Molybdenum titration

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Molybdenum trioxide titration from yellow ppt. M0O3 0.047979... 

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. 

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. 

Fig. 4. Anaerobic titration of xanthine oxidase with xanthine at pH 8.2 with a reaction time of 2 min. at about 20°. The integrated intensity of the Rapid molybdenum EPR signals (in arbitrary units) is plotted against the number of moles of xanthine added per mole of active enzyme. Activity/A4jo for the enzyme samples used was 112 corresponding to an active enzyme content of 57%. Thus the molar ratios of xanthine/total xanthine oxidase have been multiplied by 1.76 to refer to the active form only. Some of the EPR spectra (recorded at about — 130° and 9.3 GHz) are reproduced to show the changes in signal type as the amount of xanthine is increased. (Data re-calculated from ref. 88, with intensities corrected for variations in tube diameter and enzyme concentration calculated in terms of active enzyme.)...

If existence of a persulphide or other potentially electron accepting sulphur group is confirmed, this might explain why redox titration experiments have shown the number of electron equivalents which the xanthine oxidase molecule can accept to be greater than is required for reduction of the three non-protein components (58, 91). Certainly, this interpretation seems more probable than the original suggestion (58, 91) that the molybdenum can be reduced to lower oxidation states than Mo(IV) by some substrates. 

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. 

Chlorpromazine was determined by diluting 5 mL of a 0.01 M solution with 25 mL of H2O and 20 mL of ethanol, and titrating to a conductimetric endpoint with 0.05N acid or 0.05 N NaOH [63]. Chlorpromazine was determined by titration with KBr03, NaN03, or Ce(S04)2, the determination being made with the use of a molybdenum anode and a graphite, platinum, or tungsten cathode [64]. 

Epoxyoctane. A solution of 1 gram of f erf-butyl hydroperoxide (97% purity), 3 ml. of octene-1, 3 ml. of benzene, and 0.005 gram of molybdenum hexacarbonyl was sealed in a pressure tube and allowed to react for 1 hr. in a constant temperature bath at 90°C. The tube was removed and cooled in ice water. Hydroperoxide and epoxide were analyzed by iodometric titration and GLC. There was a 75% conversion of the hydroperoxide and a 92% yield of the epoxide based on the hydroperoxide conversion. 

In a definitive series of experimental investigations H. N. Wilson showed that the quinolinium salt, (C isNJ fPCV I2M0O3]3- was anhydrous, contained exactly 12 moles of molybdenum trioxide per mole of phosphate, that the precipitate had a negligible solubility and could be dried to constant weight in two hours at 105 °C. This precipitate also lent itself to a precise alkalimetric titration. In the presence of citric acid interference by silica was inhibited so that the method was admirably suitable for the analysis of basic slags or fertilizers.34... 

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

Assays of bromate and perbromate concentrations are required during the procedure. Bromate concentrations that are at least comparable to the perbromate may be determined iodometrically by reaction with sodium iodide in acid solution containing molybdate, followed by titration with standardized thiosulfate. After reduction of the bromate the solution should be ca. 0.1 M each in H+ and in free iodide ion. Perchloric, hydrochloric, or sulfuric acids may be used. The molybdenum(VI) concentration should be ca. 10 3 M. 

The postulated catalytic cycles for pterin-containing molybdenum enzymes involve a two-electron change at the molybdenum atom (Mo(VI) Mo(IV)). Microcoulometric titrations of nitrate reductase Chlorella vulgaris) (76), milk xanthine oxidase (77), and sulfite oxidase (78) show that their molybdenum centers are reduced by two electrons. The reduction potentials for the molybdenum center of chicken liver sulfite oxidase are strongly dependent upon pH and upon anion concentration (78). 

Xanthine is converted to uric acid at the molybdenum center of the enzyme, and the electrons are removed from the enzyme by oxidation of the flavin center. From early reductive titrations of xanthine oxidase with sodium dithionite, it was proposed that reducing equivalents were equilibrated among the four redox-active centers (Mo-co, two separate Fe2S2 centers, flavin) at a rate that was rapid relative to the overall catalytic rate of substrate turnover (243). Under such conditions, the flux of reducing equivalents through the enzyme should be influenced by the relative reduction potentials of the redox centers involved (244). Any effects of pH and temperature on the reduction potentials of individual redox components would affect the apparent rates of intramolecular transfer of the enzyme. 

The potentials of the redox centers of xanthine oxidase have been investigated by titrations in the presence of redox mediator dyes. An early study (245) used dithionite to generate reducing equivalents and quantified the reduced species by EPR measurements at low temperature. Subsequent studies as a function of pH showed that the potential of the molybdenum center was sensitive to pH (246). Concern over the effect of temperature on the observed potentials led to redox titrations monitored by room temperature CD and EPR spectroscopy (247). These experiments indicated that the redox potentials of all of the prosthetic... 

Microcoulometric titrations of sulfite oxidase suggest that both the magnitude and the direction of the driving force for intramolecular electron transfer between the molybdenum and the iron centers of sulfite oxidase can be modulated by pH and anion concentration (78). At low pH and/or high chloride the Mo(VI) center appears to be reduced... 

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