Molybdic acid reagent

Molybdic acid reagent 16 g of (NH4)6Mo7024 4H20 is dissolved in approximately 600 ml of water add, with mixing, 55 ml of concentrated H2SO4 and dilute to 1 liter... 

Analytical System. The manifold schematic is illustrated in Fig. 2. An unmeasured aliquot is transferred to the sample cups. Samples are aspirated at a rate of 60 specimens/hour and added to an air-segmented stream of molybdic acid reagent followed by mixing. The stannous chloride reagent is then added to the reaction mixture. After mixing and a 3-4-minute time delay, the absorbance is measured at 660 mp., using a tubular flow cell with a 15-mm light path. 

A solution of ursodeoxycholic acid (reference substance) in methanol (20 mg/ml) in a white graduated flask was allowed to stand in a window from ca. 2.0 p.m. untU ca. 10.0 a.m. the following day. From this reference substance and from a new batch to be tested, test solutions of equal concentration and also a diluted reference solution were prepared. The reference substance (100 pg and 0.5 pg) was apphed to two TLC silica gel 60 plates without fluorescence indicator, and both plates were dried for 3 min with a warm-air fan heater (grade I). One plate was then exposed to direct sunhght for 5 min. The development was then performed in the solvent system described in the DAC 1992. Figure 106 shows the two plates after derivatization with the phospho-molybdic acid reagent. 

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. 

Where the ionization constant and solubility are not known, as for example at some unusual pH, Van Lier (114) has shown how the ion concentration and solubility can be calculated easily from data on pH and total soluble silica, which includes both monomer and ionic silica as determined by the molybdic acid reagent. In general terms their method is as follows. 

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 ... 

For most purposes I have found, it is possible to make up a dilute molybdic acid reagent solution that is stable for a week. Thus to determine silica it is necessary only to add the sample solution. Standards should be run each day. 

G) Molybdic acid reagent 30 ml solution F, 8.4 ml solution D, and 1.6 ml HjO mixed to give 40 ml reagent for each determination. A larger quantity can be made up for use on any one day. A water bath at 20°C or other suitable temperature is needed for rate measurements. 

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.

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. 

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...

A peculiar phenomenon has been noted by Her. When a small amount of NaF is added to a polysilicic acid solution at pH 2 it converts an equivalent amount of the silica to SiF , which, when molybdic acid is then added reacts as though it were monomer. However, if the same amount of NaF is added v/iih or after the addition of molybdic acid reagent it does not depolymerize an equivalent amount of silica, but instead acts as a catalyst for the depolymerization of polysilicic acids. When NaF is added before the molybdic acid so that it is converted to SiF, ", then when the latter reacts with molybdic acid, the fluoride ion combines irreversibly with molybdenum so that is is no longer free in the system. When added later, the molybdic acid reacts with monomer as it is developed, but does not inactivate the fluoride, which at the low pH is probably present as HF. 

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 initial sol made at pH 2.2 (Sample 1, Table 3.10) reacted very rapidly with molybdic acid reagent. However, a mere 2 min exposure to pH 8.5 greatly reduced the depolymerization rate in both reagents, which is evidence of rapid particle growth. 

The formation of particles in the first stage has been investigated by potentiometric methods [14], trimethylsilylation [15], molybdic acid reagent [16], paper chromatography [17], and most recently Si NMR. (See, e.g. Refs. 18-21.) The potentiometric results were explained on the basis of the cyclic tetramers, Si40s(0H)6 and Si40g(0H)4 , along with the three mononuclear species previously discussed. (See Fig. 1.)... 

Unfortunately, few data are available to determine unambiguously the effect of solvent type on the condensation rate. Artaki et al. [89] investigated the effects of protic and aprotic solvents on the growth rates of polysilicates prepared from TMOS (/ = 10) with no added catalyst (pH measured immediately after mixing the reactants equalled 6-7). Using Raman spectroscopy and molybdic acid reagent methods (see, e.g., ref. 1), qualitative information related to the size of the polysilicate species was obtained as a function of the reduced time, r/fgeii for five solvent systems methanol, formamide, dimethylformamide, acetonitrile, and dioxane. Table 9 lists the stabilized... 

Jonas, Artaki, Zerda and co-workers [68,110,117] have combined Raman spectroscopy, Si NMR, and the molybdic acid reagent technique to monitor the polymerization process of silicate systems as a function of solution pH (1-9) and solvent composition (methanol or methanol plus formamide). Raman and Si NMR investigations were also performed at pressures ranging from ambient to 5 kbar. As in the previously cited examples, it was possible to correlate some of the Raman bands with specific Si resonances. Table 14 lists the assignments of Raman bands observed in the 600-1050 cm" region of the spectrum [110]. 

Phosphatides in lecithin separation by class Silica Gel G, 0.2 mm 39 18 4.5 CHCl3/MeOH/40% methylamine or 30 12 6 9 3 CHCli/acetone/MeOH/HOAc/ H2O 25 10 0.75 CHCl3/Me0H/H20 I2 vapor or molybdic acid reagent 80... 

Lecithin in food extract Silica Gel 60 F-254 65 25 4 CHCl3/Me0H/H20 Molybdic acid reagent 50... 

Phosphatides in lecithin Merck Silica Gel 60, impregnated with H3PO4 65 25 4.3 CHCl3/MeOH/0.2 M acetate buffer, pH 4 Molybdic acid reagent 82... 

Lecithins are most readily identified by a spray reagent specific for phosphorus compounds, usually molybdic acid reagent (81,82). Synthetic phosphatides may be analyzed by TLC techniques similar to those applied to lecithin (83). The plates should be stored in a controlled atmosphere, since the separation is sensitive to humidity. 

Dittmer-Lester molybdic acid reagent Dissolve 40 g M0O3 in 1 L 12.5 M sulfuric acid solution by heating. Separately, dissolve 1.78 g molybenum in 500 mL of the first solution. Mix equal volumes of the two solutions. Stable indefinitely. (A similar solution is commercially available from Sigma, St. Louis, MO). 

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