1- EDTA

EDTA is the most widely used chelator in analytical chemistry. By direct titration or through a sequence of reactions, virtually every element of the periodic table can be analyzed with EDTA. A titration based on complex formation is called a complexo-metric titration. 

EDTA is a hexaprotic system, designated The highlighted acidic hydrogen 

HO2CCH2 CH2CO2H applies at 25°C and ionic strength = 0.1 M, except 

One mole of EDTA reacts with one mole of metal ion. 

Neutral EDTA is H4Y. A common reagent is the disodium salt, Na2H2Y 2H2O, which attains the dihydrate composition upon heating at 80°C. 

Simple calculations showed that EDTA does not complex metal ions such as magnesiu-m(II) and calcium(II) at pH 4 but it does complex many other metal cations. Therefore, experiments were performed in which ETDA is added to the metal ion sample and the column was eluted with ethlenediammonium tartrate as before [13]. The amount of EDTA used was more than enough to complex the metal ions present, but an unduly high 

Samples containing a large excess of iron(Ill) give extremely wide pseudo peaks when the ethylenediammonium tartrate is used. This excess of iron(IIl) will totally obscure the magnesium peak while calcium and strontium appear on the tail of the pseudopeak . 

The very weak complexing of iron(ll) by tartrate suggested that iron(II) might be determined quantitatively by cation chromatography. This was proved to be true by Fritz and Sevenich [14] who determined iron(II) in the presence of iron(III) and several other metal ions. Total iron in solution was determined after a preliminary reduction to iron(II) with ascorbic add. 

Ethylenediaminetetraacetic acid (EDTA) is an industrial and analytical reagent because of its ability to complex with many divalent and trivalent metals up to a 

Nitrilotriacetic acid (NTA) is similar to EDTA (N and COOH functional groups) and forms a maximum of four bonds (tetradentate) with metal ions. It is used for reducing metal toxicity in humans and in aquatic and microbial life. 

In general, the concept of metal ion coordination polyhedra is a molecular shorthand which condenses real molecular assemblies into a simpler structural form. As such, it is a convenient tool to describe molecular entities. 

This compound is the most widely-used of the family of aminopolycarboxylic acids, which act as strong metal ion-complexing agents. Its near-ubiquity in surface waters has begun to cause concern. OH-induced degradation represents one way to cope with this problem. Radiolytic studies focusing on the action of OH have been undertaken recently [54, 55]. 

Two distinct primary transients have been observed by optical spectroscopy in its pulse radiolysis [54]. One of these is not affected by O2 and has been attributed to an A-centered radical cation, A/ N-EDTA , directly bridged to the second EDTA nitrogen. Using strong reductants as probes, e.%. N,N,N N -tetramethylphenylenediamine, G(A/ A/ -EDTA) = 1.6 x 10 mol J has been obtained. Besides generating iV iV-EDTA, the OH radicals produce C-centered radicals by H-abstraction. These have reducing properties and are rapidly oxidized by tetranitromethane, giving rise to nitroform anion, G(NF ) = 4.2 x 10 mol J . The C-centered radicals react rapidly k = 7.6 x 10 dm mol s ) with O2, and subsequent fast 02 elimination. The Schiff bases thus formed hydrolyze to the final products. 

In the presence of oxygen, the following products (G values in units of 10 mol j in parentheses) were observed after y-radiolysis formaldehyde (1.6), CO2 (1.6), glyoxylic acid (4.2), iminodiacetic acid (2.1), ethylenediaminetri-acetic acid (detected, not quantified). 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

As discussed previously, only chelates with a slow exchange rate remain stable in vivo. Indium-113m chelates with EDTA and DTPA have been utilized for the detection of brain tumors and for the study of renal funcions (26.27). Indium-113m chelates with ethylenediamine tetra(methylene phosphonic acid) (EDTMP) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP) have been utilized to study bone tumors (28. 2Q). These agents also have promise for the detection of myocardial infarcts (3il). 

The major uses of lndlum-111 in medicine are listed in Table 2. Indium-111 labeled DTPA is the preferred agent for the study of cerebral spinal fluid kinetics (cisternography)( ). Indium-labeled bleomycin has been used for tumor scanning C2.), although citrate has achieved greater clinical use. It appears that indium bleomycin is in fact a weak chelate and the in vivo distribution is very similar to that of indium transferrin. 

