Stannous molybdate

Phosphate. Phosphoms occurs in water primarily as a result of natural weathering, municipal sewage, and agricultural mnoff The most common form in water is the phosphate ion. A sample containing phosphate can react with ammonium molybdate to form molybdophosphoric acid (H2P(Mo202q)4). This compound is reduced with stannous chloride in sulfuric acid to form a colored molybdenum-blue complex, which can be measured colorimetrically. SiUca and arsenic are the chief interferences. 

This analytical procedure is based on an optimum analysis condition for segmented continuous flow analysis. The sample is combined with a molybdate solution at a pH between 1.4 and 1.8 to form the //-molybdosilicic acid. After an appropriate time for reaction, a solution of oxalic acid is added, which transforms the excess molybdate to a non-reducible form. The oxalic acid also suppresses the interference from phosphate by decomposing phosphomolyb-dic acid. Finally, a reductant is added to form molybdenum blue. Both ascorbic acid and stannous chloride were tested as reductants. 

Elemental composition H 4.58%, P 46.94%, O 48.49%. The hypophosphite ion may he oxidized to orthophosphate by careful oxidation. The orthophophate, PO4 ion, may he measured by colorimetry either hy using ammonium molybdate and vanadium (yellow color), ammonium molybdate and stannous chloride (blue color), or ammonium molybdate, potassium anti-monyl tartrate and asorbic acid (an intense blue color). Absorbances of the solution are read at 400 (or 470), 650 (or 690) and 880 nm, respectively. Hypophosphite ion alternatively may be identified by ion chromatography. 

Elemental composition P 43.64%, 0 56.36%. The pentoxide is dissolved in water and the ultimate hydrolysis product, H3PO4, is analyzed for PO4 by ion chromatography. Alternatively, the solution is treated with ammonium molybdate—ammonium vanadate reagent to produce a yellow colored vanado-molybdophosphoric acid. Absorbance or transmittance of the solution may be measured at a wavelength between 400 to 490 nm, depending on. concentration of P04. The solution must be diluted for analysis. The solution may further be reduced with stannous chloride to form an intensely colored molybdenum blue for measuring absorbance or transmittance at 690nm. 

This method for determining arsenic is particularly useful in biological and toxicological studies.8 The material under test is oxidised with a mixture of sulphuric and nitric acids and perhydrol, the arsenic is precipitated as sulphide, which is then oxidised and the arsenic determined colorimetrically after addition of sodium molybdate and stannous chloride. The formation of the molybdenum blue compound is also applied to the micro-determination of arsenic in soil extracts.9... 

Stannous and stannic molybdates have not been isolated. The addition of alkali molybdates to solutions of stannous salts causes precipitation of a blue oxide containing tin, possibly as stannic molybdate. 

Our laboratory has been using a similar procedure for the past 2 years. Digestions are carried out on isopropanolic extracts with sulfuric acid-peroxide, after the procedure of Youngburg and Youngburg (Yl). The colorimetric portion of the analysis is automated, utilizing the molybdic acid-stannous chloride technic of Kutter and Cohen (K8). The procedure is performed as follows. 

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. 

K8. Kuttner, T., and Cohen, H. R., Microcolorimetric studies. I. A molybdic acid, stannous chloride reagent. The microestimation of phosphate and calciiun in pus, plasma, and spinal fluid. J. Biol. Chem. 76, 517-531 (1927). 

After extraction, in the filtered extract, phosphorus is estimated colorimetrically by adding ammonium molybdate and thereafter reducing the molybdenum-phosphate complex in acidic medium with a reducing agent for which stannous chloride is used. The intensity of the blue colour molybdenum blue is directly related to the quantity of orthophosphate ion and thus provides a measure for the concentration of P in test solution. The absorbance or transmittance is measured spectrophotometrically at 660 mp,. wavelength. 

Tin molybdenum catalyst was prepared by precipitation of a solution of stannous chloride with aqueous ammonia. It was filtered and washed till chloride free. A solution of ammonium molybdate, potassium hydroxide and chromic oxide were added to above precipitate and was evaporated to dryness on sand bath. It was dried at I I0°C and calcined at... 

Most of the catholytes have been aqueous or aqueous alcoholic solutions of mineral acids. In most cases the promoters used have been stannous chloride, cupric chloride, titanium chloride, vanadyl sulfate, and molybdic acid. Copper, nickel, lead, carbon, and mercury cathodes have been used. 

