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An indirect method for determining phosphorus in aluminium alloys by atomic-absorption spectrometry
0339.9140/81/070469-04102.00/O Copyright 0 1981 Pergamon PressLtd
Tnlanto, Vol. 28. pp. 469 to 472. 1981 Printed m Great Bntain. All nghts reserved
SHORT
COMMUNICATIONS
AN INDIRECT METHOD FOR DETERMINING PHOSPHORUS IN ALUMINIUM ALLOYS BY ATOMIC-ABSORPTION SPECTROMETRY* J. L. BERNAL, M”. J. de1 NOZAL, L. DEBAN and A. J. ALLER Department of Analytical Chemistry, Faculty of Science, Prado de la Magdalena, Valladolid, Spain (Received 28 January 1980. Revised 28 October 1980. Accepted 23 December 1980) Summary-An
indirect method is described for the determination of phosphorus in aluminium alloys. Ammonium molybdate is added to a solution of the aluminium alloy and the molybdophosphoric acid formed is selectively extracted into n-butyl acetate. The twelve molybdenum atoms associated with each phosphate ion are determined by direct atomic-absorption spectrometry with the n-butyl acetate phase in a nitrous oxideacetylene flame, with measurement at 313.2 nm. The most suitable conditions have been established and the effect of other ions has been studied.
Most methods for the determination of phosphate are based on the formation of heteropoly acids.’ The sensitivity and selectivity of these methods are enhanced, if the complex is extracted into a suitable organic solvent2-6 before measurement. Such methods have been used in atomic-absorption spectrophotometry7 in place of direct flame atomic spectrometric methods8 the phosphorus content being determined indirectly by formation and extraction of phosphomolybdic acid and determination of the molybdenum in the organic phase by atomic absorption.g-‘2 An alternativer3*14 is to decompose the heteropolymolybdate in the separated extract and to strip the molybdenum into an aqueous phase for subsequent determination. The selectivity of the extraction, and the factors which affect it (principally the pH) have been widely investigated. Despite the difficulties related to the use of organic solvents in atomic-absorption work, many applications have been made of this method for determination of phosphorus, e.g., in biochemical,17-2’ agricultural,22 metallurgical,14*23-25 water10*26P2* and rockI analysis. This paper describes a method for the determination of phosphorus in aluminium-based alloys by indirect atomic-absorption, taking into account the possible interference of other ions. EXPERIMENTAL A double-beam Pye-Unicam SP-1900 atomic-absorption spectrometer was used in conjunction with a Unicam molybdenum hollow-cathode lamp. The instrument conditions used throughout were: slit-width 0.15 mm; wave* Taken in part from the Ph.D. work of A. J. Aller.
length 313.2 nm; lamp current 7 mA; a lo-cm burner was used with the air-acetylene flame and a 5-cm burner with the nitrous oxide-acetylene flame, both burners having a 0.7-mm wide slot. Reagents Ammonium molybdate solution. Dissolve 10.69 g of ammonium molybdate tetrahydrate, (NH4),Mo,02, .4H20, in distilled water and dilute to 1 litre. Standard phosphorus solution. Dissolve 0.1098 g of potassium dihydrogen phosphate, KH,PO,, in distilled water and dilute to 1 litre. Wash solution. Nitric acid diluted 1:1 with distilled water. Use analytical-reagent grade chemicals and store all solutions in polyethylene bottles to avoid contamination with silicon. Procedure
Dissolve the sample [large enough to give a final phosphorus concentration of 0.1-l pg/ml, but not less than 0.5 g (to avoid problems of inhomogeneity)] in 25 ml (more if necessary) of a 10: 10: 1 v/v mixture of nitric acid (1 + l), hydrochloric acid (1 + 1) and hydrogen peroxide (30%) by heating in a covered Teflon flask. For phosphorus contents >O.Ol’~, make this solution up to a suitable standard volume (e.g., 50 ml) and use a suitable aliquot for the subsequent analysis. Dilute this solution to about 50 ml with demineralized water, add 5 ml of ammonium molybdate solution and swirl the mixture for 3 min. Adjust to pH 1, transfer to a lOO-ml glass separating funnel, then shake with 20 ml of n-butyl acetate for 1 min. Discard the aqueous phase and wash the organic phase with three lo-ml portions of wash solution. Spray the organic phase directly into the flame and determine the molybdenum content. Use the organic solvent for determining the blank. RESULTS AND
DISCUSSION
Extraction of phosphomolybdic acid The extraction of molybdic acid into various solvents was investigated by shaking vigorously 20 ml of 469
470
SHORT COMMUNlCATIONS
PH Fig. 1. Relationship between degree of extraction and pH for the organic solvents used. -ODiethyl ether; --ct Acetophenone; -OMIBK; e b-butyl acetate; an-butanol; -w+ n-amyl alcohol; -A- isoamyl alcohol.
