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Novel indirect spectrophotometric methods for determination of phosphate and arsenate using polyoxometalates and micellar medium
Journal of Molecular Liquids 118 (2005) 51 – 55 www.elsevier.com/locate/molliq
Novel indirect spectrophotometric methods for determination of phosphate and arsenate using polyoxometalates and micellar medium A.B. Vishnikin* Department of Analytical Chemistry, Dnipropetrovsk National University, per. Nauchnij, 13, Dnipropetrovsk 49050 Ukraine Available online 1 February 2005
Abstract Five various methods based on the use of polyoxometalates (POMs) have been developed for indirect spectrophotometric determination of phosphate or arsenate. Amplification procedure involving formation of the 12-molybdophosphate, its extraction with butyl acetate, and stripping with ammonia differs from known method by applying sensitive reaction of molybdenum(VI) with phenylfluorone in presence of the nonionic surfactant neonole at the last step of analysis. The calibration curve for phosphate is linear over the range of 2108 to 2107 M. The 5 molar absorptivity was 1.68106 l mol1 cm1 at 524 nm. Ion-exchange separation of metal ion excess from mixed POMs PMeW11O39 [Me– Co(II), Zn(II)] allows to realize indirect determination of phosphorus on metal atoms by means of spectrophotometry or atomic absorption spectrometry. Stability of POMs with Keggin structure is greatly increased by solubilization in micelles of nonionic surfactant. An explanation for this effect was proposed, and two analytical procedures based on this phenomenon were created. They include determination of phosphate or arsenate by means of measurement of absorbance in near UVand visible region of the spectrum as well as another amplification procedure which is based on destroying the molybdenum(VI) complexes with alizarin S or brompyrogallol red due to formation of POMs in the presence of phosphate. Finally, it was found that reaction of Keggin POMs with cyanine dyes leads to significant changes in colour of the ion associate by contrast with other dyes. The value of molar absorptivity is equal to 9.0104 l mol1 cm1 at 580 nm. D 2004 Elsevier B.V. All rights reserved. Keywords: Phosphate determination; Arsenate determination; Polyoxometalate; Micellar medium
1. Introduction Analytical signal measured on the last step of analysis arises usually from a compound containing atoms of the analyte. Then, we can say about direct determination. Indirect methods are used not so often. In this case, analytical signal comes from compound which does not contain an analyte. In this paper, several new ways allowing to realize indirect determination of the phosphate or arsenate will be considered. In addition, other problems concerning indirect methods, e.g., methods of separation of polyoxometalates (POMs) from reagent excess and the role of micelle surfactants in these systems will be taken into account. Applying indirect methods, it is possible more freely to choice suitable equipment or method of the determination * Tel.: +380 562 361864; fax: +380 562 466152. E-mail address: [email protected]. 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.07.012
and strongly improve sensitivity. It is important that selectivity of the determination depends in this case mainly on the separation step. Moreover, determination becomes more complex and less reproducible. Indirect methods are fairly often applied for determination of some nonmetals including phosphorus, arsenic, and silicon [1]. The reactions of the formation of POMs correspond well to these purposes because one can be easily found conditions for the formation of a unique compound with known composition and for the separation of reagent excess.
2. Experimental 2.1. Apparatus and reagents Spectrophotometric measurements were carried out by means of a Specord M-40 and SF-46 UV, VIS
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A.B. Vishnikin / Journal of Molecular Liquids 118 (2005) 51–55
spectrophotometers. Atomic-absorption measurements were made with an S-115 PKS flame spectrophotometer (Sumi, Ukraine). Analytical-grade reagents were used whenever possible. Mo(VI) and W(VI) (0.1 M ) stock solutions were prepared by dissolving of the recrystallized preparates of Na2MoO4d 2H2O and Na2WO4d 2H2O and stored in a polyethylene bottles. Phosphorus(V) stock solutions were prepared from potassium dihydrogen phosphate. Analytical grade hydrogen arsenate was used for the preparation of arsenic(V) solutions, standardized by iodometric titration. Sulfuric, hydrochloric acids, ammonia solutions were prepared by dilution of the concentrated solutions, and standardized by titration. Commercial n-butylacetate, metylisobutylketone were purified by distillation. Organic reagents phenylfluorone, astrazon violet, alizarine red S, nitrozo-R salt, pyridilazoresorzine were used as received. The 1% neonole (polyoxyethylene nonylphenole) was prepared fresh weekly. One percent ascorbic acid solution was prepared fresh every 2 days. Strong acid cation ionexchange resin CU-2 was converted into Na+ form by washing it with 1 M Na2SO4, then with water and finally drying in air.
2.2.2. Procedure II. Atomic absorption and spectrophotometric determination of phosphate using 11-tungstocobalto(II)(zinc(II))phosphate after ion-exchange separation To a 25-ml flask, add sample solution containing 8–60 Ag of phosphorus(V), 2.2 ml of 0.1 M Na2WO4, 2 ml of acetate buffer solution with pH 4.0–4.5, 0.8 ml of 0.1 M CoSO4(ZnSO4), dilute to the mark. Pass solution through a cationic ion-exchange column CU-2 in Na+ form (15 cm long, 2 cm bore). Elute HPA with water, dilute to the 50 ml. Determine the content of the cobalt(zinc) in two ways. For determining cobalt, add to a 50 ml flask 35 ml of eluted solution, 8 ml of 2% NH3, 2 ml of 1% citric acid, 1 ml of 0.2% nitrozo-R-salt, dilute to the mark. Measure the absorbance at 470 nm in a 3-cm cell. Zinc was determined with pyridilazoresorzine (PAR) according following procedure. Transfer an aliquot of zinc solution to a 25-ml standard flask, add 0.5 ml of 2103 M PAR, 1 ml of carbonate buffer (0.1 M Na2CO3+0.1 M NaHCO3), 1 ml of 103 M cethylpiridiniumchloride. Measure the absorbance at 501 nm in a 3-cm cell. To determine cobalt or zinc concentration by atomic-absorption spectroscopy measure the absorption at 240.7 or 213.9 nm, respectively, under the optimum operating conditions of the instrument, using an air-acetylene flame.
