1-Silicates, 12-molybdic acid

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

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.)... 

Silicon (Si, at. mass 28.09) occurs in its compounds in the IV oxidation state, e.g., in silica (Si02) or in silicic acids. A characteristic feature of silicon(rV) is its ability to form the tetrafluoride (Sip4) and heteropoly acids, e.g., with 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.)...

Effects of diluting silicate solutions Before proceeding to the main experiments involving silicic acid solutions made from silicates by methods already described, some preliminary tests were made by injecting samples of the higher ratio silicates directly into the molybdic acid reagent. In this case very small samples of one to ten microliter volumes were injected suddenly into rapidly stirred molybdic acid solution and the development of the yellow color recorded. 

By this method concentrated silicate solutions (6 M Si02) were compared with the same silicates that had been diluted with water at 0 C to silica coneeutrations of 0.33 and 0.6 M within 3 minutes before being added to the molybdic acid. 

This direct injection method was abandoned because such small volumes of concentrated, viscous silicate solutions were difficult to measure and dilute reproducibly into the molybdic acid. Also samples of the corresponding silicic acids were needed for the extractions into THF and for ultrafiltration studies ... 

Figure 6, Monosilicic acid, reacted with molybdic acid in 2 min, in silicic acid sols made from concentrated ("5 M Si02) sodium silicate solutions. Conditions pH 1.7 116.6 mM Si02 aged 5 min (A) and 10 d (B). Silicate ratios indicated.

The reaction data curves A, B and C in Figure 8 for the sols from diluted silicates are entirely different from the counterpart curves A, C and E in Figure 3 for sols made from concentrated silicates. Obviously the silicic acids from diluted silicates reacted much more rapidly with molybdic acid and thus consist of smaller particles. 

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. -... 

In a patent issued to Balthis (212b) a method is described whereby a sample of deionized silica sol is put into an excess of 0.01 IV NaOH solution at 30 C and over a period of 90 min samples of the solution are removed and the amount of monomeric silica is determined by reaction with the molybdic acid reagent. The rate of dissolution of the silica to form monomeric silicate in the alkaline solution was related to A, the specific surface area of the silica (in square meters per gram) determined by other means, by the following equation ... 

The history and use of the silicomolybdate method for analyzing for soluble silica has been discussed in Chapter 1. It is sufficient here to point out that molybdic acid reacts only with monomeric silica to form the yellow silicomolybdic acid. It is fortunate that the reaction with molybdic acid occurs at pH 1-2, where silicic acid polymerizes least rapidly. Thus polymeric silica must first depolymerize before it can react hence the higher the degree of polymerization, the longer the time required for depolymerization and color development. This is reviewed in detail in Chapter 3. 

These solutions were compared by measuring the rate of formation of the yellow silicomolybdic acid when microsamples were suddenly mixed with molybdic acid solution at low pH. For each sample, the percent of the total silica, P, which had reacted at time t was noted at 2.5, 5, 10, 15, and 20 min at 25 C. A solution of sodium silicate of 3.3 1.0 M ratio was also included and found to react in about the same way as the lithium silicate solution of corresponding ratio. 

A method that is said to distinguish alpha and beta from gamma silicic acid was developed by Nemodruk and Bezrogova (73a), who defined thz gamma silicic acid as that which did not react with molybdic acid reagent at 100°C in 20 min, whereas beta reacted completely. 

In some cases the polysilicic acid acid must be liberated from a crystalline silicate in acid at 2 C, or even in methanol-HCl, to obtain a solution stabilized long enough to take a sample for the molybdate test. The reaction of molybdic acid with disilicic or linear trisilicic acid is rapid because these depolymerize to monomer within a few minutes at pH 3. Schwartz and Knauf (21) prepared the pure methyl esters of these acids and found that by the time they had completely hydrolyzed in water in 4 and 10 min, respectively, only monomer was present in solution. 

The reaction rate of molybdic acid with specific polysilicate anions has been measured after obtaining a solution of the free polysilicic acid by dissolving water-insoluble, but acid-soluble, crystalline silicates of known crystal structure. Wicker... 

As sources of the silicic acids, crystalline acid-soluble satis of monosilicic, disilicic, and cyclic tri-, tetra-, and hexasilicic acids were dissolved rapidly in metha-nolic HCl, in which the silicic acids are more rapidly dissolved yet are more stable against further polymerization than in water. The liberated silicic acids were reacted at once with molybdic acid reagent at 20°C. 

Since several investigators have used nearly the same molybdic acid reagent solution as used by Alexander (24), a number of values for the constants can be compared for monomer and polymers, e.xcluding those of Funk and Frydrych, who used other reaction conditions. Each polysilicic acid in Table 3.2 was prepared from a particular crystalline silicate known to contain that polysiiicate anion, by dissolving it under conditions that avoided changing the structure. 

Table 3.1. Reaction Rate Constants of Silicic Acids with Funk and Frydrych s Molybdic Acid Reagent...

Goto and Okura (81) were the first to recognize that the depolymerization of silicic acid is catalyzed by the presence of molybdic acid. Thus at pH 1-2 in the presence of HCI alone, polysilicic acid formed monomer only very slowly, as shown by adding molybdic acid after 50 min. The rate of formation of silicomolybdate was then the same as when molybdate was added at the start. However, it is not known whether the molybdic acid is actually involved as a catalyst by direct interaction with the polymer or whether it simply reduces the concentration of monomer in solution to such a low level that an equilibrium between polymer and monomer is displaced. 

Although the rate of reaction of molybdic acid with individual polysilicic acid species obtained from crystalline silicates can be measured, the results are of no... 

Low molecular weight silicic acids were separated by Baumann (82) with paper chromatography using a mixture of isopropyl alcohol, water, and acetic acid as the moving liquid and the molybdic acid reaction to locate the separate species. 

This method for the detection of germanium is only decisive in the absence of certain other materials. Apart from compounds which reduce molybdates directly—e.g., Sn, Fe, As, and Se —arsenic acid, phosphoric acid and silicic acid should not be present, as they also form heteropoly molybdic acids, which enter into the same redox reaction with benzidine. The germanium can, however, be distilled out of hydrochloric acid solution (3.5-4.0 N) as Ge v chloride. The molybdate-benzidine test is then carried out with the distillate. 

Soluble silicates also react with molybdic acid under the above conditions forming a soluble silicomolybdic acid. This product, in its turn, reacts with benzidine similarly to the phosphoric acid compound, thus impairing the decisive nature of the test for phosphate. 

It is possible to detect phosphoric acid in the presence of silicic and arsenic acids by utilizing the fact that, under suitable conditions, the formation of silicomolybdic and arsenimolybdic acids can be prevented by tartaric acid. This masking depends on the formation of a stable complex compound of molybdic and tartaric acids, in which the molybdic acid does not react with arsenic and silicic acids, but is not masked toward phosphoric acid. 

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