Molybdate blue reaction

Figure 2. Time study on oxoacids of phosphorus molybdate blue reaction.

Early colorimetric methods for arsenic analysis used the reaction of arsine gas with either mercuric bromide captured on filter paper to produce a yellow-brown stain (Gutzeit method) or with silver diethyl dithiocarbamate (SDDC) to produce a red dye. The SDDC method is still widely used in developing countries. The molybdate blue spectrophotometric method that is widely used for phosphate determination can be used for As(V), but the correction for P interference is difficult. Methods based on atomic absorption spectrometry (AAS) linked to hydride generation (HG) or a graphite furnace (GF) have become widely used. Other sensitive and specihc arsenic detectors (e.g., AFS, ICP-MS, and ICP-AES) are becoming increasingly available. HG-AES, in particular, is now widely used for routine arsenic determinations because of its sensitivity, reliability, and relatively low capital cost. 

Arsenic, oxidised to As(V) (e.g. by evaporation to dryness with HNO3) is made to react in a suitably acid solution (0.1-0.3 M H2SO4) with ammonium molybdate to form the molybdoarsenic heteropoly acid. The molybdoarsenic acid is then reduced [the Mo(Vl) atoms linked with As(V) are reduced to a lower oxidation degree] and a blue reaction product, the arsenomolybdenum blue is formed. The absorbance is measured either in the aqueous solution, or after extraction into oxygen-containing organic solvent [29-33]. The reaction conditions are so selected that the unreacted molybdate ions are not reduced. 

Skclch a flow injection system that could he used for the determination ot phosphate in a river water sample by means of the molybdenum blue reaction (phosphate -t molybdate i ascorbic acid in acid solution give phosphoniolybdonum blue, which absorbs at 66() nm),... 

Phosphate is spectrometrically determined using the molybdenum blue reaction. In this reaction, orthophosphate and molybdate ions condense in an acidic medium to form molybdophosphoric acid (or phos-phomolybdic acid), which is reduced to produce a blue colored compound (molybdenum blue), measured at 830 nm. 

This analytical procedure is based on an optimum analysis condition for segmented continuous flow analysis. The sample is combined with a molybdate solution at a pH between 1.4 and 1.8 to form the //-molybdosilicic acid. After an appropriate time for reaction, a solution of oxalic acid is added, which transforms the excess molybdate to a non-reducible form. The oxalic acid also suppresses the interference from phosphate by decomposing phosphomolyb-dic acid. Finally, a reductant is added to form molybdenum blue. Both ascorbic acid and stannous chloride were tested as reductants. 

Modras (51) reported spot test reactions to differentiate hydralazine from closely related drugs. Reagents used were aqueous copper (I) chloride, aqueous ammonium molybdate, iodine in potassium iodide solution, aqueous cobalt (II) nitrate, alcoholic ninhydrin, and alcoholic bromophenol blue. The tests were performed on paper or on Silica Gel G. 

Phthalic anhydride and urea, together with copper(I)chloride and ammonium molybdate, are heated to 200°C in trichlorobenzene. The ratios between the components are the same as in the baking process. Carbon dioxide and ammonia are released to yield Copper Phthalocyanine Blue. The reaction is complete after 2 to 3 hours, producing a yield between 85% and more than 95%. 

Under acid conditions, molybdate reacts with orthophosphate, P04 to form a blue heteropoly acid, molybdophosphoric acid. A similar reaction occurs with arsenate ion, As04. In the presence of vanadium, the product is yellow vanadomolybdophosphoric acid. These reactions are used for colorimetric analyses of phosphate, arsenate, and many other substances. Colloidal molybdenum blue has limited apphcations such as dyeing silk. It readily absorbs onto surface-active materials. 

Reactions based on the presence of the 21-hydroxy-20-keto functionality (ketol group) are much more specifie for corticosteroids, and these methods have been used for the determination of corticosteroids in pharmaceutical formulations or in biological fluids [72]. For instance, only 21-unesterified corticosteroids react with sodium molybdate in acetic acid medium [81]. The blue color obtained by reducing the ketol group allows the determination of 40-200 pg of these steroids. 

Several gas detector tubes are used in conjunction with common colorimetric reactions to detect butadiene. The reactions include the reduction of chromate or dichromate to chromous ion and the reduction of ammonium molybdate and palladium sulfate to molybdemun blue (Saltzman Harman, 1989). 

