Vanadium titration

Ahmad and Shukla [11] determined primaquine and other antimalarial aminoqui-nolines by vanadium titration. The drugs were determined by oxidation with aqueous ammonium vanadate solution and back titration of the unconsumed reagent with aqueous acidic ammonium ferrous sulfate with V-phenyl anthranilic acid indicator. 

The oxidimetric method, which involves the use of soUd vanadium pentoxide as oxidant (103). The vanadium is reduced quantitatively by butyUithium and is determined potentiometricaUy by titration with standard sulfatoceric acid [17106-39-7]. This method gives a direct measure of the actual carbon-bound lithium present when compared to the total titrated alkalinity. 

Discussion. Molybdates [Mo(VI)] are quantitatively reduced in 2M hydrochloric acid solution at 60-80 °C by the silver reductor to Mo(V). The reduced molybdenum solution is sufficiently stable over short periods of time in air to be titrated with standard cerium(IV) sulphate solution using ferroin or /V-phenylanthranilic acid as indicator. Nitric acid must be completely absent the presence of a little phosphoric(V) acid during the reduction of the molybdenum(VI) is not harmful and, indeed, appears to increase the rapidity of the subsequent oxidation with cerium(IV) sulphate. Elements such as iron, copper, and vanadium interfere nitrate interferes, since its reduction is catalysed by the presence of molybdates. 

The liberated iodine and the excess of iodide is determined by titration with standard potassium iodate solution the hydrochloric acid concentration must not be allowed to fall below 7JVf in order to prevent re-oxidation of the vanadium compound by iodine chloride. 

With the exception of iron(II) and uranium(IV), the reduced solutions are extremely unstable and readily re-oxidise upon exposure to air. They are best stabilised in a five-fold excess of a solution of 150g of ammonium iron(III) sulphate and 150 mL of concentrated sulphuric acid per litre [approximately 0.3M with respect to iron] contained in the filter flask. The iron(II) formed is then titrated with a standard solution of a suitable oxidising agent. Titanium and chromium are completely oxidised and produce an equivalent amount of iron(II) sulphate molybdenum is re-oxidised to the Mo(V) (red) stage, which is fairly stable in air, and complete oxidation is effected by the permanganate, but the net result is the same, viz. Mo(III)- Mo(VI) vanadium is re-oxidised to the V(IV), condition, which is stable in air, and the final oxidation is completed by slow titration with potassium permanganate solution or with cerium(IV) sulphate solution. 

Vanadium pentoxide and mercuric oxide were used as catalysts for the hydrogen peroxide oxidation of bis(phenylthio)methane to its monooxide 17a31 (equation 5). From the synthetic point of view, it is interesting to note that vanadium pentoxide, in addition to its catalytic action, functions also as an indicator in this reaction. In the presence of hydrogen peroxide, the reaction mixture is orange while in the absence of hydrogen peroxide a pale yellow colour is observed. Thus, it is possible to perform the oxidation process as a titration ensuring that an excess of oxidant is never present. 

An indirect oxidimetric titration method for alkylhthium compounds may be carried out, taking advantage of the fact that vanadium pentoxide causes fast and complete oxidation of alkyllithiums in solution. After the end of this reaction the reduced vanadium compounds are dissolved in sulfuric acid and titrated with Ce(IV) sulfate, using as indicator Ee(II) and 1,10-phenanthroline (64) sulfate. This method is inadequate for aryllithium compounds because the lithium phenoxide impurities reduce vanadium pentoxide. ... 

Elemental composition H 4.11%, Mo 48.94%, N 14.29% O 32.65. (NH4)2Mo04 is digested with nitric acid and the molybdenum metal is analyzed by atomic absorption or emission spectrophotometry. It is dissociated to ammonia, which may be measured by titration or by an ion-specific electrode technique (see Ammonia). Ammonium molybdate reacts under acid conditions with dilute orthophosphate solution to form molybdophosphoric acid which, in the presence of vanadium, forms yellow vanadomolybdophosphoric acid the intensity of the yeUow color may be measured by a spectrophotometer at 400 to 490 nm and is proportional to the trace amount of ammonium molybdate. 

Elemental composition V 56.02%, O 43.98%. An acid solution is analyzed for vanadium (See Vanadium). Pentoxide in sulfuric acid may he converted to vanadyl sulfate hy reduction with SO2 (See Reactions) or ferrous ammonium sulfate (FAS). The excess FAS may he destroyed with ammonium persulfate. Vanadyl sulfate is then titrated with a standard solution of potassium permanganate ... 

