Molybdic acid colloidal

Metallic potassium and sodium are explosively converted into the hydroxides when brought into contact with concentrated solutions of hydrogen peroxide many of the heavier metals such as zinc and iron, and especially aluminium,3 are readily changed into their respective hydroxides, whilst chromium, arsenic, and molybdenum are oxidised respectively to chromic, arsenic, and molybdic acids. Colloidal tellurium yields telluric acid with very dilute solutions of peroxide4 the crystalline modification reacts slowly with 60 per cent, peroxide at 100° C. 

Colloidal Molybdic Acid.—When solutions of sodium molybdate (1 molecule) and hydrochloric acid (4 molecules) are warmed together, or when a solution of molybdic acid dihydrate is evaporated over sulphuric acid under diminished pressure at 20° C., colloidal molybdic acid is formed. From the solution of the hydrosol so obtained, molybdic acid is precipitated by electrolytes. Graham considered that, by dialysis of a solution of sodium molybdate in hydrochloric acid, he obtained colloidal molybdic acid. ... 

Both concentrated and somewhat diluted solutions of sodium silicates of different ratios were examined by suddenly injecting samples into rapidly stirred dilute H2S0 to convert the ionized species to the corresponding silicic acids. These were at once characterized by reaction with molybdic acid. Also colloidal species were separated from monomer and oligomers by extraction into tetrahydrofuran (THF). 

Figure 1. Typical data graphs for reaction of molybdic acid with silicic acid mono-mer, oligomers, and small colloidal particles. Line E for polymer that was extracted from sol E extrapolates through 100% untreated silica, thus indicating no monomer is present. Slope of E is the same as that of E after 20 min, showing that the polymer reacts the same as before being extracted. Particle diameter of the colloid is estimated from k, assuming anhydrous Si02 particles. (Reproduced, with permission, from Ref. 10. Copyright 1980, Academic Press.)...

As solubility equilibrium is approached, the monomer concentration rises to the solubility of the colloidal particles. Since the reaction rate with molybdic acid, expressed as k, is related to the particle size, then in aged sols there arises a relation between k and the concentration of monomer. Her has shown a linear relation between log Jk and log monomer concentration, providing equilibrium has been approached (23). 

Her examined the pH-tltration behavior of silicic acid in the presence of 2-hydroxypyridine 1-oxide by titrating 16 mA/ (1000 ppm SiO ) silicic acid silica from pH 10.5 to 3.0 in the presence and absence of a 43 mA/ concentration of the N-oxide. At no point did the titration curves differ, indicating that no complex had formed. In another experiment, a solution of Si(OH) containing 100 ppm as SiOj was mixed with a 200-fold excess of the above -oxidc at pH 6.15 and aged for a few hours. Tests with molybdic acid showed that the reaction rate with silica monomer was the same as a control, indicating either that no complex was formed at this pH or that it dissociated very rapidly. However, the rate of dissolution of monomer from colloidal silica particles at pH 1.4 was apparently doubled in the presence of a 20 mA/ concentration of the yV-oxide, indicating some type of interaction at low pH. -... 

It is logical to assume that the rate of depolymerization or dissolution of colloidal silica particles would be proportional to the specific surface area of the silica particles. Thus different solvents such as dilute hydrofluoric acid, molybdic acid, or dilute alkali, all of which convert the monomeric silica permanently to other silicon com-. 

Instead of an alkaline solution, a dilute solution of NaF and HCl to generate HF was employed by Goto (216a) to measure the specific surface area of colloidal silica from the rate of dissolution in this acid medium. The samples from the reaction mi.x-ture were removed, the reaction was stopped by adding aluminum salt to combine with the fluoride, and the dissolved silica was then determined by the molybdic acid method. 