Orthophosphate. The method used on San Francisco Bay water is a modification of the technique suggested by the AASGP (2) in which a membrane-filtered sample reacts with ammonium molybdate in acid solution and the resulting phosphomolybdic acid complex is extracted into an isoamyl alcohol/benzene mixture. Stannous chloride is used to reduce the extracted phosphomolybdic acid to molybdenum blue which is measured colorimetrically. 

A prerequisite for the molybdenum blue method is that all the arsenic has to be present as arsenate. After digestion with oxidizing acids, such as nitric acid, all the arsenic is converted into arsenate when appropriate heating time and temperatures are applied. The principle of this determination is the reaction of arsenate with ammonium molybdate in acidic medium to form an arsenate containing molybdenum heteropolyacid that can be reduced to molybdenum blue with stannous chloride, hydrazine, or ascorbic acid. Best results are obtained with hydrazine sulfate. The absorption maximum of the blue solution is between 840-860 nm (15). The most severe interferences for this method derive from phosphates and silicates. To remove interfering ions, distillation of arsenic as AsCb or AsBrs is often recommended (12,15). 

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. 

In the second step, the reductant stannous chloride (tin(II) chloride) is added to the solution. The yellow ammonium phosphomolybdate is thereby reduced to a substance called molybdenum blue. The structure of this amorphous substance of ammonium, phosphate, and molybdate has not been elucidated, but the corresponding molybdate blue is presented with the formula [Mo03]i54(H20)7o]y. ... 

If ammonium molybdate is added to an acidified orthophosphate solution, followed by a reducing agent such as stannous chloride or ferrous sulphate, an intense blue colour ( molybdenum blue ) will develop. A more satisfactory and sensitive version of this test is to use benzidine as the reducing agent in which case an intense blue colour arises from both the molybdenum blue and a reduced product from the benzidine which is also blue. 

Mild reduction of a yellow phosphomolybdate solution with stannous chloride yields phosphomolybdenum blue (above). This forms the basis of a sensitive method for estimating P in a variety of materials. Other reducing agents may be employed. A single reagent consisting of ammonium molybdate, hydrazine sulphate and sulphuric acid can be used. 

For visualization of the sugar phosphates, the plates are sprayed successively with benzidine tricMoroacetate (Rgt. No. 21) to detect the hexose 6-phosphates, and with the molybdate reagent (Rgt. No. 166) which detects all phosphates. The lower limits of detection of the two reagents are 10 and 5 [x moles, respectively. Stannous chloride reagent also reveals the sugar phosphates. 

Detection Blue spots were yielded by sprayiug the dried chromatograms first with 1 % aqueous ammonium molybdate and then with 1 % stannous chloride in 10% HCl. The blue may appear rather later with hypophosphite. 

Molybdate (ammonium)-stannous chloride for phosphoric acids. 

As discussed on page 210, molybdenum has an increased reactivity in the complex phosphomolybdic acid. It can be smoothly reduced to molybdenum blue by certain substances which have little or no effect on normal molybdates. Antimony salts also have this property when warmed but they react only with free phosphomolybdic acid or its soluble salts, and not with insoluble phosphomolybdates (difference from stannous chloride, see page 483). 

If metallic tin is fumed with ammonium bromide, volatile stannous bromide results. The latter can be identified by the immediate blue color (molybdenum blue) that forms on treatment with ammonium molybdate or, better, with a solution of phosphomolybdic acid that has been decolorized with excess ammonium acetate. 

Stannous chloride reduces not only molybdates to the colored lower oxides,2 but also reacts with phosphomolybdic acid and its salts. Molyb-denum blue results. It is an important fact, especially for the detection of tin in the presence of antimony salts, that stannous chloride will reduce not only the soluble phosphomolybdic acid as does antimony trichloride (compare page 105), but also the insoluble phosphomolybdates (e.g., the potassium or ammonium salt). Concerning molybdeniun blue see copper test 5, page 210. 

Principle Silicate reacts with molybdate under acidic conditions to form yellow p-molybdosilicic acid. This add is subsequently reduced with stannous chloride to form a heteropoly blue complex, which has an absorbance maximmn at 820 nm. Oxalic acid is added to reduce the interference from phosphate. 

Principle Soluble silica species react with molybdate at 37°C and pH of 1.2 to form a yellow silicomolybdate complex. This complex is subsequently reduced with stannous chloride to form a heteropoly blue complex, which has an absorbance maximum at 820 nm. The intensity of the color is proportional to the concentration of molybdate-reactive silica. Though the method is written for brackish and seawater, it is also applicable to nonsaline sample matrixes. The method is calibrated using standards prepared in deionized water. Once calibrated, samples of varying salinities (0 to 35 ppt) may be analyzed. The determination of background absorbance is necessary only for samples, which have color absorbing at 820 nm. 

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