2.2. Procedures Five different procedures for indirect determining of phosphorus(V) and arsenic(V) are described below in the same order in which they are placed in the next section. 2.2.1. Procedure I. Method for determining phosphate in waters by amplification reaction using reduced 12-molybdophosphate Reduced form of 12-molybdophosphate was obtained according Ref. [2]. To a sample of water containing 2108 to 2107 M of ortophosphate add 4 ml of reagent (0.034 M Na2MoO4, 2.5 M H2SO4, 5104 M KSbOC4H4O6) and 1 ml of 2% ascorbic acid. Set aside the solution for 10 min. Transfer obtained heteropolyblue solution to a dry 250-ml separatory funnel, mix with 34 ml of 0.65 M H2SO4, add 10 ml butylacetate and shake 100 times. Allow the two layers to separate and discard the lower aqueous layer. Add 10 ml of 1.0 M HCl, shake 70 times, discard the aqueous layer. Repeat the washing. Transfer organic layer to a second dry 100-ml separatory funnel. Extract complex into water shaking 70 times with 10 ml of ammonia (1:1000). Heat for 5 min in a boiling water bath and then cool to 20 8C. Dilute the solution to 25 ml in a graduated flask. In a 25-ml flask mix 3 ml of 0.5 M H2SO4, 6 ml of 1% neonole, 1.5 ml of 4104 M ethanolic phenylfluorone solution, 6 ml of the sample solution and dilute to the 18 ml. Measure the absorbance at 524 nm in a 5-cm cell. The fitted equation was A=(0.05F0.02)+[(5.2F0.2)106]C P.
2.2.3. Procedure III. Spectrophotometric determination of arsenic(V) using 12-molybdoarsenate HPA and modification by neonole Place into 50-ml flask 4 ml of 0.01 M Na2MoO4, 0.25– 6.0 ml of 104 M Na3AsO4, 5 ml of 0.5 M H2SO4, 2 ml of 1% neonole, dilute to the volume with water. Measure the absorbance at 320 nm in a 5-cm quartz cell or at 350 nm in a 5-cm glass cell against a reagent blank. 2.2.4. Procedure IV. Spectrophotometric determination of phosphate in the copper alloys based on the ion association 5complex of PTi(IV)Mo11O40 with astrazon violet Transfer a suitable weight of sample to a 250-ml beaker and dissolve it in 10 ml of concentrated nitric acid. Cool and dilute to 250 ml. Transfer an aliquot of this solution containing not more than 0.6 Ag of P(V) to a 25-ml calibrated flask, add 5 ml of 0.5 M H2SO4, 0.5 ml of 0.002 M Ti2(SO4)3, 1.4 ml of 110-4 M astrazon violet, 2 ml of 0.1 M Na2MoO4, allow to stand for 5 min, dilute to the mark. Measure absorbance at 580 nm in a 5-cm cell against a reagent blank. 2.2.5. Procedure V. Indirect spectrophotometric determination of phosphate by nonextractive amplification reaction Into a series of 25-ml standard flasks, transfer 0.25, 0.5, 1.0, 1.5, 2.0, 2.5 ml of 1.0104 M KH2PO4, and then 0.75 ml of 0.01 M Na2MoO4, 0.75 ml of 0.5 M H2SO4, 1 ml of 1% neonole, allow to stand for 5 min, add 0.75 ml of 0.01 M alizarine red S, dilute to the mark with distilled water. After 15 min, measure absorbance of reagent blank against a sample solution at 490 nm in a 1-cm cells.
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phosphorus can be determined in stainless steels without separation.
3. Results and discussion 3.1. Amplification reaction
3.2. Ion-exchange separation method Improvement of sensitivity is the most obvious advantage of indirect methods. The so-called amplification reaction becomes possible by using POMs [1]. 12-Molybdophosphate heteropolyanion is formed on the first step of the proposed procedure . Only one complex with wellknown Keggin structure PM12O403 (M=Mo, W) prevails if the big excess of reagent is used. Separation of molybdate (tungstate) is carried out by extraction with butyl acetate. By using this organic solvent overall determination becomes very selective. Heteropolyanion (HPA) is destroyed after stripping with ammonia, and Mo or W are determined by the sensitive reaction with phenylfluorone (e 524=1.4105 l mol1 cm1 for Mo and e 515=1.0105 l mol1 cm1 for W) in the presence of neonole. It is clear that each mole of phosphorus leads in the end to formation of 12 mol of molybdenum complex with organic reagent. Thus, a high value of molar absorptivity is obtained for P that equals to 1.68106 l mol1 cm1. The double wash of the extract with 0.5 M sulfuric acid is required to make molybdate concentration lower than 107 M. Phosphorus content in water and in reagents is low and cannot interfere to the determination. In principle, determination of phosphorus(V) concentrations lower than 108 M is possible but the very big ratio of molybdate to phosphate (up to million times) makes this difficult. Rests of molybdate in the separating funnel and a lot of separation steps strongly lower reproducibility. Keeping all precautions, phosphate or arsenate ions can be determined over the concentration range from 2108 to 2107 M. The method is applicable for the determination of phosphate in natural, drinking and high pure waters (Table 1). As a rule, only molybdenum HPAs are used for determination of phosphorus. It is important that tungsten and some mixed HPAs can also be utilized. Due to the higher stability of these complexes, selectivity is significantly improved. For example,
Table 1 Determination of phosphate in waters by amplification reaction (n=5, a=0.05) Water
P found (AM)
Sr
Drinking water (from water pipe)
2.86F0.06 2.90F0.09a 1.27F0.07 1.28F0.05a 1.22F0.07 0.11F0.01 62.2F2.2 10.2F0.7 0.058F0.006
0.02 0.03 0.05 0.03 0.05 0.07 0.03 0.06 0.09
Artesian (village Tsarichanka) Mineral (bTsarichanskajaQ) Aerated (bBon BouissonQ) River Orel (Dnepr tributary) River Samara Distilled a
Determined by molybdenum blue method [2].