Rate measurements on fine amorphous powder and colloids have been made by Doremus et al. (224) and by Friedberg (225). The effects of pH, temperature, and presence of salts were similar to those reported by others. More than 50 years ago, Dienert and Wandenbulcke (226) reported the basic facts that colloidal silica passed into solution as soluble silica, which was detectable colorimetrically with molybdic acid, and that alkalinity and salts were good catalysts for dissolution. They made an observation which apparently has never been followed up. They claimed that when salt is present, the dissolution rate is faster in a quartz container than in platinum and that in the absence of added salt, colloidal silica would pass into the soluble state when heated with water in quartz, but not in platinum. However, pH measurements were not made. 

The reaction of molybdic acid with monomeric SKOH) to give the yellow silicomolybdic acid is indispensable in investigating the behavior of soluble and colloidal silica. The literature is too voluminous to cover here. Morozyuk (293) made a chronological list of the literature to 1971. 

Clear solutions of silica may contain polysilicic acids or small colloidal particles that will not react completely with molybdic acid. Thus before determining total silica, depolymerization to monomer is necessary. 

Figure 2.4. Reaction of colloid species of silica in lithium polysilicate solutions of different ratios of SiO tLijO (indica d on curves) with molybdic acid reagent.

Figure 2.5. Relative rates of reaction of colloid species in lithium polysilicates with molybdic acid reagent versus (R - 2)" , where R is the molar ratio of Si0j Li,0.

Figure 3.42. The relative proportion.s of silica present as monomer, oligomer, and high polymer (colloidal particle.s) as a 400 ppm solution of SilOllh polymerizes at 30°C at pH. V yt, monomer B, oligomer C high polymer" or colloidal particles D, reciprocal of the reaction velocity constant, k, of oligomer with molybdic acid. [From VVcitz. Franck, and Ciller (MS).]...

Her has carried out a hitherto unpublished investigation of the rates at which extremely small particles of colloidal silica depolymerize to monomer both directly in the molybdic acid reagent and in dilute alkali in which the monomer is determined on separate samples by reaction with molybdic acid. The size of the particles was estimated from the specific surface area, which was determined by the Sears alkali titration method corrected for monomer. The measurements were made as the particles grew in size at pH 8.5 and also as they became aggregated at pH 5.9. 

The term active silica has sometimes been used in referring to polysilicic acid. For example, a distinction has been made by Rule (160) between active silica and other forms of polymeric or colloidal silica. Active silica is defined as any silica in molecular or colloidal aqueous solution, in such a state of polymerization that when diluted with sodium hydroxide solution to a pH of 12, and concentration of about 0.02 percent SiOj, at 30 C, the silica will be depolymerized substantially completely to monomer in not more than 100 minutes. The monomer is determined by the molybdic acid method. 

Figure 4.9. Reaction of colloidal silica with molybdic acid. [After Goto and Okura (170).]...

The question has been raised whether at equilibrium at pH 9-10, there are polysilicate ions as well as HSiO, and Si(OH) present. Her finds that in an aged sot, the rate of reaction of silica particles with molybdic acid is not changed by removing the colloid by centrifuging and rediluting with water, indicating that no polysilicates are present,. ... 

It is logical to assume that the rate of depolymerization or dissolution of colloidal silica particles would be proportional to the specific surface area. Thus different solvents such as dilute hydrofluoric acid, molybdic acid, or dilute alkali, all of which convert the monomeric silica permanently to other silicon compounds, may therefore be employed to measure surface area. The difficulty, however, is that the types of silica particles studied have been so varied and ill-defined that this approach has not been of any value until recently, when sols of uniform discrete solid particles have become available. Even then the value of the method is in doubt unless one can be sure that the samples of silica under study all have the same composition and structure. Variables in the rate of dissolution per unit of surface area include the following ... 

Dumanski has determined the molecular weight of the crystalloidal blue molybdenum oxide obtained by the method of G. Marchetti,1f and has found the value 440. The formula is supposed to be MosOs 5 H2O. When this solution is treated with salt colloidal molybdic acid is formed. 

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. 

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