Vanadyl sulfate may be analyzed by titration with a standard solution of potassium permanganate (See Vanadium Pentoxide, Analysis) or any suitable oxidizing agent. An aqueous solution may be analyzed for vanadium by AA or ICP (See Vanadium). 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

Virtually all amphoteric oxides are converted to monomeric anions in sufficiently strong basic media. Rieger and co-workers studied strongly basic oxovanadium(IV) solutions by ESR, optical and Raman spectroscopy.472 Raman absorption at 987 cm-1 confirms the presence of the vanadyl group in solution, and from ESR spectra and titration, the authors concluded that vanadium(IV) exists as a monomer [VO(OH)3] for pHs 12. 

Pervanadic acid appears, therefore, to be formed from metavanadic acid, HVOa. This view of the reaction and the formula for pervanadic acid are further supported by titrating the red solution with caustic soda, when it is found that two molecules of caustic soda are required for each molecule of vanadium pentoxide. The monobasicity of the acid is confirmed by measuring the equivalent conductivities of solutions of potassium pervanadate. 

Estimation of Vanadium.1—Volumetric Methods.—The most convenient and the usual method for the estimation of vanadium is a volumetric process. The vanadium is first obtained in acid solution as vanadate, and reduced to the tetravalent state by one of several reducing agents which are available. The solution is then titrated in the presence of sulphuric acid with hot potassium permanganate solution, which quantitatively oxidises the lower vanadium salt to the vanadate. Using sulphur dioxide to effect the reduction, the following reactions take place —... 

The estimation of a vanadate solution by direct titration with ferrous sulphate or ferrous ammonium sulphate solution has been worked out, and is found to be specially applicable to the analysis of vanadium alloys. The vanadate is again reduced to tire tetravalent state by the ferrous salt, the end point being obtained by the use of potassium ferricyanide as internal indicator alternatively, a known excess of the ferrous salt solution is added to the vanadate, the amount unused... 

Other volumetric processes which have been w orked out include the use of potassium ferrocyanide,8 potassium ferricyanide,10 titanous chloride,11 and stannous chloride.12 According to Rosenheim and Yang,13 vanadium pentoxide is best determined in solution by addition of excess of caustic soda and back titration with sulphuric acid at 100° C., using a-naphthophthalein as indicator. 

Analytical Reagents. Various chelators give specific color changes on combining with different metal salts, such as vanadium, iron, cobalt, nickel, copper, and palladium salts and thus identify the corresponding metal ions [74], Other chelators, such as C.I. Mordant Black 11 (Eriochrome Black T) (2 see Section 3.11.1), are employed as indicators for complexometric titrations. 

An indirect vanadometric method of assay for camphor was developed. The method involves the formation of 2, -dinitrophenylhydrazone of camphor. The nitro groups in the hydrazone are then reduced to amines by treatment with vanadium sulfate (VSOl ) and the excess of the reagent is back titrated with sodium dichromate (NapCrpO-y). The entire procedure is carried out in a modified separatory funnel into which standard vanadate, 6tJ and zinc amalgam... 

Oxygen chemisorption methods were used to titrate surface vanadium sites in these studies. Raman, X-ray diffraction and isotopic labeling were done to support the dispersion results from chemisorption. A further conclusion was that as the % V increased for ethane oxidation reactions that the catalytic activity and selectivity was similar to that of unsupported vanadia. 

Figure 1.7 (a) UV-VIS absorption spectra of 100 /vanadium chloroperoxidase at pH 8.3. The lower solid line shows the apo-enzyme, the upper solid line the holo-enzyme after addition of 100 /xM vanadate. The dotted line is the peroxo-form of the enzyme after addition of 100 /optical absorbance at 316 nm ( ). Absorbance of free vanadate under the same buffer conditions (O). Reprinted with permission from Renirie, R., Hemrika, W., Piersma, S.R. and Wever, R. (2000). Biochemistry, 39, 11 33-1141. Copyright 2000 American Chemical Society. 

V2O3 or VO2 sample was dissolved in a mixed solution of phosphoric-sulfuric acid for determination of and total vanadium content. The content of was determined using ferroin as an indicator [11]. The total vanadium content was titrated by a FeS04 solution. X-ray diffraction (XRD) experiments were carried out on a D/max-3A diffractometer using Cu Kai radiation (A, = 0.154050 nm). The morphologies of the products were observed on a JSM-6330F field emission... 

In this cell, the copper compartment contains a copper electrode and [Cu2+] = 1.00 M, and the vanadium compartment contains a vanadium electrode and V2 + at an unknown concentration. The compartment containing the vanadium (1.00 L of solution) was titrated with 0.0800 M HjEDTA2-, resulting in the reaction... 

The adsorption of sulfur dioxide of the catalysts rapidly decreased with the vanadium content of the samples. As sulfur dioxide is not chemisorbed on bulk vanadium pentoxide and also appears not to chemisorb on vanadate species, on V20s/Y-Al203 catalysts this probe can be considered to be selective for the titration of the basic sites of vanadium-free alumina. Thus, by selectively probing uncovered alumina, SO2 further allows the distinction between the acid-base features of the vanadium oxide layer and those of uncovered alumina. 

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