Phosphorus molybdate blue reaction

Figure 2. Time study on oxoacids of phosphorus molybdate blue reaction.

The phosphorus concentration in each operationally defined fraction is typically detected using the phosphomolybdate-blue colorimetric reaction and spectrophoto-metric detection (American Public Health Association, 1995), with or without pre-treatment and/or filtration. The term total phosphorus applies to samples subjected to hydrolysis and/or oxidation pretreatment intended to convert all forms of phosphorus to phosphate (Maher and Woo, 1998 Monaghan and Ruttenberg, 1999). Phosphorus that reacts with molybdate in untreated samples is referred to as reactive phosphorus . Samples may be unfiltered or filtered. When samples are unfiltered, the operationally defined fractions are total... 

Fig 3.16. Uptake of phosphorus. Presence of glucose is detected by a the reaction with molybdate creating a blue colour. The dashed line shows the outline of the fungal colony. Note that the fungus takes up basically all phosphorus under the colony. 

Molybdenite (M0S2) forms soft, thin flakes, which are very similar to graphite. Molybdenum trioxide (M0O3) is a white powder that is yellow in heat and insoluble in water and most acids. It is easily soluble in alkaline solution, with molybdates being formed. Ammonium molybdate together with phosphates forms a yellow-colored complex and precipitates in nitric acid solutions. If acidified molybdate solutions are treated with reducing agents, they become deep blue (molybdenum blue). Both reactions are used in the colorimetric determination of phosphorus. 

The absorbance of the 12-MPA dimer may then be measured, or it may be reduced to form the highly colored PMB using a variety of reductants. The optimum wavelength and sensitivity are a function of both molybdate and acid concentrations [77. Because molybdenum blue may also form through direct reduction of Mo(VI) if the pH is <0.7, even in the absence of phosphorus, the acid concentration is kept within the range of 0.3-0.5 N [77]. The selectivity of this reaction for phosphate is also highly pH-dependent, and again the reaction acidity must be strictly controlled. 

Ru(OH)Cls] - + 3H2O + 103 where the rate-determining step is the reduction of ruthenium(iv) by the amine with rapid re-oxidation of the metal complex by the oxidant. The oxidation of iodide by isopolymolybdic acids in the presence of germaniumfiv) and phosphorus(v) has been reported, the two-stage reactions involving the formation of a Ge " (or P ") complex with the molybdate, followed by a secondary reaction of this intermediate with I to yield the blue heteropolyacid. The two-electron reaction of indium(i) with tri-iodide ions,... 

Specific chemical methods can be used to reveal phospholipids, glycolipids, sterols or their esters as well as compounds with quaternary nitrogens or vicinal diols. Particularly useful for many membrane extracts is the reaction of ammonium molybdate with inorganic phosphate released from phospholipids. The phosphomolybdic acid so produced is then reduced to give an intense blue colour. The method can be adapted to a spray reagent or, more often, used to detect as little as 1 jug of phosphorus in scraped samples. 

---