Indirect determination of P(V) or As(V) can be based not only on molybdenum atoms but also on metal atoms in mixed HPAs. In this case, conditions for formation of vacant Keggin heteropolyanion XM11O39n (M=Mo or W) must be achieved at the first step of analysis. Filling of vacancy with metal ion leads to mixed complex in which molar ratio of heteroatom to metal ion is 1:1. This idea was used in atomic absorption spectroscopy [3]. Phosphorus was determined on V, Bi, Fe atoms after extraction of mixed HPAs by methylisobutylketone. Because of the much better sensitivity of AAS determination of abovementioned metals in comparison with molybdenum, the lowest determination limit was achieved for phosphorus. However, this method has some disadvantages. Most of the mixed anions, especially of Me(II), are unstable in strongly acid medium created during extraction. With the exception of phosphorus, other elements cannot be determined by this method since only mixed HPA of P(V) can be extracted by oxygen-containing solvents. We propose to use ion-exchange method for removal of metal ions excess. It can be applied to any kind of mixed HPAs. Cation-exchange resin in the Na+ form absorbs all uncomplexed metal ions. On the contrary, mixed heteropolyanions are completely eluted from ion-exchange resin by water. Vacant PW11O397 HPA is formed in low acid solution (pH 3–5) and in the presence of excess of the tungstate. Mixed HPA is formed completely if a 15-fold excess of Co(II) or Zn(II) is used. Co or Zn atoms containing in mixed HPA are determined using atomic absorption spectroscopy, spectrophotometry or any other appropriate method. It is also important that a more safe and cheap air-acetylene flame can be used. The detection limit is 5107 M for phosphorus(V). 3.3. Micellar solubilization The third variant of indirect determination is based on using of surfactant-based micro heterogeneous organized systems. Application of micellar solutions of surface-active substances allows to improve characteristics of many existing methods of separation, preconcentration and determination, to propose their new variants. We may call micellar extraction, chromatography, phosphorescence at room temperature. Modification of coloured organic reagents in spectrophotometry leads to favourable changes in acid–base properties, high sensitivity and selectivity of the determination of various ions [4]. Only some types of POMs can interact with nonionic surfactants. In presence of nonionic surfactant, absorbance of an acidified aqueous mixture of molybdate and phosphate
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Table 2 Effect of nonionic surfactants on spectra of certain POMs
(arsenate) can increase up to 25 times in near visible and UV region (300–400 nm) (see Table 2). We believe that this phenomenon cannot be explained by formation of new complexes. Most probable reason of absorbance increasing is higher stability of HPA in micellar solutions and displacement of equilibrium to HPA side. Addition of surfactant has an effect which is analogous to that observed when solutions of HPA and oxygen-containing solvents are mixed. As shown earlier [5], this effect consists in more complete formation of HPA. A number of cations (including those of the alkali metals, alkali earth metals and perhaps H3O+) may be multiply complexed by bridging polyether oxygen atoms of nonionic surfactant thus facilitating the sorption of heteropolyanions by surfactant micelles [6]. Associates obtained in this way could be concentrated in micelles owing to hydrophobic and electrostatic interaction. The influence of water molecules is weakened in such environment and HPA become more stable. A resembling cation-chelating mechanism was proposed for explanation of the sorption of metal complexes including POM on polyurethane [7]. In order to determine whether neonole monomers or micelles are responsible for sensitization, absorption spectra for the arsenic HPA system were obtained over
a wide range of surfactant concentrations. Results are summarized in Fig. 1, which shows how the absorbance of the sensitized HPA complex depends on neonole concentration. All absorbances are close to each other at low concentrations of the complex, and neonole does not affect the curve shape. Only as the surfactant concentration increases through the broad critical micelle concentration region an increase in absorbance is observed. Carrying out the experiment at lower complex concentration does not change the saturation point. These results clearly indicate that the presence of micelles is necessary for sensitization by neonole. It was found that HPA in micellar solutions could not be extracted with oxygencontaining organic solvents or be reduced by ascorbic acid. This fact confirms that HPAs are encapsulated in micelles. It is noteworthy that spectra of beforehand-synthesized complex (e.g., H5PV2Mo10O40d nH2O) do not show any absorbance increasing in presence of nonionic surfactants. Moreover, formation of mixed HPAs is prevented in micellar solutions even in presence of big excess of vanadate 3 ions. For example, PMo12O40 is formed instead of 4 PVMo11O40 . 3 The maximum of the difference spectrum of AsMo12O40 recorded against a reference solution of identical acid and molybdenum concentration was found at 316 nm (Fig. 2), and corresponding molar absorptivity was 2.1104 l mol1 cm1. About the same values of band maxima and molar coefficients were observed for other molybdenum HPAs. For example, in water (for a-isomer) and water–acetonitrile (for h-isomer) solutions corresponding wavelengths of 3 band maxima are equal to 310 nm for a-PMo12O40 3 4 (e=2.4410 ), 327 nm for a-AsMo12O40 [5] and 319 nm 3 for h-AsMo12O40 (e=3.1104). Because of the increased stability of HPA in micellar solutions, it is not necessary to use big excess of reagents. It is enough to create the 7104 M molybdenum(VI) concentration at the arsenic(V) concentration of 8106
Fig. 1. Dependence of absorbance (at 350 nm) of solutions of the molybdoarsenate HPA on neonole concentration. CAs=4.0106 M (1); CAs=8.0106 M (2); CMo=8.0104 M, CH=0.2 M, l=5 cm.
