EDTA-cobalt complex

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. 

As we have seen earlier that the trivalent metal complexes are normally bound still more firmly due to the formation of four rings (unlike three rings with divalent metal complexes) and stable in strongly acidic solutions, for instance cobalt (Co2+) EDTA complex is fairly stable in concentrated hydrochloric acid ( 11.5 N). 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

Cobalt(ll)-EDTA complex, hydrogen peroxide determination, 628, 639 Cobalt(ll)-hexacyanoferrate, hydrogen peroxide determination, 651 Cobalt(lll)-phthalocyaninetetrasulfonate, hydroperoxide determination, 677 CocrystaUization, alkyl hydroperoxides-ether, 111, 113... 

The final product is ferrocyanide and cobaltic EDTA, but this goes through an intermediate which can be isolated, and which is an adduct of these twro. Dr. Wilkins tried this system out in his rapid flow rate system and found a rate of association which was about right for substitution rates on a cobaltous ion. So this seemed to be a case where perhaps the nitrogen end of a cyanide was able to coordinate into a cobaltous complex, with either concomitant cr subsequent charge transfer. Yet no transfer of ligand occurs in the overall reaction. 

Ion chromatography has been applied to the determination of cobalt, nickel, copper, zinc and cadmium as their EDTA complexes using anion separation and suppressor columns and 0.03pm sodium bicarbonate0.03gm sodium carbonate [28] eluant and a conductiometric detector. 

The first example of a chiral copper photosensitizer is [Cu(dmp)((R,R-diop))]+ [R,R-diop = (R,R)-2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphe-nylphosphino)-butane dmp = 2,9-dimethyl-1,10-phenanthroline], in which two chiral centers are introduced in the (R,R)-diop ligand. This complex was applied to the stereoselective photoreduction of [Co(edta)]- [25]. After the reaction, the CD spectrum exhibits a positive peak at 590 nm and a negative one at 515 nm, which indicates the presence of excess A-fCo(edta)]. This means that A-[Co(edta)] more rapidly reacts with the photoexcited copper complex than does the A-enantiomer, where the stereoselectivity, defined as the ratio of the conversion rate, is 1.17. However, the photoreduction of Co(acac)3 and [Co(bpy)3]3+ occurs without stereoselectivity. This is probably because the electrostatic attraction between [Cu(dmp)((R,R-diop))]+ and [Co(edta)] is favorable for the stereoselection, but such interaction does not exist between [Cu(dmp)((R,R-diop))]+ and the other cobalt(III) complexes. 

Brooks, S.C., Taylor, D.L., and Jardine, P.M., Reactive transport of EDTA-complexed cobalt in the presence of ferrihydrite, Geochim. Cosmochim. Acta, 60, 1899, 1996. 

The ionic pair of [Co(AMMEsar)] + cation with an anthracene carboxylate anion (A-Co(III)) was used as both a photosensitizer and an ETA in the photodecomposition of water to produce hydrogen [387]. The photoreduction of encapsulated cobalt(III) ion to cobalt(II) ion occurs on excitation of anthracene chromophore (v< 25 000 cm-i). The A-Co(III) complex shows almost no fluorescence (0<2x 10 ), whereas the A-Co(II) complex produces specific violet fluorescence (Fig. 66). The cobalt(II) complex is formed in the presence of EDTA on light irradiation of the A-Co(III) solution (v> 25 641 cm i). The visible band at 21 276 cm- disappeared, and violet fluorescence was observed. The quantum yield of cobalt(II) complex formation was... 

Cobalt (Co, at. mass 58.93) occurs predominantly in the II oxidation state. In some complexes it is readily oxidizable to Co(III). The hydroxide Co(OH)2 is precipitated at pH 7.5 and is insoluble in excess of NaOH. Cobalt forms ammine, cyanide, tartrate and EDTA complexes. Blue chloride complexes are formed in fairly concentrated chloride solutions. 

Faujasite-X zeolite (NaX) (Si/Al = 1.23, ca. 2 pm particle size, from Aldrich Chemical Company), ammonium hydroxide (assay 29+%, from Fisher Chemical Company), cobalt(II) chloride (99+ % assay) and copper nitrate hydrate [Cu(N03). H20, 101.7 % by EDTA complexation, from J.T. Baker Chemical Company] were used. 

RACEMIC AND OPTICALLY ACTIVE COBALT(III) COMPLEXES OF CDTA, EDTA, AND PDTA IN NON-AQUEOUS SOLVENTS... 

An essentially nonaqueous procedure is described for preparing the racemic and optically active forms of the heavy alkali metal complexes (potassium, rubidium, and cesium) of the cobalt(UI) complexes of cdta, edta, and pdta. The... 

EDTA (ethylenediaminetetraacetic acid) forms stable metal chelates with a number of metal ions. Using this reagent as a complexing- agent, arsenic, bismuth, and selenium can be determined without any interference in the presence of nickel and cobalt. The cobalt-EDTA chelate is stable in 5 M HCl solution, whereas the corresponding bismuth complex is not. The influence of copper on the determination of arsenic can also be eliminated with EDTA, but not in the determination of selenium. Thiourea has been used to eliminate the influence of copper in the determination of antimony and sodium oxalate to eliminate the influence of copper and nickel in the determination of tin. An addition of thiosemicarbazide and 1,10-phenanthro-line reduces the interference of copper, nickel, platinum, and palladium in the determination of arsenic. 

The first step in the reaction of the cydta complex of cobalt(ii) with cyanide is reversible formation of a 1 1 seven-co-ordinate adduct. Formation of this adduct follows a second-order rate law and has the extraordinarily low activation energy of 0-8 kcalmol". Further reaction to [Co(CN)5] " is slow. The rate law for reaction of the 1 1 adduct with cyanide is first-order in adduct, second-order in cyanide. Thus the overall reaction is third-order in cyanide, like the related reactions of the edta complexes of cobalt(n) and of nickel(n). The mono-ida and -mida complexes of nickel(ii) react with cyanide by rapid reversible addition of two cyanides the rate-determining step en route to [Ni(CN)4] is reaction with a third cyanide. The overall reaction is thus again third-order in cyanide concentration. [Ni(ida)2] and [Ni(mida)2] react slowly with cyanide by parallel dissociative and associative paths the Ni(ida) and [Ni(ida)(CN)] so formed (or their mida analogues) then react rapidly with further cyanide to give [Ni(CN)4]. Reaction of [Ni(trien)] + with cyanide is fifth-order overall first-order in complex, fourth-order in total cyanide concentration. ... 

---