Molybdenum blue reduction

A sulfuric acid solution of the oxide (25-75% solution) can be reduced with tin, copper, zinc, and other reducing agents forming a blue solution of molybdenum blue which are hydrous oxides of non-stoichiometric compositions (see Molybdenum Blue). Reduction with atomic hydrogen under carefully controlled conditions yields colloidal dispersion of compounds that have probable compositions Mo204(OH)2 and Mo40io(OH)2. Reduction with lithium aluminum hydride yields a red compound of probable composition MosOtIOEOs. Molybdenum(Vl) oxide suspension in water also can be reduced to molybdenum blue by hydriodic acid, hydrazine, sulfur dioxide, and other reductants. 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

A. Molybdenum blue method Discussion. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric acid (phosphomolybdic acid), which upon selective reduction (say, with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. The intensity of the blue colour is proportional to the amount of phosphate initially incorporated in the heteropoly acid. If the acidity at the time of reduction is 0.5M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting blue complex exhibits maximum absorption at 820-830 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. 

Phosphorus is extracted from soil at 20 1°C with a solution of sodium bicarbonate at pH 8.5. The absorbance of the molybdenum blue complex produced by the reduction with ascorbic acid of the phosphomolybdate formed when acid ammonium molybdate reacts with phosphate is measured using a spectrophotometer at 880 nm. 

In the case of giant wheel (molybdenum blue) compounds, the general synthetic strategy involves the acidification (pH 1) and reduction of an aqueous molybdate(VI) solution [possible reducing agents iron powder, tin(II) chloride, molybdenum(V) chloride, ascorbic acid, cysteine, hydroxylamine, hypophos-phorous acid, sodium dithionite, or hydrazine sulfate].On the other hand, an icosahedral ball-shaped cluster can be formed in an aqueous Mo(VI)... 

The determination of phosphorus after precipitation and solvent extraction as molybdophos-phoric acid (MPA) and reduction to molybdenum blue is a classical procedure,30 40 while cerium can be determined directly as molybdocerophosphoric acid (MCPA emax 7300 at 318 nm). A more selective method is to strip excess of MPA by extraction with chloroform, then to decompose residual MCPA and determine the phosphate liberated therefrom as MPA after extraction into isobutyl acetate. Alternatively AAS can be used to determine the amount of molybdenum. 

Physical and chemical measurements were made weekly at a central station in each side of the lake. Water samples were filtered through Whatman GF/C or Gelman A/E glass-fiber filters (1.0- xm pore size). N03 was measured by reduction to N02" in a cadmium column and formation of a pink azo dye, NH4+ was measured by using a phenol-hypochlorite method, and soluble reactive phosphate was measured by a molybdenum blue method. After 1990 nutrients were measured by using similar methods on a Technicon Auto Analyzer (83). 

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. 

The chemical form of phosphorus in the water column available for uptake by biota is important. The biologically available phosphorus is usually taken to be soluble reactive phosphorus (orthophosphate) , i.e. which, upon acidification of a water sample, reacts with added molybdate to yield molybdophosphoric acid, which is then reduced with SnCl2 to the intensely-coloured molybdenum blue complex and is determined spectrophotometrically (Imax = 882 nm). Reduction in inputs of phosphate, for example from point sources or by creating water meadows and buffer strips to contain diffuse runoff, has obviously been one of the major approaches to stemming eutrophication trends and... 

Molybdenum Sulphates.— Reduction of a solution of molybdic acid in sulphuric acid by means of hydrogen sulphide is stated to yield the compound M0O3.M0O0.2SO3 as a black substance, soluble in water to an unstable brown solution reduction with alcohol is said to yield soluble blue crystals of the sulphate 7MoO3.2MoO3.7SO3.aq. 

As(V) by the addition of aqueous iodine, followed by conversion to molybdoarsenate and reduction to molybdenum blue by boiling under reflux with a sulphuric acid solution of ammonium vanadate and hydrazine sulphate. The method is sufficiently sensitive to determine down to 3 fig of Lewisite per millilitre of sample. 

Heteropoly acids [oxygen compounds of Mo(VI), W(VI), Si, P(V), As(V), Ge, and other elements] and their reduction products (molybdenum blues) are extracted into oxygen-containing solvents by a mechanism similar to that above. 

Germanium(rV) forms heteropoly acids with molybdate and other ions. The method for determining germanium, based on yellow germanomolybdic acid [35-37] is insensitive (e = 2.0-10 at 430 nm), but reduction of the heteropoly acid to germano-molybdenum blue [38,39] considerably increases the sensitivity (e = 1.0-10 at 800 nm). 

Ions besides (P +) which may act as the central coordinating atom to form 12-fold heteropoly acids with molybdate include arsenic (As +), silicon (Si +), germanium (Ge +) and under some conditions molybdenum (Mo +) and boron (B +). Tungstate can also be coordinated about P as central atoms but with less avidity. The heteropolycomplexes, before reduction give a yellow hue to their water solution. With high P concentrations, a yellow precipitate is formed. In solution of low enough concentration to be suitable for determination by reduction to form the blue colour, the yellow colour is so faint that, it is not noticed and spectrophotometric measurements is done without any problem. The molybdenum blue colour is produced when either molybdate or its heteropolycomplexes are partially reduced. Some of the molybdenum ions are reduced from h-6 to a low valence state, probably h-3 and/or h-5, involving unpaired electrons due to which spectrophotometric resonance (blue colouration) would be expected. 

---