Fig. 2. Absorption spectra of molybdoarsenate HPA at several As(V) concentrations: 4.0106 M (1), 2.0106 M (2), 1.0106 M (3), 5.0107 M (4).
POM
Surfactant
k max (nm)
e (without surfactant, mol1 l cm1)
e (in the presence of surfactant, mol1 l cm1)
PMo12O3 40
neonole, sintanole Triton X-100 OP-10 neonole
320
200
22,000
314
6700
18,200
310 316
9200 3500
19,000 21,000
PMo10W2O3 40 PVMo11O4 40 AsMo12O3 40
A.B. Vishnikin / Journal of Molecular Liquids 118 (2005) 51–55
M for complete formation of the Keggin HPA. Without a surfactant, this reaction occurs at a molybdenum concentration of 0.01 M.
55
nation of phosphorus in copper, aluminum and many other alloys without separation. 3.5. Nonextractive amplification reaction
2 þ 3 AsO3 4 þ 12MoO4 þ 24H fAsMo12 O40 þ 12H2 O
After formation molybdoarsenate HPA is stable in wide acidity range from pH 1 to 2 M on sulfuric acid. The calibration curve is linear over the concentration range 5107 to 1.2105 M of arsenic(V). 3.4. Ion associates of POMs with cyanine dyes Reactions of heteropolyanions with basic dyes in presence of nonionic surfactants are widely used for determination of some nonmetals [8]. HPA can bind up to five cations of basic dye. Thus, in this case, principles of multiplication and indirect determination are also realized. Up to the present, three types of dyes have found wide application: triphenylmethane, rhodamine, and antipyrine. Typically, spectra of dyes and ion association complexes are fully identical. Therefore, it is necessary to separate the excess of dye as well as its ion association complex with isopolymolybdates. Reaction of Keggin HPAs with cyanine (polymethyne) dyes leads to considerable changes in spectra. The intensity of the main band of the dye is slightly diminished new band is appeared at 580 nm. For ion association complex (Az)5PTi(IV)Mo11O39 (where Az is astrazon violet) value of e 580 is equal to 9.0104 l mol1 cm1. Gelatin or nonionic surfactants have been used for solubilization of obtained precipitate. At low phosphorus concentrations, solutions of the ion association complex are stable without using of surfactant. It is important to note that isopolymolybdates do not form stable ion association complexes with cyanine dyes. Besides excluding the solvent extraction, simplification of analysis, acceleration of determination, high reproducibility are important for the analysis. Interaction of HPAs with micelles of the surfactant and formation of stable mixed complex favourable changes selectivity. The proposed method can be applied to the fast and simple determi-
Another new method for indirect determining phosphorus using micellar solubilization may be proposed. It is based on the absorbance decreasing caused by reaction between phosphate and complexes of molybdenum with the organic reagents alizarin red S or brompyrogallol red (BPR). In view of the fact that each mole of phosphate ions can destruct up to 12 mol of molybdenum complex bamplificationQ reaction has been realized in studied system. Such reactions go to completion only in the presence of the nonionic surfactant. In this case HPA is more stable than complex of molybdenum(VI) with the organic reagent, and equilibrium can be displaced to HPA side in reaction: nMo(R)2+PfPMon+2nR By using brompyrogallol red as R, the coefficient of amplification n is equal to 9 and effective molar absorptivity for phosphate is 6.7105 l mol1 cm1. In summary, it can be stated that application of indirect methods of analysis using POMs and modern methods of separation and modification allow further to develop this area of analytical chemistry.
References [1] W.J. Williams, Handbook of Anion Determination, Butterworths, London, 1979. [2] J. Murphy, J.P. Riley, Anal. Chim. Acta 27 (1962) 31 – 36. [3] O.M. Trokhimenko, N.F. Falendysh, J. Anal. Chem. 53 (1998) 475 – 478. [4] S.N. Shtykov, J. Anal. Chem. 55 (2000) 687 – 697. [5] A. Halasz, E. Pungor, Talanta 18 (1971) 569 – 575. [6] A.Yu. Nazarenko, M.M. Tananajko, G.A. Totradze, Dokl. Akad. Nauk Ukr. SSR, Ser. B Geol. Him. Biol Nauki 11 (1983) 50 – 52. [7] R.F. Hamon, A.S. Khan, A. Chow, Talanta 29 (1982) 313 – 326. [8] I.P. Alimarin, E.N. Dorokhova, V.P. Givopiszev, E.G. Bondareva, P.G. Kazmin, J. Anal. Chem. 39 (1984) 965 – 981.
