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Phosphorus
Chapter 37. Phosphorus Phosphorus (P, at. mass 30.97) occurs mainly in the V oxidation state as phosphate (derived from orthophosphoric acid) and the condensed forms, pyro-, meta-, and polyphosphate. Phosphoric acid gives stable heteropoly acids with Mo(VI), W(VI), V(V) etc. Phosphorus occurs also in the III, I and -HI oxidation states in phosphite, hypophosphite and phosphine (PH3), respectively.
37.1. Methods of separating phosphate When a sample is dissolved, the phosphorus usually passes into solution as P(V). Rather than isolate the phosphate, it is often better to isolate the interfering elements, leaving the phosphate to be determined in the mother liquor. Examples of such separations include distillation of Si, As, and Ge as volatile halides [1] or of boron as trimethyl borate [2], precipitation of heavy metals as sulphides from an acid medium, retention of cations on a strongly acidic cation exchanger, and electrolytic separation of metals. The separation of P(V) from other elements, in particular from Si, is often achieved by extracting phosphorus as a heteropoly acid from a slightly acidic solution of pH --1.4. Higher alcohols, esters, and ethers are suitable extractants [3-5]. By extraction with a mixture of butanol and chloroform, molybdophosphoric acid can be separated from molybdoarsenic acid [6]. Isobutyl acetate extracts molybdophosphoric acid, but not molybdosilicic acid, from a solution at pH 0.3-1.0. In the determination of traces of P in silicon tetrachloride, shaking the sample with concentrated sulphuric acid causes the phosphorus to pass into the acid layer [7]. Trace amounts of phosphate can be co-precipitated with AI(OH)3, Fe(OH)3, or Be(OH)2 as collector [8-10]. Traces of phosphate, co-precipitated with AI(OH)3 at pH 8.5, have been floated with Na oleate, by passing a stream of nitrogen [ 11 ]. Traces of phosphate were concentrated on a cation-exchanger impregnated with Fe(III) [12,13] or with barium chloranilate [14]. Mixtures of P(V), As(V), and Si(IV) were separated on an anion-exchange column [15].
37.2. Methods of determining phosphate Microgram quantities of phosphorus are conventionally determined by the phosphomolybdenum blue method. The molybdovanadophosphoric acid method is suitable for determining relatively large quantities of P(V). Sensitive spectrophotometric methods based on ion-associates with basic dyes deserve attention.
37.2.1. Phosphomolybdenum blue method In an acid medium containing excess of molybdate, orthophosphate forms pale yellow molybdophosphoric acid ("Mo-P"). This reaction is useful for determining phosphate at fairly high concentrations, but the sensitivity can be increased considerably by the use of aqueous acetone [ 16-18]. A sensitive spectrophotometric method for determining phosphorus is based on the
37.2. Methods of determining phosphate
327
reduction of molybdophosphoric acid to phosphomolybdenum blue ("P-Mo") under mild conditions to prevent reduction of the free molybdic acid [19-24]. Reductants employed include hydrazine, SnC12, ascorbic acid, sulphite, and other reducing agents [25,26]. In the preparation of "P-Mo" blue, a single reagent consisting of ammonium molybdate, hydrazine sulphate, and H2SO4 is convenient (see procedure below). Molybdophosphoric acid is reduced either in aqueous medium (-~0.5 M H2SO4) or in the organic phase (usually n-butanol) after molybdophosphoric acid has been extracted. Alternatively, the phosphomolybdenum blue may be formed in the aqueous phase, then extracted into n-butanol [27]. Aqueous acetone medium is also used [28]. The absorbance of the phosphomolybdenum blue depends on the medium, its acidity, and the kind of the reducing agent used. The molar absorptivity of the blue solution in butanol after hydrazine reduction is 2.5.104 (a = 0.81) at ~max 780 nm. Extraction of phosphomolybdenum blue displaces the absorption maximum slightly towards shorter wavelengths. Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr's salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. Oxalic-, tartaric-, and citric acids, and EDTA affect the completeness of reduction of "Mo-P" acid [31 ]. Before the determination of phosphate, any nitrate must first be reduced to ammonia, which is then distilled from the alkaline medium [32]. Phosphomolybdenum blue may be extracted with CHC13 in the presence of dioctylamine, trioctylamine, or propylene carbonate [33]. The FIA technique has also been used in the determination of phosphorus(V) [15,34].
