthanides can be separated by exploiting differences in their EDTA complex formation constants. 

A new aminocarboxylate chelator of potential therapeutic value, 77(2-hydroxybenzy -Al -benzylethylenediamine-A Al -diacetate, reacts as LH4 and LH3 with Fe(OH)2q by dissociative activation with rate constants of 770 and 13 300 M s-1, respectively. These rate constants are similar to those for reaction of Fe(OH)2q with edta and with nta. These formation reactions are, however, considerably faster than with simple ligands of identical charge thanks to the zwitterionic properties of ami-nocarboxyl ates (334). 

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. 

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. 

Note that Kf for EDTA is defined in terms of the species Y4 reacting with the metal ion. The equilibrium constant could have been defined for any of the other six forms of EDTA in the solution. Equation 12-5 should not be interpreted to mean that only Y4 reacts with metal ions. Table 12-2 shows that formation constants for most EDTA complexes are quite large and tend to be larger for more positively charged cations. 

You can see from the example that a metal-EDTA complex becomes less stable at lower pH. For a titration reaction to be effective, it must go to completion (say, 99.9%), which means that the equilibrium constant is large—the analyte and titrant are essentially completely reacted at the equivalence point. Figure 12-9 shows how pH affects the titration of Ca2+ with EDTA. Below pH 8, the end point is not sharp enough to allow accurate determination. The conditional formation constant for CaY2" is just too small for complete reaction at low pH. 

With the conditional formation constant we can treat EDTA complex formation as if all the free EDTA were in one form. 

For a titration with EDTA, you can follow the derivation through and find that the formation constant, Kf, should be replaced in Equation 12-11 by the conditional formation constant, K, which applies at the fixed pH of the titration. Figure 12-12 shows a spreadsheet in which Equation 12-11 is used to calculate the Ca2+ titration curve in Figure 12-11. As in acid-base titrations, your input in column B is pM and the output in column E is volume of titrant. To find the initial point, vary pM until V, is close to 0. 

Equation 12-18 states that the effective (conditional) formation constant for an EDTA complex is the product of the formation constant, Kf. times the fraction of metal in the form M" + times the fraction of EDTA in the form Y4- K" = aM, + aY4 h t. Table 12-1 told us that aY4- increases with pH until it levels off at 1 near pH 11. 

In a direct titration, analyte is titrated with standard EDTA. The analyte is buffered to a pH at which the conditional formation constant for the metal-EDTA complex is large and the color of the free indicator is distinctly different from that of the metal-indicator complex. 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

Fig. 14.12 Formation constants at 25 C for 1 1 chelates of Lrrl+ ions with various aminepoly-carboxylate ions (ida, iminodiacetate nta, nilnlo-triacetate , A -hydroxy-elhyleihylenediaminetri-acelute edta, ethylene-dinminetetraacelate cdta, traru-1,2-cycIohcxane-diamineletraacctale dtps, diethylenetnammepenta-acetate). [From Moeller. T. J. Chem. Educ. 1970. 47, 417-430. Reproduced with permission.]...

The general interests in chelation of metals have continued to lead to new forms of polyaminopolycarboxylic acids. Examination of the acid dissociation constant of ethyl-ene-bis-N,N -(2,6-dicarboxy)piperidine (9) showed that they were nearly identical with those of EDTA. However, the formation constants for metal chelates of (9) are lower by 0.5 to 3.7 log 3 units. 

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