Molybdate method

Flynn and Meehan [191] have described a solvent extraction phospho-molybdate method using isoamyl alcohol for monitoring the concentration of 32 P in sea and coastal waters near nuclear generating stations. 

When a quantitative determination of the phosphoric acid and chlorine is required, a fresh quantity of ash is prepared from a weighed amount of the meat sample mixed with an alkali such as milk of lime, sodium carbonate, etc. The phosphoric acid and the chlorine are determined in the nitric acid solution of the ash, the former by the ammonium molybdate method, and the latter either volumetrically or gravimetrically as silver chloride. 

In an aliquot part of this solution (100 or 150 c.c.) the phosphoric acid is determined either by the ammonium molybdate method or as magnesium pyrophosphate (see Vol. I, p. 132). 

Aliquots of the clear liquid phase above the sediment were used to measure Si and A1 concentrations by atomic absorption spectrometry (Perkin-Elmer atomic absorption spectrometer, model 3030B) and for the determination of the degree of Si polycondensation in the liquid phase by molybdate method (20). 

During each blood pressure measurement session, five measurements are recorded for each animal. The blood pressure is taken as the mean of the last 3 recordings. Urinary protein is determined by the Pyrogallol Red-molybdate method (RA-1000 Tech-nicon). Urinary sodium, creatinine and urea and serum electrolytes, creatinine, albumin, cholesterol and triacylglycerols are measured by a standard autoanalyser technique. Kidney samples are fixed in formalin and embedded in paraffin. Sections are stained with the periodic acid/Schiff technique. Focal glomerular sclerosis is scored semiquantitatively by light microscopy. 

Small quantities of phosphorus may be estimated quickly by the molybdate method, the amount of phosphomolybdate being estimated colorimetrieally by comparison in Nessler glasses or test-tubes with a standard prepared under conditions which are made identical as far as possible. An account of this estimation is given under Phosphoric Acid, p. 182. 

The silicate anion structures of C-S-H(I) preparations appear to be affected by how long the material remains in contact with its mother liquor and by how strongly it is subsequently dried. Experiments using the molybdate method showed that the anions in precipitates made from CaCl, and sodium silicate solutions at 0 "C were mainly those present in the silicate solution, and thus monomeric if the latter is sufficiently dilute (S45). By allowing such products to stand in contact with their mother liquors at O C. and drying at — 10°C, preparations with Ca/Si ratios of 1.2-1.5 were obtained that contained only dimeric silicate anions. 

A second method for separating enzymatic and nonenzymatic Fe(III) reduction by H2S is to block S04 reduction with molybdate (Mo04 ). The technique has been used effectively to demonstrate the importance of enzymatic reduction in marine and freshwater sediments (Section 8.08.6.4.4). As with all inhibitor techniques, there is the possibility that molybdate additions directly or indirectly affect processes other than S04 reduction. For example, it could overestimate biotic Fe(III) reduction if the enzymatic process was stimulated by a cessation of competition with H2S for Fe(III) substrates, or underestimate if S04 reduction was not completely blocked. Despite these potential limitations, the molybdate method produces patterns that are consistent with other types of geochemical data, and it is therefore widely used. 

The kinetics of the silicic acid condensation was investigated using the molybdate method introduced by Alexander [14]. Upon addition of a molybdate solution to a silicate solution, monomeric and dimeric silicic acid react to form the yellow molybdosilicate [SiMoi204o] [5]. The change in the absorption of this solution reflects the decrease in monomeric and dimeric silicic acid due to the condensation reaction. Because of the reaction equilibrium between silicic acid and the molybdate as well as between monomeric, dimeric, and oligomeric silicic acids, it is important to adhere to a well-defmed, constant, and reproducible time protocol for the formation of the molybdosilicate. 

The pH value of the reaction mixture shows a relation to the condensation rate the higher the pH value, the faster the condensation reaction. Of course, this relation is well known from simpler silica chemical systems. It thus appears that polyamines do not directly influence the first steps of the condensation reaction, for example by catalytic action or by coordinative interaction between the amine and the silicic acid anions, but that the accelerating effect the polyamines exert is related to the increase of the pH value, i.e. to their basicity. As the molybdate method probes only the first steps of the condensation reaction, significant interactions may, however, occur between larger silicate species and the amines. 

