Reduced molybdate complexes

Discussion. Small quantities of dissolved silicic acid react with a solution of a molybdate in an acid medium to give an intense yellow coloration, due probably to the complex molybdosilicic acid H4[SiMo12O40]. The latter may be employed as a basis for the colorimetric determination of silicate (absorbance measurements at 400 nm). It is usually better to reduce the complex acid to molybdenum blue (the composition is uncertain) a solution of a mixture of l-amino-2-naphthol-4-sulphonic acid and sodium hydrogensulphite solution is a satisfactory reducing agent. 

Figure 1. Characteristic EPR signals of Fe(II)Fe(III) sites in semimethemerythrinj (a), semimethemerythrinQ (b), reduced uteroferrin (c), reduced uteroferrin-molybdate complex (d), reduced bovine spleen purple acid phosphatase (e), reduced component A of methane monooxygenase (f). (Reproduced with permission from ref. 26. Copyright 1987 Elsevier.)...

Oxyanions also affect the coordination chemistry of the metal center (84). Molybdate and tungstate are tightly bound noncompetitive inhibitors (Ki s of ca. 4 (iM) (85). These anions bind to the reduced form of the enzyme, changing the rhombic EPR spectrum of the native enzyme to axial (Figure 1) and affecting the NMR shifts observed (84,85). Comparisons of the ENDOR spectra of reduced uterofenin and its molybdate complex show that molybdate binding causes the loss of iH features which are also lost when the reduced enzyme is placed in deuterated solvent (86). These observations suggest that molybdate displaces a bound water upon complexation. 

Riley, 1962 Oscarson et al 1980 Strickland and Parson, 1968). This method is based on the spectrophotometric measurement of the blue As(V) and P(V) molybdate complexes. Phosphate is measured in one sample after reducing As(V) to As(III), which does not form the color complex. In a second sample, both P(V) and As(V) are measured and the As(V) concentration determined by subtracting the P(V) concentration. A third sample is oxidized to convert As(III) to As(V), and the As(lll) concentration obtained by subtracting As(V) and P(V). 

ESR evidence has been obtained suggesting that HA can reduce molybdate to Mo(V) and complex the Mo(V) species in strongly acidic media (Lakatos et al., 1977a Goodman and Cheshire, 1982). The ESR spectrum of a peat soil HA complex with Mo(V) enriched in Mo (nuclear spin, I = ) features two distinct components, each split into two six-line hyperfine patterns at g and g , consistent with two different axially symmetric Mo(V)—HA complexes. 

Many methods for sulphide and H2S are based on the reducing properties of S(-II). Hydrogen sulphide reduces molybdate in acid medium to molybdenum blue, and the molybdophosphate to phosphomolybdenum blue [52]. Iron(III) reduced by H2S in the presence of 1,10-phenanthroline gives the orange Fe(phen)3 complex [2,53], Hydrogen sulphide may be determined after conversion into thiocyanate by the reaction with Fe(III) [54]. Sulphide has been determined also by a colour redox reaction with nitroprusside [55-57], In another sensitive reaction the sulphide ions decompose the Ag complex with Cadion 2B and Triton X-100 (e = 2.5-10 ) [58], In another indirect method sulphide releases the chloranilate ion from the Hg(Il) chloranilate [59]. Sulphide has also been determined by a method based on its reaction with bromate, followed by bromination of 2 ,7 -dichlorofluorescein by the bromine released [60]. 

Standard methods for phosphates, polyphosphates, and organic phosphates in environmental samples are predominantly nonchromatographic methods, which are based upon the molybdenum blue method. Within this colorimetric method ammonium molybdate and antimony potassium tartrate react under acidic conditions with dilute solution of phosphorous to form an antimony-phospho-molybdate complex which is then reduced to an intensely blue-colored complex by ascorbic acid. U.S. EPA Methods 365.1 to 365.4 are based upon this chemistry. 

Reactivity studies of the (O)Mo (S) groups (103) in the oxo/thio-molybdate complexes clearly show the greater comparative reactivity of the Mo =S group relative to the Mo =0 group. The results of these studies have led to the suggestion (103) that the Mo=S group may be a site of exceptional reactivity in the (0)Mo (S) chromophore in xanthine oxidase. The presence of the latter in the Mo cofactor has been suggested on the basis of analysis of Mo extended X-ray absorption fine structure (EXAFS) data (176). The data (for the reduced form of the cofactor) show that the first coordination sphere around the Mo atom contains the Mo(0)(S) unit (Fig. 34). 

