EDTA oxidation, masking

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. 

HO2, was considered as a reactive intermediate in both cases. The addition of radical scavengers strongly retarded the oxidation of the phosphinate ion confirming the radical type mechanism. It was also demonstrated that the reaction ceased when the catalyst was masked with EDTA. 

Thiourea masks Cu2+ by reducing it to Cu+ and complexing the Cu+. Copper can be liberated from thiourea by oxidation with H202. Selectivity afforded by masking, demasking, and pH control allows individual components of complex mixtures of metal ions to be analyzed by EDTA titration. 

In order to determine S(IV) in rain water, it is necessary to prevent the oxidation of S(IV) between sampling and analysis. The suppressive effect on the oxidation of S(IV) was investigated by the addition of EDTA (Ethylenediaminetetraacetate) or TEA (Triethanolamine) as masking reagents for FeJ+ and Mn. Table III shows the suppressive effect of EDTA or TEA at various pH values. The suppressive effect was not observed between pH 3 and 5, because neither EDTA nor TEA chelate with Fe3+ and Mnz+ at these pH values. EDTA and TEA were found to be very effective for the suppression of the oxidation of S(IV) in solutions having neutral and basic pH values. 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

Calamine, which is used for relief of skin irritations, is a mixture of zinc and iron oxides. A 1.022-g sample of dried calamine was dissolved in acid and diluted to 250.0 mL. Potassium fluoride was added to a 10.00-mL aliquot of the diluted solution to mask the iron after suitable adjustment of the pH, Zn " " consumed 38.71 mL of 0.01294 M EDTA. A second... 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

The mercury-TMK complex is not formed in the presence of iodide. Many complexing agents (chloride, bromide, sulphate, acetate, citrate, tartrate, and EDTA) do not interfere. Some of these substances may be used to mask hydrolysable metals. Of the cations, only Pd(II), Pt(II), and Au(III) interfere. Reductants and oxidants should be absent. 

The optimal acidity corresponds to pH 3.0 0.2. Only Au, Hg, and Pt interfere in determination of palladium. The reaction of TMK with Hg(ll) is slower (about 2 h). Oxidants also interfere the reagent behaves as a reductor. Chloride masks silver ions. EDTA can be used to prevent hydrolysis of some metal ions. 

The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02-10" (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3 -diaminobenzidine method. 

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu " can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa"] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

Macro quantities of selenium can be determined gravimetrically after reduction to the elemental form by various reagents such as tin (II) chloride, potassium iodide, or ascorbic acid (I). Ooba described a technique whereby the element is precipitated from perchloric acid solution with hydrazine (2). Selenium may be titrated with standard solutions of sodium thiosulfate, iodide, and ferrous, chromous, or trivalent titanium salts after oxidation to Se(VI) (I). Photometric and fluorometric methods based on formation of the piaselenol with diaminobenzidine or 2,3-diaminonaphthalene has been used for the determination of selenium (I, 3,4,5). Interfering elements such as As, Co, Cr, Cu, Fe, Hg, and Ni, are masked with EDTA or other chelating agents. 

Mn(IV) mineral will mask the detection of the Mn(III) or Mn(II) reduction products. This was the case for experiments performed with a lOx higher concentration of Mn02 coated silica (1.0% vs. 0.1% coating) and Co(II)EDTA (2.0 mM vs. 0.2 mM). Even though the ratio of the solid phase Mn and the reductant were constant, Mn reduction products were not detected due to the overwhelming signal of the Mn(IV) (data not shown). These sets of experiments add credibility to the notion that oxidation of Co(II)EDTA by Mn-oxides is a surface-mediated process that is restricted to the outer molecular layers of the mineral structure. 

Alum interferes with the direct titration of zinc with EDTA. Triethanolamine has been used to mask aluminium in the determination of magnesium by complexometric titration" and a similar method is used in the B.P.C. for the determination of zinc oxide in this preparation. 

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