Molybdenum blue method

This experiment uses the molybdenum-blue method to determine the concentration of phosphate in a phosphate/sodium chloride mixture. Elow-injection analysis is used to increase the speed of analysis, allowing students to... 

Phosphorus from organophosphorus compounds, which are combusted to give mainly orthophosphate, can be absorbed by either sulphuric acid or nitric acid and readily determined spectrophotometrically either by the molybdenum blue method or as the phosphovanadomolybdate (Section 17.39). 

Molybdenum blue method. When arsenic, as arsenate, is treated with ammonium molybdate solution and the resulting heteropolymolybdoarsenate (arseno-molybdate) is reduced with hydrazinium sulphate or with tin(II) chloride, a blue soluble complex molybdenum blue is formed. The constitution is uncertain, but it is evident that the molybdenum is present in a lower oxidation state. The stable blue colour has a maximum absorption at about 840 nm and shows no appreciable change in 24 hours. Various techniques for carrying out the determination are available, but only one can be given here. Phosphate reacts in the same manner as arsenate (and with about the same sensitivity) and must be absent. 

A. Molybdenum blue method Discussion. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric acid (phosphomolybdic acid), which upon selective reduction (say, with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. The intensity of the blue colour is proportional to the amount of phosphate initially incorporated in the heteropoly acid. If the acidity at the time of reduction is 0.5M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting blue complex exhibits maximum absorption at 820-830 nm. 

B. Phosphovanadomolybdate method Discussion. This second method is considered to be slightly less sensitive than the previous molybdenum blue method, but it has been particularly useful for phosphorus determinations carried out by means of the Schoniger oxygen flask method (Section 3.31). The phosphovanadomolybdate complex formed between the phosphate, ammonium vanadate, and ammonium molybdate is bright yellow in colour and its absorbance can be measured between 460 and 480 nm. 

Apparent indicator constant 264, 267 Apparent stability constant 59 Aqua regia 111 Arc alternating current, 764 direct current, 763, 771 sensitivities of elements, (T), 766 Aromatic hydrocarbons analysis of binary mixtures, 715 Arsenates, D. of (ti) 357 Arsenic, D. of as silver arsenate, (ti) 357 as trisulphide, (g) 448 by iodine, (am) 634, (ti) 397 by molybdenum blue method, (s) 681 by potassium bromate, (ti) 406 by potassium iodate, (ti) 401 in presence of antimony, (s) 724 Arsenic(III) oxide as primary standard, 261... 

Johnson and Pilson [229] have described a spectrophotometric molybdenum blue method for the determination of phosphate, arsenate, and arsenite in estuary water and sea water. A reducing reagent is used to lower the oxidation state of any arsenic present to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will not form the molybdenum complex. This results in an absorbance value for phosphate only. 

Particularly in autoanalyser methods this wide variation in chloride content of the sample can lead to serious salt errors and, indeed, in the extreme case, can lead to negative peaks in samples that are known to contain ammonia. Salt errors originate because of the changes of pH, ionic strength and optical properties with salinity. This phenomenon is not limited to ammonia determination by autoanalyser methods it has, as will be discussed later, also been observed in the automated determination of phosphate in estuarine samples by molybdenum blue methods. 

In the method for [17] inorganic arsenic the sample is treated with sodium borohydride added at a controlled rate (Fig. 10.1). The arsine evolved is absorbed in a solution of iodine and the resultant arsenate ion is determined photometrically by a molybdenum blue method. For seawater the range, standard deviation, and detection limit are 1—4 xg/l, 1.4%, and 0.14 pg/1, respectively for potable waters they are 0-800 pg/1, about 1% (at 2 pg/1 level), and 0.5 pg/1, respectively. Silver and copper cause serious interference at concentrations of a few tens of mg/1 however, these elements can be removed either by preliminary extraction with a solution of dithizone in chloroform or by ion exchange. 

For ammonia, the commonly employed molybdenum blue method was examined. In this case, there were a number of issues. For example, the standard method requires the use of phenol and hypochlorite. Phenol is unsuitable for health, safety and environmental reasons, and hypochlorite is commonly regarded as unstable. We found that salicylate could be substituted for phenol, with little affect on sensitivity and a relatively small movement of the absorbance maximum, and hypochlorite is stable if stored carefully, and there is very low contamination by certain catalytic metals that accelerate decomposition, such as copper and iron [20]. 

Table 12.13 compares results obtained by this method with those obtained by a molybdenum blue method of spectrophotometry [115, 116]. Values obtained by atomic absorption spectrometry are higher than those obtained by the molybdenum blue method and this is believed to reflect the greater inherent accuracy of the former method. A UK standard method also... 

