Determination of Phosphorus Based on the Formation of a Reduced Keggin-Type 12-Molybdophosphate Complex in an Aqueous-Organic Solution

Abstract

Extensive studies have been devoted to the determination of phosphorus as phosphate ion by the molybdenum blue method. There have been many reports on various modifications of this method in order to make the sensitivity and stability high. Various reductants, such as 1-amino-2-naphthol-4-sulfonic acid, stannous chloride(II), and ascorbic acid have been used to reduce the molybdophosphate complex. 1-11 However, it is known to be difficult for methods using 1-amino-2-naphthol-4-sulfonic acid and stannous chloride(II) to control both the pH and the temperature. 12 It is also well known that solvent extraction with ethyl acetate, 1-butylalcohol, 2-methyl-1-propylalcohol, and so on, makes the sensitivity high. 5,[13][14][15][16][17][18][19][20][21] Motomizu et al. reported that the formation of colored ionassociate between molybdophosphate and Malachite Green could be utilized to determine phosphorus in the ultra-trace range from 0.018 to 1.0 ng cm -3 . 22These highly sensitive methods contain a few careful procedures and require skill to determine P-PO4 3-with both high accuracy and precision. Molybdenum blue methods containing other metal ions, such as Sb(III), so-called phosphoantimonylmolybdenum blue method, has also usually been used as manual batch procedures. 2,3,23 Blomqvist and Westin reported in detail the effect of Cr(VI), Ge(VI), W(VI), and V(V) ions that have easily reacted with heteropolyoxometalate on the absorbance measurement in the phosphoantimonylmolybdenum blue method. 24 Various flowinjection analysis methods, that analyze phosphorus automatically, have also been developed. 2,3,25 It was also reported that organic phophorus and hydrolyzable phosphorus were converted into the orthophosphate ion for determining P-PO4 3-. 3,4,[25][26][27] In a previous report, the formation conditions of molybdophosphate and -arsenate complexes in aqueous solution were confirmed. 28 A few kinds of molybdophosphate complexes have been formed under high acidic conditions. The addition of water-miscible organic solvents, such as acetonitrile and acetone, made only 12-molybdophosphate stable. Maeda et al. examined the electrochemistry of 12-molybdophosphate in aqueous-organic solution. 29 The reduced form of 12-molybdophosphate was found to be more stable in aqueousorganic solution than in aqueous solution. Based on these results, optimizing the formation conditions of the reduced 12-molybdophosphate complex in aqueous-organic solution seems to enable one to determine a wider range of concentrations of P-PO4 3-than the phosphoantimonylmolybdenum blue method as a manual batch procedure. ExperimentalThe absorption spectra were recorded on a Hitachi Model U-2000 spectrophotometer using silica cells with path lengths of 0.1 and 1 cm. Spectroscopic measurements under various conditions were made at 25 ± 0.1˚C without any baseline correction; those under the optimum determining conditions were made at room temperature. All of the reagents were of analytical grade, and were used as received. Results and Discussion Optimizatio...

Voltammetric and Spectroscopic Studies of α- and β-[PW12O40]3–Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-Bu4N)3][PW12O40] Salts

“…The chemical and physical properties of polyoxometalates (POMs) have been studied extensively . POMs exhibit a wide range of structures, and many have been used in catalysis, analytical chemistry, biochemistry, and materials chemistry. − POMs also exhibit extensive and often complicated redox behavior. The ability to exhibit multielectron and proton-transfer reactions facilitates their applications as oxidation catalysts in organic synthesis and water splitting. ,,, In general, Keggin and Wells–Dawson-type POMs, such as [XM 12 O 40 ] n − and [X 2 M 18 O 62 ] n − , respectively, exhibit a series of well-resolved one-electron transfer reactions in neutral or slightly acidic aqueous media that merge into two or more electron transfer reactions in strongly acidic aqueous solutions .…”

A Simple Spectrophotometric Determination of Phosphate in Sugarcane Juices, Water and Detergent Samples

“…The proposed method is working on simple and straight forward principles of the reduction of phosphomolybdate 11,17,23,24 by sodium sulphide leading to molybdenum blue which was monitored at, λ max 715 nm The developed method found to be having the following advantages over the reported methods. [14][15][16][17]23,25,27,32 . (i) It is more sensitive when compared to the reported method 17 .…”

“…(ii) The preparation of reducing agent in this method is very simple when compared to that in the official method 25 . (iii) It neither involves extraction 19 nor sophisticated instruments like HPLC, 16 flow injection spectrophotometer 15 and atomic spectroscopy 14 , nor requires strict control of pH 23,27,32 or temperature or heat treatment 32 .…”

Effects of Organic Solvents and Salts on the Isomerization Reaction (β→α) of Keggin-type 12-Molybdophosphate Anion

“…4 Recently improved methods for the determination of phosphate have been developed that use water-miscible organic solvents, such as acetonitrile and ethylene glycol. 5,6 The structures of many polyoxometalates are based on a Keggin-or Dawson-type structure. The Keggin-type polyoxometalates, XM12O40…”