Novel indirect spectrophotometric methods for determination of phosphate and arsenate using polyoxometalates and micellar medium A.B. Vishnikin* Department of Analytical Chemistry, Dnipropetrovsk National University, per. Nauchnij, 13, Dnipropetrovsk 49050 Ukraine Available online 1 February 2005
Abstract Five various methods based on the use of polyoxometalates (POMs) have been developed for indirect spectrophotometric determination of phosphate or arsenate. Amplification procedure involving formation of the 12-molybdophosphate, its extraction with butyl acetate, and stripping with ammonia differs from known method by applying sensitive reaction of molybdenum(VI) with phenylfluorone in presence of the nonionic surfactant neonole at the last step of analysis. The calibration curve for phosphate is linear over the range of 2108 to 2107 M. The 5 molar absorptivity was 1.68106 l mol1 cm1 at 524 nm. Ion-exchange separation of metal ion excess from mixed POMs PMeW11O39 [Me– Co(II), Zn(II)] allows to realize indirect determination of phosphorus on metal atoms by means of spectrophotometry or atomic absorption spectrometry. Stability of POMs with Keggin structure is greatly increased by solubilization in micelles of nonionic surfactant. An explanation for this effect was proposed, and two analytical procedures based on this phenomenon were created. They include determination of phosphate or arsenate by means of measurement of absorbance in near UVand visible region of the spectrum as well as another amplification procedure which is based on destroying the molybdenum(VI) complexes with alizarin S or brompyrogallol red due to formation of POMs in the presence of phosphate. Finally, it was found that reaction of Keggin POMs with cyanine dyes leads to significant changes in colour of the ion associate by contrast with other dyes. The value of molar absorptivity is equal to 9.0104 l mol1 cm1 at 580 nm. D 2004 Elsevier B.V. All rights reserved. Keywords: Phosphate determination; Arsenate determination; Polyoxometalate; Micellar medium
1. Introduction Analytical signal measured on the last step of analysis arises usually from a compound containing atoms of the analyte. Then, we can say about direct determination. Indirect methods are used not so often. In this case, analytical signal comes from compound which does not contain an analyte. In this paper, several new ways allowing to realize indirect determination of the phosphate or arsenate will be considered. In addition, other problems concerning indirect methods, e.g., methods of separation of polyoxometalates (POMs) from reagent excess and the role of micelle surfactants in these systems will be taken into account. Applying indirect methods, it is possible more freely to choice suitable equipment or method of the determination * Tel.: +380 562 361864; fax: +380 562 466152. E-mail address: [email protected]. 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.07.012
and strongly improve sensitivity. It is important that selectivity of the determination depends in this case mainly on the separation step. Moreover, determination becomes more complex and less reproducible. Indirect methods are fairly often applied for determination of some nonmetals including phosphorus, arsenic, and silicon [1]. The reactions of the formation of POMs correspond well to these purposes because one can be easily found conditions for the formation of a unique compound with known composition and for the separation of reagent excess.
2. Experimental 2.1. Apparatus and reagents Spectrophotometric measurements were carried out by means of a Specord M-40 and SF-46 UV, VIS
52
A.B. Vishnikin / Journal of Molecular Liquids 118 (2005) 51–55
spectrophotometers. Atomic-absorption measurements were made with an S-115 PKS flame spectrophotometer (Sumi, Ukraine). Analytical-grade reagents were used whenever possible. Mo(VI) and W(VI) (0.1 M ) stock solutions were prepared by dissolving of the recrystallized preparates of Na2MoO4d 2H2O and Na2WO4d 2H2O and stored in a polyethylene bottles. Phosphorus(V) stock solutions were prepared from potassium dihydrogen phosphate. Analytical grade hydrogen arsenate was used for the preparation of arsenic(V) solutions, standardized by iodometric titration. Sulfuric, hydrochloric acids, ammonia solutions were prepared by dilution of the concentrated solutions, and standardized by titration. Commercial n-butylacetate, metylisobutylketone were purified by distillation. Organic reagents phenylfluorone, astrazon violet, alizarine red S, nitrozo-R salt, pyridilazoresorzine were used as received. The 1% neonole (polyoxyethylene nonylphenole) was prepared fresh weekly. One percent ascorbic acid solution was prepared fresh every 2 days. Strong acid cation ionexchange resin CU-2 was converted into Na+ form by washing it with 1 M Na2SO4, then with water and finally drying in air.
2.2.2. Procedure II. Atomic absorption and spectrophotometric determination of phosphate using 11-tungstocobalto(II)(zinc(II))phosphate after ion-exchange separation To a 25-ml flask, add sample solution containing 8–60 Ag of phosphorus(V), 2.2 ml of 0.1 M Na2WO4, 2 ml of acetate buffer solution with pH 4.0–4.5, 0.8 ml of 0.1 M CoSO4(ZnSO4), dilute to the mark. Pass solution through a cationic ion-exchange column CU-2 in Na+ form (15 cm long, 2 cm bore). Elute HPA with water, dilute to the 50 ml. Determine the content of the cobalt(zinc) in two ways. For determining cobalt, add to a 50 ml flask 35 ml of eluted solution, 8 ml of 2% NH3, 2 ml of 1% citric acid, 1 ml of 0.2% nitrozo-R-salt, dilute to the mark. Measure the absorbance at 470 nm in a 3-cm cell. Zinc was determined with pyridilazoresorzine (PAR) according following procedure. Transfer an aliquot of zinc solution to a 25-ml standard flask, add 0.5 ml of 2103 M PAR, 1 ml of carbonate buffer (0.1 M Na2CO3+0.1 M NaHCO3), 1 ml of 103 M cethylpiridiniumchloride. Measure the absorbance at 501 nm in a 3-cm cell. To determine cobalt or zinc concentration by atomic-absorption spectroscopy measure the absorption at 240.7 or 213.9 nm, respectively, under the optimum operating conditions of the instrument, using an air-acetylene flame.