Reagents Molybdenum reagent. Solution (1). Dissolve 1.0 g of ammonium molybdate in 100 ml of 2 M H2804. Solution (2). Dissolve 0.10 g of hydrazine sulphate in 100 ml of water. Immediately before use, mix 10 ml of solution (1) with 10 ml of solution (2), and dilute to 100 ml with water. Solutions (1) and (2) should not be stored longer than 4 days. Standard phosphorus(V) solution: 1 mg/ml. Dissolve in water 4.3900 g of potassium dihydrogen phosphate (KH2PO4) dried at l l0~ add 1 ml of CHC13 (to prevent the formation of mould), and dilute with water to 1 litre. Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2.
Procedure
Extractive separation of P. Evaporate the sample solution freed from As (e.g., by extraction as AsC13, cf. Section 8.1.1) nearly to dryness, dilute with 20 ml of water, add 2 ml of the ammonium molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel and extract the "Mo-P" acid with two 10-ml portions of butanol. Wash the alcoholic extract with 0.05 M H2SO4. Determination of P. Evaporate an aliquot of the extract (or an aqueous sample solution
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37. Phosphorus
freed from As, Ge, and Si), containing not more than 30 ~tg of P, to dryness in a beaker with nitric acid. Add 12 ml of the molybdenum reagent, and place the beaker on a boiling waterbath for 10 min. Transfer the cooled solution to a separating funnel, and extract the phosphomolybdenum blue with two portions of n-butanol. Dilute the extract to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 720 nm, using the solvent as reference. Notes. 1. The absorbance of the P-Mo blue may be measured in the aqueous solution. In this case, the coloured aqueous solution is diluted to the mark in the volumetric flask with the molybdenum reagent. 2. Molybdophosphoric acid may be extracted, then stripped with dilute ammonia solution (1 +50), the solution acidified with nitric acid and evaporated, and the complex reduced to the heteropoly blue.
37.2.2. Molybdovanadophosphoric acid method Addition of molybdate to an acidic solution containing orthophosphate and vanadate, results in the formation of the yellow-orange molybdovanadophosphoric acid having the Mo:V:P ratio of 11:1:1 [35,36]. The absorption maximum of the compound is at 314 nm (e = 2.0-104). At 400 nm, e = 2.5.103 (specific absorptivity 0.08). In the molybdovanadophosphoric acid method, the absorbance is measured either at 315 nm (sensitivity as high as that in the "P-Mo" blue method), or at 400-470 nm (much lower sensitivity). The colour depends on the acidity of the solution and on the concentrations of the reagents used. The optimum acid concentration is 0.5-1.0 M HNO3 (H2SO4, HC104, or HC1). In insufficiently acid solutions, the yellow colour is produced even in the absence of orthophosphate; in excessively acid solutions, the formation of molybdovanadophosphoric acid proceeds too slowly. The concentration of V(V) and Mo(VI) in the final solution should be --0.002M and --0.01M, respectively. Since the reagents also produce a slight colour in the absence of phosphate, the absorbance must be measured against a reagent blank solution. In --0.8 M HNO3 silicon does not interfere provided it is not present in greater amount than phosphorus. In more acidic media, even more silicon can be tolerated. At higher concentrations, silicic acid can be converted into the inert polymeric form by heating the sample solution to fumes with conc. HC104. Large amounts of Fe(III) interfere, but may be masked with fluoride, the excess of which is complexed with boric acid. Reductants and certain coloured metal ions [e.g., Cr(VI), Ni, Co, Cu, and U(VI)] also interfere. Molybdovanadophosphoric acid may be separated from many coloured ions by extraction with oxygen-containing organic solvents [35]. Reducing agents must be absent.