Inflammatory Site Imaging Lymph Node Imaging Lymphocyte Kinetics 

Molecular formula C10H16N2O8 Molecular weight 292.24 CAS Registry No 60-00-4 Merck Index 13,3546 

Sample preparation Place 0.5 cm of a dried blood stain in 50-100 (jlL 25 mM cop-per(II) sulfate, let stand for 3 h, vortex, centrifuge at 3000—9000 rpm for 10 min, filter (0.2 Am), inject a 25 (jiL aliquot. Alternatively, dilute 200 ixL whole blood with 2 mL water, mix with an equal volume of 50 mM copper(ll) sulfate, centrifuge for 7 min, inject an aliquot of the supernatant. 

Miller, M.L. McCord, B.R. Martz, R. Budowle, B. The analysis of EDTA in dried bloodstains by electrospray LC-MS-MS and ion chromatography, J.Anal.Toxicol., 1997, 21, 521-528. 

Mobile phase MeCN water ammonium hydroxide 80 20 0.03 

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EDTA Efhylenediamineletra-acetic acid. EFA Essential fatty acids. ... 

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

Figure 5.8. Paramagnetic ion-induced spin-lattice relaxation rates (rp) of the protons of 5.1c and 5.1 f in CTAB solution and of CTAB in the presence of 5.1c or 5.1 f, normalised to rpfor the surfactant -CH-j. The solutions contained 50 mM of CTAB, 8 mM of 5.1c or 5.1f and 0 or 0.4 mM of [Cu (EDTA) f ...

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

Hedta, edta coordinated ions derived trien triethylenetetraamine... 

Tabie 11.34 Formation Constants of EDTA Compiexes at 25°C, ionic Strength... 

NaB(CgHg)4 (aqueous) metals from dilute HNO3 or HOAc solution (pH 2), or pH 6.5 in presence of EDTA. ... 

Among the complexing agents that find use as titrating agents, ethylenediamine-A,A(A, A-tet-raacetic acid (acronym EDTA, and equation abbreviation, H4Y) is by far the more important, and it is used in the vast majority of complexometric titrations. The successive acid values of H4Y are pKi = 2.0, pisTj = 2.67, = 6.16, pTCt = 10.26 at 20°C and an ionic strength of 0.1. The fraction... 

The formation constants of EDTA complexes are gathered in Table 11.34. Based on their stability, the EDTA complexes of the most common metal ions may be roughly divided into three groups ... 

The more stable the metal complex, the lower the pH at which it can be quantitatively formed. Elements in the first group may be titrated with EDTA at pH 1 to 3 without interference from cations of the last two groups, while cations of the second group may be titrated at pH 4 to 5 without interference from the alkaline earths. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

The nickel ion freed may then be determined by an EDTA titration. Note that two moles of silver are equivalent to one mole of nickel and thus to one mole of EDTA. 

Manganese(II) can be titrated directly to Mn(III) using hexacyanoferrate(III) as the oxidant. Alternatively, Mn(III), prepared by oxidation of the Mn(II)-EDTA complex with lead dioxide, can be determined by titration with standard iron(II) sulfate. 

The equivalent amount of cadmium ion exchanged for the silver ion can readily be determined by EDTA titration procedures. 

Standard EDTA Solutions. Disodium dihydrogen ethylenediaminetetraacetate dihydrate is available commercially of analytical reagent purity. After drying at 80°C for at least 24 hr, its composition agrees exactly with the dihydrate formula (molecular weight 372.25). It may be weighed directly. If an additional check on the concentration is required, it may be standardized by titration with nearly neutralized zinc chloride or zinc sulfate solution. 

Water. Distilled water must be (a) redistilled in an all-Pyrex glass apparatus or (b) purified by passage through a column of cation exchange resin in the sodium form. For storage, polyethylene bottles are most satisfactory, particularly for very dilute (0.00 lAf) EDTA solutions. 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Destruction of the masking ligand by chemical reaction may be possible, as in the oxidation of EDTA in acid solutions by permanganate or another strong oxidizing agent. Hydrogen peroxide and Cu(II) ion destroy the tartrate complex of aluminum. 

Salicylic acid 2-Hydroxybenzoic acid LeSCN2+ at pH 3 is reddish-brown Typical uses Pe(III) titrated with EDTA to colorless iron-EDTA complex... 

Br , citrate, CE, CN , E, NH3, SCN , S20 , thiourea, thioglycolic acid, diethyldithiocarba-mate, thiosemicarbazide, bis(2-hydroxyethyl)dithiocarbamate Acetate, acetylacetone, BE4, citrate, C20 , EDTA, E , formate, 8-hydroxyquinoline-5-sul-fonic acid, mannitol, 2,3-mercaptopropanol, OH , salicylate, sulfosalicylate, tartrate, triethanolamine, tiron... 

Citrate, cyclohexanediaminetetraacetic acid, V,V-dihydroxyethylglycine, EDTA, E , SO , tartrate... 

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