2.2. Procedures Five different procedures for indirect determining of phosphorus(V) and arsenic(V) are described below in the same order in which they are placed in the next section. 2.2.1. Procedure I. Method for determining phosphate in waters by amplification reaction using reduced 12-molybdophosphate Reduced form of 12-molybdophosphate was obtained according Ref. [2]. To a sample of water containing 2108 to 2107 M of ortophosphate add 4 ml of reagent (0.034 M Na2MoO4, 2.5 M H2SO4, 5104 M KSbOC4H4O6) and 1 ml of 2% ascorbic acid. Set aside the solution for 10 min. Transfer obtained heteropolyblue solution to a dry 250-ml separatory funnel, mix with 34 ml of 0.65 M H2SO4, add 10 ml butylacetate and shake 100 times. Allow the two layers to separate and discard the lower aqueous layer. Add 10 ml of 1.0 M HCl, shake 70 times, discard the aqueous layer. Repeat the washing. Transfer organic layer to a second dry 100-ml separatory funnel. Extract complex into water shaking 70 times with 10 ml of ammonia (1:1000). Heat for 5 min in a boiling water bath and then cool to 20 8C. Dilute the solution to 25 ml in a graduated flask. In a 25-ml flask mix 3 ml of 0.5 M H2SO4, 6 ml of 1% neonole, 1.5 ml of 4104 M ethanolic phenylfluorone solution, 6 ml of the sample solution and dilute to the 18 ml. Measure the absorbance at 524 nm in a 5-cm cell. The fitted equation was A=(0.05F0.02)+[(5.2F0.2)106]C P.
2.2.3. Procedure III. Spectrophotometric determination of arsenic(V) using 12-molybdoarsenate HPA and modification by neonole Place into 50-ml flask 4 ml of 0.01 M Na2MoO4, 0.25– 6.0 ml of 104 M Na3AsO4, 5 ml of 0.5 M H2SO4, 2 ml of 1% neonole, dilute to the volume with water. Measure the absorbance at 320 nm in a 5-cm quartz cell or at 350 nm in a 5-cm glass cell against a reagent blank. 2.2.4. Procedure IV. Spectrophotometric determination of phosphate in the copper alloys based on the ion association 5complex of PTi(IV)Mo11O40 with astrazon violet Transfer a suitable weight of sample to a 250-ml beaker and dissolve it in 10 ml of concentrated nitric acid. Cool and dilute to 250 ml. Transfer an aliquot of this solution containing not more than 0.6 Ag of P(V) to a 25-ml calibrated flask, add 5 ml of 0.5 M H2SO4, 0.5 ml of 0.002 M Ti2(SO4)3, 1.4 ml of 110-4 M astrazon violet, 2 ml of 0.1 M Na2MoO4, allow to stand for 5 min, dilute to the mark. Measure absorbance at 580 nm in a 5-cm cell against a reagent blank. 2.2.5. Procedure V. Indirect spectrophotometric determination of phosphate by nonextractive amplification reaction Into a series of 25-ml standard flasks, transfer 0.25, 0.5, 1.0, 1.5, 2.0, 2.5 ml of 1.0104 M KH2PO4, and then 0.75 ml of 0.01 M Na2MoO4, 0.75 ml of 0.5 M H2SO4, 1 ml of 1% neonole, allow to stand for 5 min, add 0.75 ml of 0.01 M alizarine red S, dilute to the mark with distilled water. After 15 min, measure absorbance of reagent blank against a sample solution at 490 nm in a 1-cm cells.
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phosphorus can be determined in stainless steels without separation.
3. Results and discussion 3.1. Amplification reaction
3.2. Ion-exchange separation method Improvement of sensitivity is the most obvious advantage of indirect methods. The so-called amplification reaction becomes possible by using POMs [1]. 12-Molybdophosphate heteropolyanion is formed on the first step of the proposed procedure . Only one complex with wellknown Keggin structure PM12O403 (M=Mo, W) prevails if the big excess of reagent is used. Separation of molybdate (tungstate) is carried out by extraction with butyl acetate. By using this organic solvent overall determination becomes very selective. Heteropolyanion (HPA) is destroyed after stripping with ammonia, and Mo or W are determined by the sensitive reaction with phenylfluorone (e 524=1.4105 l mol1 cm1 for Mo and e 515=1.0105 l mol1 cm1 for W) in the presence of neonole. It is clear that each mole of phosphorus leads in the end to formation of 12 mol of molybdenum complex with organic reagent. Thus, a high value of molar absorptivity is obtained for P that equals to 1.68106 l mol1 cm1. The double wash of the extract with 0.5 M sulfuric acid is required to make molybdate concentration lower than 107 M. Phosphorus content in water and in reagents is low and cannot interfere to the determination. In principle, determination of phosphorus(V) concentrations lower than 108 M is possible but the very big ratio of molybdate to phosphate (up to million times) makes this difficult. Rests of molybdate in the separating funnel and a lot of separation steps strongly lower reproducibility. Keeping all precautions, phosphate or arsenate ions can be determined over the concentration range from 2108 to 2107 M. The method is applicable for the determination of phosphate in natural, drinking and high pure waters (Table 1). As a rule, only molybdenum HPAs are used for determination of phosphorus. It is important that tungsten and some mixed HPAs can also be utilized. Due to the higher stability of these complexes, selectivity is significantly improved. For example,
Table 1 Determination of phosphate in waters by amplification reaction (n=5, a=0.05) Water
P found (AM)
Sr
Drinking water (from water pipe)
2.86F0.06 2.90F0.09a 1.27F0.07 1.28F0.05a 1.22F0.07 0.11F0.01 62.2F2.2 10.2F0.7 0.058F0.006
0.02 0.03 0.05 0.03 0.05 0.07 0.03 0.06 0.09
Artesian (village Tsarichanka) Mineral (bTsarichanskajaQ) Aerated (bBon BouissonQ) River Orel (Dnepr tributary) River Samara Distilled a
Determined by molybdenum blue method [2].