Reagents Ammonium metavanadate, 0.25% solution. Dissolve 1.25 g of NH4VO3 in 250 ml of hot water. Cool the solution and add 10 ml of conc. HNO3. Allow the solution to stand overnight, filter if necessary, and dilute with water to 500 ml. Store the solution in a polyethylene container. Ammonium molybdate, 5% solution. Dissolve 2.5 g of the reagent in 250 ml of water (at --50~ Allow the solution to stand overnight, filter if necessary, dilute with water to 500 ml, and store in a polyethylene container.
37.2. Methods of determining phosphate
329
Standard phosphorus(V) solution: 1 mg/ml. Preparation as in Section 37.2.1.
Procedure To the slightly acidic sample solution containing not more than 0.3 mg of P(V), add successively 2.5 ml of HNO3 (1+1), 2.5 ml of the vanadate solution, and 2.5 ml of the molybdate solution, mixing the solution after the addition of each reagent. Dilute the solution to volume with water in a 25-ml standard flask. After 30 min, measure the absorbance at 400 nm against a reagent blank solution.
37.2.3 Other methods With basic dyes, molybdophosphoric acid forms ion-associates which are the basis of sensitive extraction methods for determining P(V). Malachite Green [37--48], Crystal Violet [49-52], Brilliant Green [53,54], Ethyl Violet [55], Methylene Blue [56,57], Rhodamine B [58,59], and Rhodamine 6G [60,61] are among the most widely used. The ion-associates, which are sparingly soluble in water, can be extracted, and the absorbance of the extracts measured [39,40,55,56]. The associates may also be separated by flotation [44,49] or centrifugation [53,58] before being dissolved in polar solvents. Aqueous pseudo-solutions are often stabilized with, e.g., poly(vinyl alcohol) [38,41,50], Syntanol DS-10 [37], or Zephiramine [57]. The most sensitive methods include those utilizing Malachite Green (e = 3.3.105) [44] and Brilliant Green (e = 2.9.105) [53]. The derivative spectrophotometry technique has been applied in the determination of phosphate with Rhodamine 6G [61 ]. The molybdophosphate anion has been associated, in a HC1 medium, with a cationic complex of Co with 5-C1-PADAP. The sparingly soluble associate has been separated by flotation with butyl acetate and dissolved in methanol (~ - 3.4.105 at 560 nm) [62]. A method based on phosphoantimonylmolybdenum blue has also been proposed [62a]. Numerous indirect amplification methods have been devised for the determination of phosphate. The molybdenum in an extract of molybdophosphoric acid (Mo:P = 12:1) has been determined with thiocyanate [63], phenylfluorone [64], dithiol (e = 1.7-105), Sulphonitrophenol S (e - 4.6-105) [65], or 2,2'-diquinoxalyl [66]. The indirect method that involves the Fe(II)-ferrozine complex [67] is unusually sensitive (~ = 9.7.105). In another method involving the complex of Ce(III) and Arsenazo III, the phosphate gives a sparingly soluble CePO4 and liberates an equivalent quantity of Arsenazo III [68].
