Malachite green, phosphomolybdate

A number of indirect flame atomic absorption spectrometry (AAS) methods have been reported for determination of phosphate (Table 8.2). For example, phosphate was determined by measuring molybdenum after solvent extraction of phosphomolybdate [107]. A more recent variation of this method involved flotation of the malachite green-phosphomolybdate ion pair at an aqueous-diethyl ether interface [108]. After dissolution in methanol, molybdenum was determined using flame AAS (nitrous oxide flame) at 313.26 nm. The method was successfully applied to measurement of seawater containing ca. 40 pg P (1.3 pM). 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Analysis. Phosphorus is generally measured as phosphate, HNO3 oxidizing all other species to this form. The colorimetric method involving malachite green and phosphomolybdate permits the determination ofP down to 10 ppb. ICPMS gives about the same sensitivity. 

Molybdates are used in a variety of industries. Sodium molybdate is used for the synthesis of pigments such as molybdate chrome orange, which is a homogeneous mixture of lead molybdate, lead chromate and lead sulphate. This use is likely to decline because of concerns about health hazards associated with lead, but phosphomolybdates and phosphotungstomolybdates are used to complex certain dyestuffs to produce pigments. A few of the best-known of these are Malachite Green, Rhodamine Band Methyl Violet, also used as indicators in analytical chemistry. 

A green or blue pigment manufactured by precipitating basic dyestuffs such as malachite green or Victoria blue with solutions of phosphotungstic acid, phosphomolybdic acid, or a mixture of both. 

Phosphomolybdate - Malachite Green complex with absorbance at 620 nm... 

Fig. 8 Malachite green assay for phosphate detection. The enzymatic reaction produces inorganic phosphate P. A secondary reaction is used to form a phosphomolybdate-malachite green complex, which has a strong absorbance at 660 nm.

These three methods are commonly used the detection of radioactive phosphate containing [24], the colorimetric detection of a complex of phosphomolybdate and malachite green [25], and the colorimetric detection of a phosphorylated product obtained by a secondary enzymatic phosphate transfer using purine ribonucleoside phosphorylase (PNP) [26] (Figure 2). 

Inorganic orthophosphate was measured colorimetrically (9) using the complex of phosphomolybdate with malachite green. Organic phosphate was converted to orthophosphate by ignition with (NOa) and acid hydrolysis (10),... 

Another test for phosphorus (as phosphate) is based on the fact that the phosphomolybdates (and phosphotungstates) form deeply coloured insoluble lakes with triphenylmethane dyes such as methyl violet or malachite green (Chapter 12.8). The procedure is as follows a drop of test solution is placed on a filter paper which is then sprayed with a 1% solution of methyl violet. After about half a minute the paper is then sprayed with a solution of acidified ammonium molybdate and a blue spot develops. This method is sensitive to 1 part in 500,000. 

Phosphomolybdate forms strong ion association complexes with basic dyes at low pH. For example, the sensitivity of a method based on spectrophotometric determination of the 12-MPA-malachite Green complex [84,85] was approximately 30 times that of a reduced phosphomolybdate determination. Other dyes used for this purpose include Saffranin, brilliant green, Fuchsine red, methylene blue, methyl violet, and Rhodamine B [5]. Surfactants such as polyvinyl alcohol are frequently used to avoid precipitation of the ion association complex. 

Enhanced sensitivity may be achieved through solvent extraction of PMB or phosphomolybdate ion association complexes prior to spectrophotometric measurement. For example, extraction with PMB with iso-butanol enabled a detection limit of 0.2 jxg P (6 nM) to be achieved [86]. Motomizu et al. [87] extracted the phosphomolybdate-malachite green ion pair into a mixture of toluene and 2-methylpentane-2-one to obtain a detection limit of 0.1 xg P (3.2 nM). Use of solid phase extraction of PMB has also been reported [88]. 

Phosphomolybdate-malachite green ion pair—FIA method Visible spectrophotometry MRP 0.1 0.003 [84]... 

Phosphate can be measured by the reaction of phosphomolybdate with the dye malachite green. This results in a shift in the spectral characteristics of the dye which can be measured colorimetrically. 

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