Indirect determination of P(V) or As(V) can be based not only on molybdenum atoms but also on metal atoms in mixed HPAs. In this case, conditions for formation of vacant Keggin heteropolyanion XM11O39n (M=Mo or W) must be achieved at the first step of analysis. Filling of vacancy with metal ion leads to mixed complex in which molar ratio of heteroatom to metal ion is 1:1. This idea was used in atomic absorption spectroscopy [3]. Phosphorus was determined on V, Bi, Fe atoms after extraction of mixed HPAs by methylisobutylketone. Because of the much better sensitivity of AAS determination of abovementioned metals in comparison with molybdenum, the lowest determination limit was achieved for phosphorus. However, this method has some disadvantages. Most of the mixed anions, especially of Me(II), are unstable in strongly acid medium created during extraction. With the exception of phosphorus, other elements cannot be determined by this method since only mixed HPA of P(V) can be extracted by oxygen-containing solvents. We propose to use ion-exchange method for removal of metal ions excess. It can be applied to any kind of mixed HPAs. Cation-exchange resin in the Na+ form absorbs all uncomplexed metal ions. On the contrary, mixed heteropolyanions are completely eluted from ion-exchange resin by water. Vacant PW11O397 HPA is formed in low acid solution (pH 3–5) and in the presence of excess of the tungstate. Mixed HPA is formed completely if a 15-fold excess of Co(II) or Zn(II) is used. Co or Zn atoms containing in mixed HPA are determined using atomic absorption spectroscopy, spectrophotometry or any other appropriate method. It is also important that a more safe and cheap air-acetylene flame can be used. The detection limit is 5107 M for phosphorus(V). 3.3. Micellar solubilization The third variant of indirect determination is based on using of surfactant-based micro heterogeneous organized systems. Application of micellar solutions of surface-active substances allows to improve characteristics of many existing methods of separation, preconcentration and determination, to propose their new variants. We may call micellar extraction, chromatography, phosphorescence at room temperature. Modification of coloured organic reagents in spectrophotometry leads to favourable changes in acid–base properties, high sensitivity and selectivity of the determination of various ions [4]. Only some types of POMs can interact with nonionic surfactants. In presence of nonionic surfactant, absorbance of an acidified aqueous mixture of molybdate and phosphate
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A.B. Vishnikin / Journal of Molecular Liquids 118 (2005) 51–55
Table 2 Effect of nonionic surfactants on spectra of certain POMs
(arsenate) can increase up to 25 times in near visible and UV region (300–400 nm) (see Table 2). We believe that this phenomenon cannot be explained by formation of new complexes. Most probable reason of absorbance increasing is higher stability of HPA in micellar solutions and displacement of equilibrium to HPA side. Addition of surfactant has an effect which is analogous to that observed when solutions of HPA and oxygen-containing solvents are mixed. As shown earlier [5], this effect consists in more complete formation of HPA. A number of cations (including those of the alkali metals, alkali earth metals and perhaps H3O+) may be multiply complexed by bridging polyether oxygen atoms of nonionic surfactant thus facilitating the sorption of heteropolyanions by surfactant micelles [6]. Associates obtained in this way could be concentrated in micelles owing to hydrophobic and electrostatic interaction. The influence of water molecules is weakened in such environment and HPA become more stable. A resembling cation-chelating mechanism was proposed for explanation of the sorption of metal complexes including POM on polyurethane [7]. In order to determine whether neonole monomers or micelles are responsible for sensitization, absorption spectra for the arsenic HPA system were obtained over
a wide range of surfactant concentrations. Results are summarized in Fig. 1, which shows how the absorbance of the sensitized HPA complex depends on neonole concentration. All absorbances are close to each other at low concentrations of the complex, and neonole does not affect the curve shape. Only as the surfactant concentration increases through the broad critical micelle concentration region an increase in absorbance is observed. Carrying out the experiment at lower complex concentration does not change the saturation point. These results clearly indicate that the presence of micelles is necessary for sensitization by neonole. It was found that HPA in micellar solutions could not be extracted with oxygencontaining organic solvents or be reduced by ascorbic acid. This fact confirms that HPAs are encapsulated in micelles. It is noteworthy that spectra of beforehand-synthesized complex (e.g., H5PV2Mo10O40d nH2O) do not show any absorbance increasing in presence of nonionic surfactants. Moreover, formation of mixed HPAs is prevented in micellar solutions even in presence of big excess of vanadate 3 ions. For example, PMo12O40 is formed instead of 4 PVMo11O40 . 3 The maximum of the difference spectrum of AsMo12O40 recorded against a reference solution of identical acid and molybdenum concentration was found at 316 nm (Fig. 2), and corresponding molar absorptivity was 2.1104 l mol1 cm1. About the same values of band maxima and molar coefficients were observed for other molybdenum HPAs. For example, in water (for a-isomer) and water–acetonitrile (for h-isomer) solutions corresponding wavelengths of 3 band maxima are equal to 310 nm for a-PMo12O40 3 4 (e=2.4410 ), 327 nm for a-AsMo12O40 [5] and 319 nm 3 for h-AsMo12O40 (e=3.1104). Because of the increased stability of HPA in micellar solutions, it is not necessary to use big excess of reagents. It is enough to create the 7104 M molybdenum(VI) concentration at the arsenic(V) concentration of 8106
Fig. 1. Dependence of absorbance (at 350 nm) of solutions of the molybdoarsenate HPA on neonole concentration. CAs=4.0106 M (1); CAs=8.0106 M (2); CMo=8.0104 M, CH=0.2 M, l=5 cm.