37.3. Analytical applications The phosphomolybdenum blue method has been used for determining phosphorus in biological materials [69-71], milk [72], vegetables [73], wine and blood serum [74], sewage [75,76], waters [5,11,77-87], soils and plants [88-94], rocks and minerals [95], geological deposits (sediments) [96], organic compounds [97], fertilisers [98], cast iron and steel [8,26,99,100], nickel and its alloys [26,100,101], copper alloys [101], aluminium alloys [102], platinum and gold [6], gallium and its compounds [103], iron ores [104], silicon [1], boron [2], tungsten materials [9], arsenic and its oxide [105], niobium- and tantalum oxides [106], neodymium and yttrium oxides [107], silicates [108-110], glass [111], coke [112], coal dust [91], silumins [ 10], and concentrated chloride solutions [ 113]. Optimum conditions for determining P in soils, waters, and plants were studied [81]. Phosphorus was determined by the FIA technique in soils [91] and waters [87]. The method has been applied for determination of phosphate in the presence of
330
37. Phosphorus
phosphate esters [114], pyrophosphate, and polyphosphate [115]. The condensed phosphates are converted into phosphates by boiling for 15 min in 2.5 M H2804 medium. When trace amounts of phosphates were determined in water, they were preconcentrated (as the P-Mo blue) in a column packed with silane-coated glass beads; DMF was used as the eluent [116]. The P-Mo blue may also be concentrated on a nitro- or acetylcellulose membrane in the presence of dodecyltrimethylammonium bromide [ 117]. The P-Mo blue method has been applied for automatic determination of P(V) in silicate rocks [118], soils [119], sea deposits [120], and water [121]. This method has been applied also in the FIA technique [122-126]. The method involving the M o - V - P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141 ]. The methods involving basic dyes have been used for determining phosphate in natural waters [38,39,41,43,48,55,59,142], biological materials [47,50,52], soils [45,51], uranium [37], iron [46,49], and nickel, cobalt, copper, and zinc [46]. The FIA technique has also been applied [40,43,48]. Various methods were applied for determining phosphorus in calcium chloride extracts [143]. The spectrophotometric methods for the determination (and speciation) of phosphorus in natural waters have been compared with other analytical techniques [144].
37.4. Determination of other phosphorus compounds Hypophosphite can be determined, in the presence of phosphate, from the colour it gives with ammonium molybdate in H2SO4 medium [145]. Hypophosphite may also be determined by its bleaching of the colour of the Fe(III)-SCN- complex [ 146 ]. Pyrophosphate is determined by using its effects on the colour reactions of Fe with SCN- or 1,10-phenanthroline [147]. A method for separation of hypophosphite, phosphite, phosphate, pyrophosphate, and triphosphate by ion exchange chromatography, with HC1 and KC1 solutions as eluents, has been proposed [148].
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37.1. Methods of separating phosphate When a sample is dissolved, the phosphorus usually passes into solution as P(V). Rather than isolate the phosphate, it is often better to isolate the interfering elements, leaving the phosphate to be determined in the mother liquor. Examples of such separations include distillation of Si, As, and Ge as volatile halides [1] or of boron as trimethyl borate [2], precipitation of heavy metals as sulphides from an acid medium, retention of cations on a strongly acidic cation exchanger, and electrolytic separation of metals. The separation of P(V) from other elements, in particular from Si, is often achieved by extracting phosphorus as a heteropoly acid from a slightly acidic solution of pH --1.4. Higher alcohols, esters, and ethers are suitable extractants [3-5]. By extraction with a mixture of butanol and chloroform, molybdophosphoric acid can be separated from molybdoarsenic acid [6]. Isobutyl acetate extracts molybdophosphoric acid, but not molybdosilicic acid, from a solution at pH 0.3-1.0. In the determination of traces of P in silicon tetrachloride, shaking the sample with concentrated sulphuric acid causes the phosphorus to pass into the acid layer [7]. Trace amounts of phosphate can be co-precipitated with AI(OH)3, Fe(OH)3, or Be(OH)2 as collector [8-10]. Traces of phosphate, co-precipitated with AI(OH)3 at pH 8.5, have been floated with Na oleate, by passing a stream of nitrogen [ 11 ]. Traces of phosphate were concentrated on a cation-exchanger impregnated with Fe(III) [12,13] or with barium chloranilate [14]. Mixtures of P(V), As(V), and Si(IV) were separated on an anion-exchange column [15].
