Silicomolybdic acid

The preparation of silicomolybdic acid is based on a hydrolytic reaction similar to that used in the preparation of silicotungstic acid. (See synthesis 48.) Only a few modifications are necessary to avoid the precipitation or reduction of molybdic acid. 

The properties of silicomolybdic acid are like those of silicotungstic acid. Toward indicators having a pH of 5 to 6 it behaves as a tetrabasic acid. With phenolphthal-ein as the indicator it requires eight equivalents for titration in cold solution but twenty-four equivalents for titration at 100°C. With tertiary organic bases it is tetrabasic. 

The acid prepared by the following directions has about six molecules of water of hydration. Illingsworth and Keg-gin1 believe that the X-ray photographs show the penta-hydrate cubic packing. As in the case of silicotungstic acid, the amount of water of hydration is usually not important. The concentration of a solution of silicomolybdic acid can be determined by titration with a standard alkali or by evaporation followed by careful ignition. 

Fifty grams t of sodium molybdate is dissolved in 200 ml. of water, and the solution heated to 60°C. To this solution is added 20 ml. of concentrated hydrochloric acid (density 1.18). The mixture is stirred vigorously with a 

The solution and the ether form an emulsion very readily consequently, they should be shaken together rather gently, preferably with a rotary motion. If an emulsion does form, it can be broken up by bubbling a stream of washed air into it. The ether complex is diluted with one-half of its volume of water, and the ether rapidly displaced by a stream of air. Should the solution become green, the yellow color can be restored by adding a little concentrated nitric acid. Crystallization takes place readily. 

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Alternatively, it is possible to replace the phosphorus atom by silicon, which affords products among which silicomolybdic acids H4[Si(Mo3O10)4].aq and H4H4[Si(Mo207)6].aq are commercially of some significance. The second kind of formula emphasizes the fact that only 3 (or 4) out of 7 (or 8) hydrogen atoms provide possible substitution sites. 

Selenium dioxide, by oxidation of selenium by nitric acid, 119 purification, 120 Selenium hexafluoride, 121 Silica gel, platinized, use in preparation of hydrogen bromide, 152 Silicobromoform, 38 analysis of, 41 Silicoethane, hexachlor, 42 Silicoformic anhydride, 42 Silicomolybdic acid, 127 analysis, 128... 

Under acid condition (at pH 1) silica reacts with ammonium molybdate to form a yellow colored heteropoly acid, silicomolybdic acid. The reactions are shown below ... 

Phosphate reacts with ammonium molybdate too, similarly forming yellow phosphomolybdic acid. The presence of phosphate, therefore, interferes in the test. The addition of oxalic acid or citric acid destroys phosphomolybdic acid complex but not silicomolybdic acid complex. The intensity of color developed is proportional to the concentration of sihca in the sample. 

An additional color development step may often be required to confirm the yellow color of silicomolybdic acid. The latter is reduced to a dark blue substance by treating with aminonaphthalo sulfonic acid. The color of heteropoly blue formed is more intense than the yellow color of silicomolybdic acid. The latter test is more sensitive and can give a detection limit of 50 pg silica/L when using a spectrophotometer. 

Figure 11. Product distribution in MoOj/SiC>2 catalysts as determined from acidimetric titrations (a) silicomolybdic acid (o) dimolybdates ( ) polymolybdates. The estimation of MoOj (i i) is derived from the intensity ratio of the reflectance bands at 360 and 440 cm-1 [66].

Thus, heteropoly acids may not be stable under reaction conditions (Mestl et al., 2001). This statement is in line with the results of Raman investigations of supported /J-silicomolybdic acid that unambiguously demonstrated its decomposition to surface molybdena during methane oxidation (Banares et al., 1995). It was also shown that silica-supported silicomolybdic acid and silica-supported molybdenum surface oxide species with the same molybdenum loadings performed identically (Banares et al., 1995). Thus, the presence of any heteropoly acid structure during high-temperature oxidation can be ruled out. 

Phosphomolybdic and silicomolybdic acids Weakly basic medium 15 [48]... 

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. 

Phosphoric and arsenic acids form compounds analogous to silicomolybdic acid which also react with benzidine with colour formation hence these acids should be removed before applying the test. In the presence of phosphoric acid, the test is carried out as follows. Mix a drop of the test solution with 2 drops of the molybdate reagent in a micro centrifuge tube and centrifuge the mixture. Transfer the supernatant liquid to a micro crucible by means of a capillary tube, warm gently, cool and add 2 drops 05m oxalic acid solution (the latter decomposes the small quantity of residual phosphomolybdate (NH4)3[PMo12O40] but has little action on the silicomolybdic acid complex), then introduce a drop... 

Ammonium molybdate-tin(II) chloride test The silicate is separated by volatilization as silicon tetrailuoride and the latter collected in a little sodium hydroxide solution. The resulting silicate is treated with ammonium molybdate solution and the ammonium salt of silicomolybdic acid, H4[SiMo12O40], is reduced by tin(II) chloride solution to molybdenum blue . Tin(II) chloride does not reduce ammonium molybdate solution. 

Dissolved monomer determination Photometric determination of silicomolybdic acid with ammonium molybdate according to Motomizu... 

Catalytic properties were studied for two reactions namely isopropanol conversion and propene partial oxidation. The first reaction is a test reaction which allows to characterize acidic, basic or redox properties of a catalyst. One gets dehydration to propene or di-isopropylether for acid catalyst, acetone for basic catalyst in absence of air and acetone and water for redox type catalyst in presence of air. The experimental results at 100°C clearly show that at low Mo loadings acidic features are favored while redox features are favored at higher loadings. This indicates that monomeric MoOj" species are acidic (presumably as in silicomolybdic acid) while polymeric species exhibit redox properties. 

Various methods for the determination of silicon, particularly from biological samples are available. The techniques usually involve decomposition of the compound to give silica or a silicate by either wet combustion or fusion with sodium peroxide. The silicate formed can then be determined colorimetrically, volumetrically, or gravimetrically. Acidified ammonium heptamoiybdate reacts oniy with monomeric Si(OH)4 and not its oiigomers to give a yellow silicomolybdic acid complex that can be determined spectrophotometrically. This method works... 

Prepare a silica reagent by mixing equal volumes of 10 per cent ammonium molybdate and 5M nitric acid. Put 1 ml of the silica sol in one tube and 1 ml of silicate solution in another dilute both with 20 ml of water, and add 1 ml of silica reagent. Yellow silicomolybdic acid forms in both tubes, but it is much more intense in the. silicate solution. A fresh silica sol cannot be distinguished from a silicate solution by this test, though their reactions to litmus are quite different. 

When silicomolybdic acid is used as electron acceptor, O2 evolution from illuminated chloroplasts becomes almost insensitive to DCMU (18) and DCMU-like inhibitors (, 16). Ioxynil and... 

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