Vanadomolybdophosphoric acid method

Analytical Procedures. Mn was determined by atomic absorption spectrophotometry (AAS) or the formaldioxime method (27J. Ca, Mg and Fe were determined by AAS. Silicate, phosphate, sulphate and chloride were determined using techniques described in Standard Methods (28). The molybdosi1icate method was used for silicate. Phosphate was determined using the vanadomolybdophosphoric acid method. Sulphate was determined by BaSO gravimetry. Chloride was determined by the mercuric chloride method. Salicylate and phthalate were determined by UV spectrophotometry. 

In this section, a simple procedure, based on the use of a UV photo-oxidation module (previously described) and a UV-visible measurement, is presented [4]. Another procedure, based on the use of the alternative vanadomolybdophosphoric acid method with a UV-LED detection at 380 nm can also be envisaged [29]. 

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. 

An alternative approach to these gravimetric methods is determination procedme based on the formation of colored complexes. Orthophosphate in acidic solution reacts with molybdic acid and vanadic acid to form yellow-orange vanadomolybdophosphoric acid which has maximum absorption at 330 nm. Another approach is a two-stage reaction in which yellow phosphomolybdate is produced from the addition of ammonium molybdate to the orthophosphate in acidic solution, and the phosphomolybdate is then reduced by ascorbic acid to a molybdenum blue, which is measured at 890 nm. This last method is the basis for an AOAC-approved spectrophoto-metric method. 

In the presence of ammonium metavanadate and acidic ammonium molybdate, phosphate forms vanadomolybdophosphoric acid. The absorbance of this yellow-colored product is commonly measured at 470 run. The sensitivity is generally lesser than that for reduced PMB methods, but it is quite tolerant of interfering ions, and is suitable for monitoring FRP in waste and contaminated waters. A detection limit of 200 pg P (6.5 pM) using a 1 cm cuvette has been reported for this method [25]. 

The substoicheiometric extraction of anionic species has been applied to the determination of Cr in aluminium alloys at the 1 ng level by both activation and dilution methods, and to Mn in gallium arsenide, using the tetraphenyl-arsonium complex. The technique has also been applied to the determination of F in aqueous solution in the 25—150/ig range using tetraphenyl-stilbonium extraction. Comparison of the substoicheiometric activation and isotope dilution methods showed that the results from the former method were of better precision because of the high sensitivity. Substoicheiometric extraction of vanadomolybdophosphoric acid with isobutyl methyl ketone has been applied to the isotope dilution determination of phosphorus and. 

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