Properties of EDTA

Stable complexes with EDTA and its related compounds are formed with most polyvalent metal ions, hence their wide use in chelation therapy. Pure EDTA is a white crystalline solid with a relative molar mass of 292.1. It is a weak, tetrabasic acid which is sparingly soluble in water. EDTA is not metabolized in the human body and is effectively non-biodegradable in the environment. 

Each of the nitrogen atoms (Table 7.2) has an unshared pair of electrons and the four acidic hydrogens (the p/f values for the ionization equilibria being pTi i = 2.0 pA 2 = 2.67 pA3 = 6.16 pA4 = 10.26) means that one EDTA molecule has six potential sites for metal ion bonding and hence may be described as a hexadentate ligand. 

The speed and precision with which EDTA is able to donate its spare electron pair to the central metal cation has meant that it has uses not only in chelation therapy and in the commercial areas listed previously, but also in complexation titrations in analytical chemistry. 

As we learned in Chapter 3, formation constants are mathematical expressions that relate the concentration of products and reactants in any equilibrium reaction. Thus the reaction between a metal ion and EDTA can be illustrated as  

The concentration of (EDTA) , and thus the ability to complex metal ions, will depend upon the pH. A decrease in pH results in an increase in the deprotonation of EDTA and hence an increase in the concentration of the ED I A ion. The effect of this is that only metal ions with a very high affinity for EDTA will be able to form stable complexes. The stability constants for the EDTA and [diethylenetriaminepentaacetic acid] - (DTPA ) complexes with some important metal ions that are of particular interest for chelation therapy are listed in Table 7.3. It is important to note that the stability of the EDTA and DTPA complexes with toxic metals, such as lead, mercury, cadmium, or plutonium are quite similar to those with essential metals such as zinc, cobalt or copper however, the Ca complex is many orders of magnitude lower. This has important implications for chelation therapy. First, the mobilization and excretion of zinc and other essential metals are likely to be increased, along with that of the toxic metal during EDTA treatment and secondly, the chelation of the ionic calcium in the blood, that can cause tetany and even death, can be avoided by administering the chelator as the calcium salt. 

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Whalley G. Preservative properties of EDTA. Manuf Chem 1991 62(9) 22-23. 

Ge, H., and Huang, S., Microwave preparation and adsorption properties of EDTA-modified cross-linked chitosan,... 

The reaction is based on the electron-donating properties of EDTA, dyad of Scheme 14.14, the electron mediator (methylviologen), colloidal Pt, acetic acid, and Triton X-100, irradiating with a 350 W Xe lamp equipped with... 

EDTA Is a Weak Acid Besides its properties as a ligand, EDTA is also a weak acid. The fully protonated form of EDTA, HeY, is a hexaprotic weak acid with successive pKa values of... 

To correct the formation constant for EDTA s acid-base properties, we must account for the fraction, ayi-, of EDTA present as Y . 

Now that we know something about EDTA s chemical properties, we are ready to evaluate its utility as a titrant for the analysis of metal ions. To do so we need to know the shape of a complexometric EDTA titration curve. In Section 9B we saw that an acid-base titration curve shows the change in pH following the addition of titrant. The analogous result for a titration with EDTA shows the change in pM, where M is the metal ion, as a function of the volume of EDTA. In this section we learn how to calculate the titration curve. We then show how to quickly sketch the titration curve using a minimum number of calculations. 

Phosphonates exhibit all the properties of polyphosphates, such as threshold effect, crystal distortion, and sequestration, but are superior in their effectiveness. They provide good chelates for calcium, magnesium, iron, and copper and are commonly used where iron fouling is a problem. Their sequestering properties are generally superior to other common chelants, such as EDTA and NTA. 

Table 1 shows some biochemical properties of the pectolytic enzymes present in pool 1. The pectin lyase/pectate lyase activities (pool I) and polygalacturonase activity (pool II) were not significantly affected by NH4+, Na+ and K+ (0,25 - 2,5mM), while Al +, p-mercaptoethanol, Hg2+, EDTA, Ba + and Zn+2 (2,5mM) inhibited 30-100% these activities. On the other hand, Ca2+, Mg + and Mn + at 2,5mM concentration activated 20-100% pectin/pectate lyases but Ca " " and Cu " " (2,5mM) inhibited polygalacturonase activity about 42 - 70%. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

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). 

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