Table 12.13 Comparison of results for soils by atomic absorption spectrometry and molybdenum blue methods... 

Matrix effects in the analysis of nutrients in seawater are caused by differences in background electrolyte composition and concentration (salinity) between the standard solutions and samples. This effect causes several methodological difficulties. First, the effect of ionic strength on the kinetics of colorimetric reactions results in color intensity changes with matrix composition and electrolyte concentration. In practice, analytical sensitivity depends upon the actual sample matrix. This effect is most serious in silicate analysis using the molybdenum blue method. Second, matrix differences can also cause refractive index interference in automated continuous flow analysis, the most popular technique for routine nutrient measurement. To deal with these matrix effects, seawater of... 

Physical and chemical measurements were made weekly at a central station in each side of the lake. Water samples were filtered through Whatman GF/C or Gelman A/E glass-fiber filters (1.0- xm pore size). N03 was measured by reduction to N02" in a cadmium column and formation of a pink azo dye, NH4+ was measured by using a phenol-hypochlorite method, and soluble reactive phosphate was measured by a molybdenum blue method. After 1990 nutrients were measured by using similar methods on a Technicon Auto Analyzer (83). 

The advantages of high sensitivity, rapid analysis and simplicity of equipment are discussed, and the results for both types of sample material are compared with values obtained through use of the molybdenum blue method. 

Fresh water UV lamp 900 W, a few drops of 30% H202 and 0.004 M H2S04, t = 1.5-2 h P Colorimetric determination by molybdenum blue method 6... 

The accelerating effect of US on the determination of phosphate by the Molybdenum Blue method was ascribed to depolymerization of molybdate, which was thought to speed up its reaction with phosphate and inorease the sensitivity [32]. 

Rigler F. H. (1968) Further observations inconsistent with the hypothesis that the molybdenum blue method measured orthophosphate in lake water. Limnol. Oceanogr. 13, 7-13. 

The single-point standard addition method was used in the determination of phosphate by the molybdenum blue method. A 2.00-mL urine sample was treated with molybdenum blue reagents to produce a species absorbing at 820 nm, after which the sample was diluted to 100 mL. A 25.00-mL aliquot of this solution gave an absorbance of 0.428 (solution 1). Addition of 1.00 mL of a solution containing 0.0500 mg of phosphate to a second 25.0-mL aliquot gave an absorbance of 0.517 (solution 2). Use these data to calculate the concentration of phosphate in milligrams per milliliter of the specimen. 

The Ca, Sr and Fe contents were determined with a Seiko induced coupled plasma (ICP) spectrometer by first dissolving in HCl and PO was assayed by molybdenum blue method. The morphology of the particles was observed using a Joel electron microscope. X- ray powder diffraction (XRD) was done with a Rigaku high-intensity diffractometer using nickel-filtered Cu-Ko radiation (60 kV, 125 mV). X-ray photoelectron spectroscopy... 

On-line monitoring of phosphate in natural water and effluent stream by the molybdenum blue method using sequential injection analysis has been described [1]. The detection limit of 0.5 mg r P04 was reported. 

N, P Plants UV-Vis 0.05-0.5%, 0.2-5% (w/w) Sequential determinations/indophenol blue and molybdenum blue methods [20]... 

Phosphate Plant digests UV—Vis 10-60 mg IA1 Proposal for merging zones/molybdenum blue method [1]... 

Al, Si, P Digestion with diethyl ether-concentrated sulphuric acid-potassium persulphate. Silicon determined gravimetrically. Aluminium determined by titrimetric EDTA method. Phosphorus determined by spectrophotometric molybdenum blue method. 

Reactive phosphate was determined by a slightly modified molybdenum blue method (Strickland and Parsons, 1968 Presley, 1971). Precision of analysis was better than 3%. Standards were periodically made up and absorbance readings of these standards were the same throughout this study. 

Standard methods for phosphates, polyphosphates, and organic phosphates in environmental samples are predominantly nonchromatographic methods, which are based upon the molybdenum blue method. Within this colorimetric method ammonium molybdate and antimony potassium tartrate react under acidic conditions with dilute solution of phosphorous to form an antimony-phospho-molybdate complex which is then reduced to an intensely blue-colored complex by ascorbic acid. U.S. EPA Methods 365.1 to 365.4 are based upon this chemistry. 

UWfWlS 820 nm Uses molybdenum blue method for detection 74 n... 

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). 

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