solvent with the aqueous molybdate solution (at various pH values) for 1 min and measuring the amount of molybdate in the organic phase. The same solvents were tested as extractants for phosphomolybdie acid, the solvent being added to a 0.5-ppm phosphorus solution obtained by mixing 10 ml of ammonium molydate reagent with potassium of dihydrogen phosphate solution and adjusted to the desired pH with buffer. Preliminary experiments showed that shaking for 1 min was sufficient. The results are shown in Fig. 1, where it can be seen that the amount extracted increases as the pH decreases and becomes quantitative over the pH range 0.4-1.6, depending on the solvent. For further work the pH was adjusted to 1 for convenience. From the results it was deduced that under the conditions used the best solvent is n-butyl acetate, because it is the most highly selective and gives virtually lOOo/, extraction in a single step. It is rec-
ommended that before aspiration into the flame the organic phase is washed to remove any molybdate reagent mechanically carried over from the aqueous phase. Calibration graph and optimum concentration ranges When the recommended procedure is used, the calibration graph for the determination of phosphorus is linear over the range 0.01-l ppm of phosphorus in the original aqueous solution. Precision A series of absorbance values was obtained by replicate (tenfold) analysis of 25 ml of aluminium alloy solution containing 0.8 ppm of phosphorus. The average absorbance was 0.48 and the relative standard deviation was 1.2% with the air-acetylene flame and 0.8% for the nitrous oxide-acetylene flame.
471
SHORT COMMUNICATIONS
Table 1. Concentrations (ppm) of Si, As, W and Ge which do not interfere in the phosphorus determination (0.5 ppm in aqueous solution) when different organic solvents are used Solvent n-Butyl acetate MIBK Acetophenone Diethyl ether n-Amy1 alcohol Isoamyl alcohol n-Butanol
Si
As
W
Ge
4000 10 200 2000 100 60 30
3000 20 300 300 60 50 20
4000
4000 20 200 2000 80 40 50
;: 3000 300 400 60
centration) interfered with the determination of phosphorus if n-butyl acetate was used as extractant. The use of other solvents increases the interference by these elements. Application of the method to metallurgical samples
The accuracy of the method was assessed by the determination of P in B.C.S. and other aluminium alloys. Table 2 shows the results, which confirm the validity and suitability of the proposed method.
REFERENCES
Detection limits and sensitivity
Khimia Fosfora, Nauka, MOSCOW, 1. Analiticheskaya 1974. 2. A. Syty and J. A. Dean, Appl. Optics, 1968, 7, 1331. 3. J. Paul and W. F. R. Pover, Anal. Chim. Acta, 1960, 22,
With a nitrous oxide-acetylene flame the detection limit is nearly 0.01 ppm of phosphorus in the aqueous solution, and the sensitivity (concentration for 1% absorption) is of the order of 0.02 ppm. The practical concentration range is 0.0&l ppm. The determination in the air-acetylene flame has a practical range of 0.1-l ppm. The detection limit of aqueous phosphorus solutions can be lowered to 0.03 ppm and the sensitivity is 0.05 ppm (for 1% absorption).
10. W. S. Zaugg and R. J. Knox, Anal. Chem., 1966. 38,
Effect of foreign ions
11. T. Kumamaru,
185.
4. 5. 6. 7.
J. Paul, Mikrochim. Acta, 1965, 830. Idem, ibid., 1965, 836.
Idem, Anal. Chim. Acta, 1966,35, 200. M. S. Cresser, Solvent Extraction in Flame Spectroscopic Analysis, Butterworths, London, 1978. 8. D. C. Manning and S. Slavin, Atom. Abs. News/., 1969,
8, 132. 9. G. F. Kirkbright, A. M. Smith and T. S. West, Analyst, 1967, 92, 411. 1759.