37.2. Methods of determining phosphate Microgram quantities of phosphorus are conventionally determined by the phosphomolybdenum blue method. The molybdovanadophosphoric acid method is suitable for determining relatively large quantities of P(V). Sensitive spectrophotometric methods based on ion-associates with basic dyes deserve attention.
37.2.1. Phosphomolybdenum blue method In an acid medium containing excess of molybdate, orthophosphate forms pale yellow molybdophosphoric acid ("Mo-P"). This reaction is useful for determining phosphate at fairly high concentrations, but the sensitivity can be increased considerably by the use of aqueous acetone [ 16-18]. A sensitive spectrophotometric method for determining phosphorus is based on the
37.2. Methods of determining phosphate
327
reduction of molybdophosphoric acid to phosphomolybdenum blue ("P-Mo") under mild conditions to prevent reduction of the free molybdic acid [19-24]. Reductants employed include hydrazine, SnC12, ascorbic acid, sulphite, and other reducing agents [25,26]. In the preparation of "P-Mo" blue, a single reagent consisting of ammonium molybdate, hydrazine sulphate, and H2SO4 is convenient (see procedure below). Molybdophosphoric acid is reduced either in aqueous medium (-~0.5 M H2SO4) or in the organic phase (usually n-butanol) after molybdophosphoric acid has been extracted. Alternatively, the phosphomolybdenum blue may be formed in the aqueous phase, then extracted into n-butanol [27]. Aqueous acetone medium is also used [28]. The absorbance of the phosphomolybdenum blue depends on the medium, its acidity, and the kind of the reducing agent used. The molar absorptivity of the blue solution in butanol after hydrazine reduction is 2.5.104 (a = 0.81) at ~max 780 nm. Extraction of phosphomolybdenum blue displaces the absorption maximum slightly towards shorter wavelengths. Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr's salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. Oxalic-, tartaric-, and citric acids, and EDTA affect the completeness of reduction of "Mo-P" acid [31 ]. Before the determination of phosphate, any nitrate must first be reduced to ammonia, which is then distilled from the alkaline medium [32]. Phosphomolybdenum blue may be extracted with CHC13 in the presence of dioctylamine, trioctylamine, or propylene carbonate [33]. The FIA technique has also been used in the determination of phosphorus(V) [15,34].
Reagents Molybdenum reagent. Solution (1). Dissolve 1.0 g of ammonium molybdate in 100 ml of 2 M H2804. Solution (2). Dissolve 0.10 g of hydrazine sulphate in 100 ml of water. Immediately before use, mix 10 ml of solution (1) with 10 ml of solution (2), and dilute to 100 ml with water. Solutions (1) and (2) should not be stored longer than 4 days. Standard phosphorus(V) solution: 1 mg/ml. Dissolve in water 4.3900 g of potassium dihydrogen phosphate (KH2PO4) dried at l l0~ add 1 ml of CHC13 (to prevent the formation of mould), and dilute with water to 1 litre. Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2.
Procedure
Extractive separation of P. Evaporate the sample solution freed from As (e.g., by extraction as AsC13, cf. Section 8.1.1) nearly to dryness, dilute with 20 ml of water, add 2 ml of the ammonium molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel and extract the "Mo-P" acid with two 10-ml portions of butanol. Wash the alcoholic extract with 0.05 M H2SO4. Determination of P. Evaporate an aliquot of the extract (or an aqueous sample solution
328
37. Phosphorus
freed from As, Ge, and Si), containing not more than 30 ~tg of P, to dryness in a beaker with nitric acid. Add 12 ml of the molybdenum reagent, and place the beaker on a boiling waterbath for 10 min. Transfer the cooled solution to a separating funnel, and extract the phosphomolybdenum blue with two portions of n-butanol. Dilute the extract to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 720 nm, using the solvent as reference. Notes. 1. The absorbance of the P-Mo blue may be measured in the aqueous solution. In this case, the coloured aqueous solution is diluted to the mark in the volumetric flask with the molybdenum reagent. 2. Molybdophosphoric acid may be extracted, then stripped with dilute ammonia solution (1 +50), the solution acidified with nitric acid and evaporated, and the complex reduced to the heteropoly blue.