Fig. 2. Absorption spectra of molybdoarsenate HPA at several As(V) concentrations: 4.0106 M (1), 2.0106 M (2), 1.0106 M (3), 5.0107 M (4).
POM
Surfactant
k max (nm)
e (without surfactant, mol1 l cm1)
e (in the presence of surfactant, mol1 l cm1)
PMo12O3 40
neonole, sintanole Triton X-100 OP-10 neonole
320
200
22,000
314
6700
18,200
310 316
9200 3500
19,000 21,000
PMo10W2O3 40 PVMo11O4 40 AsMo12O3 40
A.B. Vishnikin / Journal of Molecular Liquids 118 (2005) 51–55
M for complete formation of the Keggin HPA. Without a surfactant, this reaction occurs at a molybdenum concentration of 0.01 M.
55
nation of phosphorus in copper, aluminum and many other alloys without separation. 3.5. Nonextractive amplification reaction
2 þ 3 AsO3 4 þ 12MoO4 þ 24H fAsMo12 O40 þ 12H2 O
After formation molybdoarsenate HPA is stable in wide acidity range from pH 1 to 2 M on sulfuric acid. The calibration curve is linear over the concentration range 5107 to 1.2105 M of arsenic(V). 3.4. Ion associates of POMs with cyanine dyes Reactions of heteropolyanions with basic dyes in presence of nonionic surfactants are widely used for determination of some nonmetals [8]. HPA can bind up to five cations of basic dye. Thus, in this case, principles of multiplication and indirect determination are also realized. Up to the present, three types of dyes have found wide application: triphenylmethane, rhodamine, and antipyrine. Typically, spectra of dyes and ion association complexes are fully identical. Therefore, it is necessary to separate the excess of dye as well as its ion association complex with isopolymolybdates. Reaction of Keggin HPAs with cyanine (polymethyne) dyes leads to considerable changes in spectra. The intensity of the main band of the dye is slightly diminished new band is appeared at 580 nm. For ion association complex (Az)5PTi(IV)Mo11O39 (where Az is astrazon violet) value of e 580 is equal to 9.0104 l mol1 cm1. Gelatin or nonionic surfactants have been used for solubilization of obtained precipitate. At low phosphorus concentrations, solutions of the ion association complex are stable without using of surfactant. It is important to note that isopolymolybdates do not form stable ion association complexes with cyanine dyes. Besides excluding the solvent extraction, simplification of analysis, acceleration of determination, high reproducibility are important for the analysis. Interaction of HPAs with micelles of the surfactant and formation of stable mixed complex favourable changes selectivity. The proposed method can be applied to the fast and simple determi-
Another new method for indirect determining phosphorus using micellar solubilization may be proposed. It is based on the absorbance decreasing caused by reaction between phosphate and complexes of molybdenum with the organic reagents alizarin red S or brompyrogallol red (BPR). In view of the fact that each mole of phosphate ions can destruct up to 12 mol of molybdenum complex bamplificationQ reaction has been realized in studied system. Such reactions go to completion only in the presence of the nonionic surfactant. In this case HPA is more stable than complex of molybdenum(VI) with the organic reagent, and equilibrium can be displaced to HPA side in reaction: nMo(R)2+PfPMon+2nR By using brompyrogallol red as R, the coefficient of amplification n is equal to 9 and effective molar absorptivity for phosphate is 6.7105 l mol1 cm1. In summary, it can be stated that application of indirect methods of analysis using POMs and modern methods of separation and modification allow further to develop this area of analytical chemistry.
References [1] W.J. Williams, Handbook of Anion Determination, Butterworths, London, 1979. [2] J. Murphy, J.P. Riley, Anal. Chim. Acta 27 (1962) 31 – 36. [3] O.M. Trokhimenko, N.F. Falendysh, J. Anal. Chem. 53 (1998) 475 – 478. [4] S.N. Shtykov, J. Anal. Chem. 55 (2000) 687 – 697. [5] A. Halasz, E. Pungor, Talanta 18 (1971) 569 – 575. [6] A.Yu. Nazarenko, M.M. Tananajko, G.A. Totradze, Dokl. Akad. Nauk Ukr. SSR, Ser. B Geol. Him. Biol Nauki 11 (1983) 50 – 52. [7] R.F. Hamon, A.S. Khan, A. Chow, Talanta 29 (1982) 313 – 326. [8] I.P. Alimarin, E.N. Dorokhova, V.P. Givopiszev, E.G. Bondareva, P.G. Kazmin, J. Anal. Chem. 39 (1984) 965 – 981.