Phosphomolybdenum Blue Method

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. 

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Phosphorus and Silicon in Waters, Effluents and Sludges [e.g. Phosphorus in Waters, Effluents and Sludges by Spectrophotometry-phosphomolybdenum blue method. Phosphorus in Waters and Acidic Digests by Spectrophotometry-phosphovanadomolybdate method. Ion Chromatographic Methods for the Determination of Phosphorus Compound, Pretreatment Methods for Phosphorus Determinations, Determination of silicon by Spectrophotometric Determination of Molybdate Reactive Silicon-1 -amino-2-naphthol-4, sulphonic acid (ANSA) or Metol reduction methods or ascorbic acid reduction method. Pretreatment Methods to Convert Other Eorms of Silicon to Soluble Molybdate Reactive Silicon, Determination of Phosphorus and Silicon Emission Spectrophotometry], 1992... 

Microgram quantities of phosphorus are conventionally determined by the phosphomolybdenum blue method. The molybdovanadophosphoric acid method is suitable for determining relatively large quantities of P(V). Sensitive spectrophotometric methods based on ion-associates with basic dyes deserve attention. 

Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr s salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. ... 

Inlerference in the phosphomolybdenum blue method comes primarily from As, Si and Ge, which also react with molybdate to form the corresponding isostructural acids which are reduced to heteropoly blues. All of these have absorption maxima close to each other (Figure 14.1). 

In the spectrophotometric determination of Si, Ge, P(V), As(V), and V(V) the yellow heteropoly acids occurring in acid solutions in the presence of an excess of molybdate or tungstate are important. The yellow heteropoly acids are the basis of less sensitive spectrophotometric methods, but the blue reduction products (e.g., phosphomolybdenum blue) are the basis of very sensitive spectrophotometric methods for determining these elements. The conditions for formation and extraction of these compounds have been investigated [133-135]. 

Caesium is separated from small amounts of rubidium and potassium as the sparingly soluble caesium tungstosilicate. The precipitate is dissolved, the tungstosilicic acid is reduced to silicotungsten blue, and the absorbance is measured at 640 nm [88], Caesium has been also determined indirectly as phosphomolybdenum blue (absorbance measured at 805 nm) after the precipitation of, and isolation of caesium molybdophosphate [89]. Caesium was also determined indirectly by precipitation of C 3Bi2l9 with subsequent determination of Bi by the iodide method [90]. 

Many methods for sulphide and H2S are based on the reducing properties of S(-II). Hydrogen sulphide reduces molybdate in acid medium to molybdenum blue, and the molybdophosphate to phosphomolybdenum blue [52]. Iron(III) reduced by H2S in the presence of 1,10-phenanthroline gives the orange Fe(phen)3 complex [2,53], Hydrogen sulphide may be determined after conversion into thiocyanate by the reaction with Fe(III) [54]. Sulphide has been determined also by a colour redox reaction with nitroprusside [55-57], In another sensitive reaction the sulphide ions decompose the Ag complex with Cadion 2B and Triton X-100 (e = 2.5-10 ) [58], In another indirect method sulphide releases the chloranilate ion from the Hg(Il) chloranilate [59]. Sulphide has also been determined by a method based on its reaction with bromate, followed by bromination of 2 ,7 -dichlorofluorescein by the bromine released [60]. 

Lowry method The biuret reaction is incorporated with the reduction reaction of Folin-Ciocalteu s reagent and the resulting phosphomolybdenum blue is quantitated spectrophotometrically at 750 nm 2-100 ng ml" ... 

A low-cost and robust FIA method for total phosphorus in wastewater is based on a two-stage photooxidation/thermal digestion procedure together with a mixed oxidizing/hydrolyzing reagent for converting phosphorus compounds to orthophosphate. The formed orthophosphate is then determined by the phosphomolybdenum blue reaction. 

The analysis of phosphorus in waters has historically been based on the photometric measurement of 12-phosphomolybdate or the phosphomolybdenum blue species, which are produced when phosphomolybdate is reduced. The majority of manual and automated methods of phosphate determination are based on the spectrophotometric determination of phosphorus as phosphomolybdenum blue, i.e.. 

Spectrophotometric methods are usually preferred for routine analysis of this parameter, most of them relying on the reaction between orthophosphate ions and molybdate in acidic medium in order to form a heteropoly acid. Color formation can be enhanced by adding vanadate to obtain the yellow vanadomolybdate complex (vanadomolybdophosphoric acid method) or by reducing the molybdo-phosphoric acid to yield strongly colored phosphomolybdenum blue species. 

Phosphomolybdenum blue—batch method Visible photometry MRP 10 0.4 0.32 0.013 100 mm cell Lower detection limit possible detection limit defined [173]... 

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