37.2.2. Molybdovanadophosphoric acid method Addition of molybdate to an acidic solution containing orthophosphate and vanadate, results in the formation of the yellow-orange molybdovanadophosphoric acid having the Mo:V:P ratio of 11:1:1 [35,36]. The absorption maximum of the compound is at 314 nm (e = 2.0-104). At 400 nm, e = 2.5.103 (specific absorptivity 0.08). In the molybdovanadophosphoric acid method, the absorbance is measured either at 315 nm (sensitivity as high as that in the "P-Mo" blue method), or at 400-470 nm (much lower sensitivity). The colour depends on the acidity of the solution and on the concentrations of the reagents used. The optimum acid concentration is 0.5-1.0 M HNO3 (H2SO4, HC104, or HC1). In insufficiently acid solutions, the yellow colour is produced even in the absence of orthophosphate; in excessively acid solutions, the formation of molybdovanadophosphoric acid proceeds too slowly. The concentration of V(V) and Mo(VI) in the final solution should be --0.002M and --0.01M, respectively. Since the reagents also produce a slight colour in the absence of phosphate, the absorbance must be measured against a reagent blank solution. In --0.8 M HNO3 silicon does not interfere provided it is not present in greater amount than phosphorus. In more acidic media, even more silicon can be tolerated. At higher concentrations, silicic acid can be converted into the inert polymeric form by heating the sample solution to fumes with conc. HC104. Large amounts of Fe(III) interfere, but may be masked with fluoride, the excess of which is complexed with boric acid. Reductants and certain coloured metal ions [e.g., Cr(VI), Ni, Co, Cu, and U(VI)] also interfere. Molybdovanadophosphoric acid may be separated from many coloured ions by extraction with oxygen-containing organic solvents [35]. Reducing agents must be absent.
Reagents Ammonium metavanadate, 0.25% solution. Dissolve 1.25 g of NH4VO3 in 250 ml of hot water. Cool the solution and add 10 ml of conc. HNO3. Allow the solution to stand overnight, filter if necessary, and dilute with water to 500 ml. Store the solution in a polyethylene container. Ammonium molybdate, 5% solution. Dissolve 2.5 g of the reagent in 250 ml of water (at --50~ Allow the solution to stand overnight, filter if necessary, dilute with water to 500 ml, and store in a polyethylene container.
37.2. Methods of determining phosphate
329
Standard phosphorus(V) solution: 1 mg/ml. Preparation as in Section 37.2.1.
Procedure To the slightly acidic sample solution containing not more than 0.3 mg of P(V), add successively 2.5 ml of HNO3 (1+1), 2.5 ml of the vanadate solution, and 2.5 ml of the molybdate solution, mixing the solution after the addition of each reagent. Dilute the solution to volume with water in a 25-ml standard flask. After 30 min, measure the absorbance at 400 nm against a reagent blank solution.