Phosphate is determined almost universally by its reaction with molybdate to form phosphomolybdate. The latter can be reduced to a blue compound that absorbs at various wavelengths, of which 640 and 820 nm are often used for colorimetric quantitation (Allen 1940 Sumner 1944 Meun and Smith, 1968). 

In the Biuret reaction, a purple colour develops when the protein is treated with alkaline copper sulphate. This reaction is dependent on peptide bonds and not on the side chains of individual amino-acids present. In the Folin-Ciocalteu reaction, the protein is treated with tungstate and molybdate under alkaline conditions and the formation of a complex such phenylalanine and tyrosine gives rise to a blue colour. Lowry developed one of the most widely used protein assays in which a combination of the above reactions is involved07, l8). 

Dimethoate Spectrophotometric flotation-dissolution reaction with molybdate and methylene blue and spectrophotometric finish [527]... 

The spot-test technique of the reaction utilizes the conversion of the silicic and fluosilicic acids by means of ammonium molybdate into silicomolybdic acid H4[SiMo12O40]. The latter, unlike free molybdic acid, oxidizes benzidine in acetic acid solution to a blue dyestuff and molybdenum blue is simultaneously produced. (DANGER THE REAGENT IS CARCINOGENIC.)... 

Ammonium molybdate-benzidine test (DANGER THE REAGENT IS CARCINOGENIC.) Silicates react with molybdates in acid solution to form the complex silicomolybdic acid H4[SiMo12O40]. The ammonium salt, unlike the analogous phosphoric acid and arsenic acid compounds, is soluble in water and acids to give a yellow solution. The test depends upon the reaction between silicomolybdic acid and benzidine in acetic acid solution whereby molybdenum blue and a blue quinonoid oxidation compound of benzidine are produced. 

Place a drop of the test solution and of the molybdate reagent upon drop-reaction paper, and warm gently over a wire gauze. Add a drop of 0 05 per cent benzidine acetate and hold the paper over ammonia vapour. A blue colouration results. 

CARCINOGENIC.) In this test use is made of the fact that benzidine, which is unaffected by normal molybdates and by free molybdic acid, is oxidized in acetic acid solution by phosphomolybdic acid or by its insoluble ammonium salt (see reaction 4 above). This reaction is extremely sensitive two coloured products are formed, viz. the blue reduction product of molybdenum compounds ( molybdenum blue ) and the blue oxidation product of benzidine ( benzidine blue ). Moreover, solutions of phosphates which are too dilute to show a visible precipitate with the ammonium molybdate reagent will react with the molybdate reagent and benzidine to give a blue colouration. 

Hydrogen sulphide No precipitate is produced in acid solution, but a blue solution (due to the production of quadrivalent vanadium ions) is formed and sulphur separates. Other reducing agents, such as sulphur dioxide, oxalic acid, iron(II) sulphate, hydrazine, formic acid, and ethanol, also yield blue vana-dium(IV) (V02+) ions (cf. Molybdates, Section VIL6). The reaction takes place slowly in the cold, but more rapidly on warming. 

The accelerating effect of US on the determination of phosphate by the Molybdenum Blue method was ascribed to depolymerization of molybdate, which was thought to speed up its reaction with phosphate and inorease the sensitivity [32]. 

Colorimetric methods Silicon is determined by the molybdenum blue spectrophotometric method after solubilization in H2O, in alkaline solutions or in concentrated HF. A flow analysis procedure for the measurement of soluble silicon with respect to the total Si concentration is used. The proposed method is applied to samples of rain water and of aerosols on filters [45]. Simultaneous determination of orthophosphate and silicate in brackish water is performed by the same technique. Molybdate/ antimony, ascorbic acid, and oxalic acid reagents are added to the samples and spectra are recorded in the wavelength range 410-820 nm after a total reaction time of 30 min [46]. 

MoO(SR)4] complexes. Sulfur coordination has been held important since early studies by Meriwether et al. revealed the generation of enzyme-like EPR signals upon reaction of molybdate with various thiols (66). The blue [MoO(SPh)4] complex (43) was among the first mononuclear oxo-Mo(V) complexes containing S-donor ligands to be... 

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