Total hydrolysable phosphate in water samples was determined after preliminary hydrolysis with sulphuric acid solution. To 50 ml samples was added 0.05 ml phenolphthalein indicator and, in cases where a red colour was observed, the color was discharged with drops of 30 % sulphuric acid solution. To each sample, 1 ml of 30 % sulphuric acid solution was added and diluted to 100ml. The samples were boiled for at least 90 minutes on a hot plate and the final digest was reduced to 25 to 50 ml. To each digest, 6 N sodium hydroxide solution was added until a faint pink color was observed. The faint pink colour was then discharged by adding 50% hydrochloric acid solution and the sample then proceeded to determination of the phosphate using vanadate-molybdate method as in the case of total reactive phosphate. Both the standards and blanks for determination of total hydrolysable phosphate were treated in the same manner as the samples. 

Lange and Vejdelek reference more than 100 articles describing molybdate methods and 30 articles describing the methodology adapted in the European Pharmacopoeia were phosphate is first boxmd as ammonium phos-phomolybdate and thereafter reduced by stannous chloride. These articles were published in the 1930s to the 1970s. 

Soluble Silicate. The molybdate method was used to measure soluble silicates [3]. 

Gold-plated stainless steel high pressure vessels were used. The samples and a small amount of water (to counterbalance the vapor pressure) were rocked at constant high temperature between 100° and 400°C for period of time of 2 to 5 hr. The vanado-molybdate method for quantitative spectrophotometric determination was used. The reagent used was that of Bridget, et al. (1). The procedure was modified slightly. 

Excess salt was added to aqueous formamlde, the mixture was shaken vigorously for at least 24 hours and then filtered through a fritted glass filter. The nitrogen content was determined by the Kjeldahl method, potassium was determined by Che Perrin method and phosphorus was determined gravimetrically by the quinoline molybdate method. 

Soluble silica as determined by the molybdate method is not necessarily present as Si(OH)4. Bogdanova (40) reported that in natural waters that contained only about 5 ppm total silica, 4-9% of the silica was polymeric but was converted to monomer by acid. It is most likely that the polymeric" silica was actually very small colloidal particles of aluminum silicate that liberated monomer when acidified.. ... 

Measurement by Alexander, Heston, and Her (158) on the solubility of a very pure amorphous silica (Linde silica) in water at different pH values (adjusted with HCl or NaOH) show the following variations, as measured by the molybdate method ... 

However, for colloids of extremely small particle size or dilute sols where an appreciable fraction of the silica is present as soluble silica, it is preferable to determine the amount of monomer by suddenly acidifying a sample and determining it by the molybdate method, after which the corresponding correction can be made. 

The mixture was immediately titrated with 0.1 N NaOH solution rapidly to pH 9.0 and the volume noted. Samples of the final solution were then analyzed by the molybdate method to determine how much silica was still present as monomer-dimer (reaction time 2-3 min). When no more than 200 ppm of monomer was added at the beginning, essentially all of it remained, showing.that it had not polymerized. Blank titrations were also run on x ml of 0.1 TV NaOH containing no silica. This blank titer was 0.3 ml. 

Depolymerization of polysilicic acid was studied by Stade and Wieker (109), who prepared a solution of polysilicic acid containing a minimum of monomer by dissolving (LijSiOj) powder in dilute acid at 0°C. Samples were then diluted to 12 ppm SiOj in water at pH 1.3-5.5 and the depolymerization to monomer followed at different pH values and temperatures. The rate of depolymerization was followed by removing samples periodically and analyzing for monomer in solution by the molybdate method of Alexander (24a). At the same time the rate at which the residual polymer reacted with molybdate was also measured. 

Bishop and Bear (141) followed the polymerization of monomeric silica at pH 8.5 at 25r45 C by measuring the decline in unreacted monomer using the molybdate method. The initial rate constant, assuming a second-order reaction, showed a peculiar variation with temperature . . . 

Special procedures for eliminating interference by phosphorus in biological samples analyzed by the molybdate method were developed by McGavack, Leslie, and Kao (412) and by King (413). 

Application of the Molybdate Method and Other Spectrophotometric Methods... 

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