37.2.3 Other methods With basic dyes, molybdophosphoric acid forms ion-associates which are the basis of sensitive extraction methods for determining P(V). Malachite Green [37--48], Crystal Violet [49-52], Brilliant Green [53,54], Ethyl Violet [55], Methylene Blue [56,57], Rhodamine B [58,59], and Rhodamine 6G [60,61] are among the most widely used. The ion-associates, which are sparingly soluble in water, can be extracted, and the absorbance of the extracts measured [39,40,55,56]. The associates may also be separated by flotation [44,49] or centrifugation [53,58] before being dissolved in polar solvents. Aqueous pseudo-solutions are often stabilized with, e.g., poly(vinyl alcohol) [38,41,50], Syntanol DS-10 [37], or Zephiramine [57]. The most sensitive methods include those utilizing Malachite Green (e = 3.3.105) [44] and Brilliant Green (e = 2.9.105) [53]. The derivative spectrophotometry technique has been applied in the determination of phosphate with Rhodamine 6G [61 ]. The molybdophosphate anion has been associated, in a HC1 medium, with a cationic complex of Co with 5-C1-PADAP. The sparingly soluble associate has been separated by flotation with butyl acetate and dissolved in methanol (~ - 3.4.105 at 560 nm) [62]. A method based on phosphoantimonylmolybdenum blue has also been proposed [62a]. Numerous indirect amplification methods have been devised for the determination of phosphate. The molybdenum in an extract of molybdophosphoric acid (Mo:P = 12:1) has been determined with thiocyanate [63], phenylfluorone [64], dithiol (e = 1.7-105), Sulphonitrophenol S (e - 4.6-105) [65], or 2,2'-diquinoxalyl [66]. The indirect method that involves the Fe(II)-ferrozine complex [67] is unusually sensitive (~ = 9.7.105). In another method involving the complex of Ce(III) and Arsenazo III, the phosphate gives a sparingly soluble CePO4 and liberates an equivalent quantity of Arsenazo III [68].
37.3. Analytical applications The phosphomolybdenum blue method has been used for determining phosphorus in biological materials [69-71], milk [72], vegetables [73], wine and blood serum [74], sewage [75,76], waters [5,11,77-87], soils and plants [88-94], rocks and minerals [95], geological deposits (sediments) [96], organic compounds [97], fertilisers [98], cast iron and steel [8,26,99,100], nickel and its alloys [26,100,101], copper alloys [101], aluminium alloys [102], platinum and gold [6], gallium and its compounds [103], iron ores [104], silicon [1], boron [2], tungsten materials [9], arsenic and its oxide [105], niobium- and tantalum oxides [106], neodymium and yttrium oxides [107], silicates [108-110], glass [111], coke [112], coal dust [91], silumins [ 10], and concentrated chloride solutions [ 113]. Optimum conditions for determining P in soils, waters, and plants were studied [81]. Phosphorus was determined by the FIA technique in soils [91] and waters [87]. The method has been applied for determination of phosphate in the presence of
330
37. Phosphorus
phosphate esters [114], pyrophosphate, and polyphosphate [115]. The condensed phosphates are converted into phosphates by boiling for 15 min in 2.5 M H2804 medium. When trace amounts of phosphates were determined in water, they were preconcentrated (as the P-Mo blue) in a column packed with silane-coated glass beads; DMF was used as the eluent [116]. The P-Mo blue may also be concentrated on a nitro- or acetylcellulose membrane in the presence of dodecyltrimethylammonium bromide [ 117]. The P-Mo blue method has been applied for automatic determination of P(V) in silicate rocks [118], soils [119], sea deposits [120], and water [121]. This method has been applied also in the FIA technique [122-126]. The method involving the M o - V - P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141 ]. The methods involving basic dyes have been used for determining phosphate in natural waters [38,39,41,43,48,55,59,142], biological materials [47,50,52], soils [45,51], uranium [37], iron [46,49], and nickel, cobalt, copper, and zinc [46]. The FIA technique has also been applied [40,43,48]. Various methods were applied for determining phosphorus in calcium chloride extracts [143]. The spectrophotometric methods for the determination (and speciation) of phosphorus in natural waters have been compared with other analytical techniques [144].
37.4. Determination of other phosphorus compounds Hypophosphite can be determined, in the presence of phosphate, from the colour it gives with ammonium molybdate in H2SO4 medium [145]. Hypophosphite may also be determined by its bleaching of the colour of the Fe(III)-SCN- complex [ 146 ]. Pyrophosphate is determined by using its effects on the colour reactions of Fe with SCN- or 1,10-phenanthroline [147]. A method for separation of hypophosphite, phosphite, phosphate, pyrophosphate, and triphosphate by ion exchange chromatography, with HC1 and KC1 solutions as eluents, has been proposed [148].
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