Figure 21. H NMR spectra of reduced uteroferrin and its tungstate and molybdate complexes. Reprinted with permission from R. C, Scarrow, J. W. Pyrz, and L. Que, Jr.,y. Am. Chem. Soc., 112, 657-665 (1990). Copyright (1990) American Chemical Society.

A very thorough investigation by Kitson and Mellon reveals that hardly any ions cause interference and only chromate and dichromate ions were found to interfere in concentration equivalent to the phosphate concentra-tion.2 Reducing agents will reduce the complex to molybdenum blue, a feature actually used in some determinations. Organic ions such as citrate, tartrate, and oxalate will form a complex with molybdate. It should be kept in mind though that these compounds are reported as potential interferences in quantitative determination. Whether they could actually eliminate a false positive reaction in an identification is questionable. When used for quantitative determinations the color is reported to be stable for 3 h, while otiiers report a stability of 24 h. ... 

The same method has been applied with little modification for the determination of silicates in water extract of black liquor and pulp by reducing the molybdate complex with ferrous sulfate in a slightly acidic media [3]. 

The effect of various interfering ions on the molybdate complex must also be considered. Only a very small amount of nitrate can be tolerated, and the permissible chloride concentration depends upon the amount of HCl removed during preparation of the reducing solution (Andersson [1962a]). Molybdate complexes with sulphuric acid when the latter is present in large amounts, and under these conditions, decomposition of the silicomolybdate must occur to a certain extent. Since the heteromolybdates of arsenic and phosphorus are less stable than the corresponding silicomolybdates, interference by these elements can, to some extent, be eliminated by use of polybasic organic acids such as oxalic and tartatic, which form complexes with molybdate. Interference by phosphate, however, cannot be eliminated completely. 

When the Claus reaction is carried out in aqueous solution, the chemistry is complex and involves polythionic acid intermediates (105,211). A modification of the Claus process (by Shell) uses hydrogen or a mixture of hydrogen and carbon monoxide to reduce sulfur dioxide, carbonyl sulfide, carbon disulfide, and sulfur mixtures that occur in Claus process off-gases to hydrogen sulfide over a cobalt molybdate catalyst at ca 300°C (230). 

Phosphate. Phosphoms occurs in water primarily as a result of natural weathering, municipal sewage, and agricultural mnoff The most common form in water is the phosphate ion. A sample containing phosphate can react with ammonium molybdate to form molybdophosphoric acid (H2P(Mo202q)4). This compound is reduced with stannous chloride in sulfuric acid to form a colored molybdenum-blue complex, which can be measured colorimetrically. SiUca and arsenic are the chief interferences. 

Molybdenum blue method. When arsenic, as arsenate, is treated with ammonium molybdate solution and the resulting heteropolymolybdoarsenate (arseno-molybdate) is reduced with hydrazinium sulphate or with tin(II) chloride, a blue soluble complex molybdenum blue is formed. The constitution is uncertain, but it is evident that the molybdenum is present in a lower oxidation state. The stable blue colour has a maximum absorption at about 840 nm and shows no appreciable change in 24 hours. Various techniques for carrying out the determination are available, but only one can be given here. Phosphate reacts in the same manner as arsenate (and with about the same sensitivity) and must be absent. 

There also exists interference from diphosphoric acid, other more highly condensed phosphoric acids, and their organic derivatives. The free phosphoric acid can be determined as a heteropolyacid complex of phosphoric acid and ammonium molybdate. Afterward the complex is reduced by stannum II chloride to molybdenum blue. The amount of this dye can be measured photometricly at 625 nm. Organic derivatives of phosphoric acid and condensed phosphoric acids do not interfere with this method. 

Pertechnetate and molybdate are reduced in acetic acid media by p-thiocresol and form complex compounds. As mentioned above the technetitun compound can be extracted by chloroform. Since the blue molybdenum complex is insoluble in this solvent separation of technetium from molybdenum can be achieved " . 

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