The effect of a number of ions on the determination of 0.5 ppm of phosphorus by the recommended -procedure has been investigated. An ion was considered not to interfere if it produced less than 5% change in the absorbances. No interference was observed from 5000 ppm of the following ions when tested individually: Bi3+, Ca2+ Cd2+, Ce3+, Cr3+, Cu’+, Fe3+, Hg*+, Mg2+, La’+, Mn2+, Nb5+, Pb2+, Ni2+, Sb3+, Se4*, Te4+, Ti4+, V’+, Zn2+, Zr4+, F-, Cl- and NO;. To study the effect of silicon, arsenic, tungsten and germanium, different quantities of these elements were added to samples of 1 g of high-purity aluminium alloy and the degree of extraction under various conditions was investigated. As seen from Table 1, none of these elements (over an appreciable range of con-
Y. Otani and Y. Yamamoto,
Chem. Sot. Japan, 1967,40.
12. T. V. Ramakrishna, J. W. Robinson and P. W. West, Anal. Chim. Acta, 1969, 45, 43.
13. T. R. Hurford and D. F. Boltz, Anal. Chem., 1968, 40. 379. 14. C. Riddle and A. Turek, Anal. Chim. Acta, 1977,92,49. 15. L. A. Trudell and D. F. Boltz, Mikrochim. Acra, 1970, 1220. 16. S. J. Simon and D. F. Boltz, Anal. Chem. 1975, 47, 1758.
17. W. S. Zaugg and R. J. Knox, Anal. Biochem.,
1967, 20, 282. 18. G. Linden, S. Turk and B. Tarodo de la Fuente, Chim. Anal. Paris, 1971, 53, 244. 19. G. Devoto., Boll. Sot. Ital. Biol. Speriment., 1968, 44,
424. 20. J. A. Parsons, B. Dawson, E. Callahan and J. T. Potts, Jr., Eiochem. J., 1970, 119, 791. 21. Y. Kidani, H. Takemura and H. Koike, Bunseki Kagaku,
1974,23,
212.
Table 2. Determination of phosphorus in aluminium alloys* Sample
Designation
Aluminium alloy II Si-Al Devarda’s alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy
BCS 181 BCS 182/l BDH L-2520 L-2521 L-2530 L-2550 L-2560 L-2561 L-2571
* Courtesy of E.N.D.A.S.A.
Bull.
429.
P found, % air-C,H, N20-C2H2 0.0012 0.0009 0.0119 0.0827 0.0763 0.0801 0.0912 0.0560 0.0360 0.0165
0.0010 0.0008
0.0115 0.0820 0.0760 0.0791 0.0911 0.0563 0.0358 0.0163
Certified P content, % 0.0012 0.0009 0.0116 0.0825 0.0760 0.0796
0.0910 0.0561 0.0355 0.0160
412 22. T. G. Carreno and 1971, 30, 501. 23. Y. Inokuma and 1026. 24. I. G. Yudelevich, and T. M. Korda.,
SHORT
F. D. Macias, J. Endo,
Anal Ed@.
Tetsu to Hagane,
COMhlUNlCATIONS
Agrobiol. 1977, 63,
L. M. Buyanova, N. F. Bakhturova Zh. Analyt. Khim., 1977, 32, 28.
25. A. M. Baialordo and A. Gomez-Coedo, Rev. Metal., 1973, 9, 35. 26. V. Rozenblum, Anal. Lett. 1975, 8, 549. 27. A. Sapek and B. Sapek, SCAN, 1976, 7, 10. 28. M. J. Fishman and D. E. Erdman, Anal. Chem., 197;. 49, 139R.
Tnlanto, Vol. 28. pp. 469 to 472. 1981 Printed m Great Bntain. All nghts reserved
SHORT
COMMUNICATIONS
AN INDIRECT METHOD FOR DETERMINING PHOSPHORUS IN ALUMINIUM ALLOYS BY ATOMIC-ABSORPTION SPECTROMETRY* J. L. BERNAL, M”. J. de1 NOZAL, L. DEBAN and A. J. ALLER Department of Analytical Chemistry, Faculty of Science, Prado de la Magdalena, Valladolid, Spain (Received 28 January 1980. Revised 28 October 1980. Accepted 23 December 1980) Summary-An
indirect method is described for the determination of phosphorus in aluminium alloys. Ammonium molybdate is added to a solution of the aluminium alloy and the molybdophosphoric acid formed is selectively extracted into n-butyl acetate. The twelve molybdenum atoms associated with each phosphate ion are determined by direct atomic-absorption spectrometry with the n-butyl acetate phase in a nitrous oxideacetylene flame, with measurement at 313.2 nm. The most suitable conditions have been established and the effect of other ions has been studied.
Most methods for the determination of phosphate are based on the formation of heteropoly acids.’ The sensitivity and selectivity of these methods are enhanced, if the complex is extracted into a suitable organic solvent2-6 before measurement. Such methods have been used in atomic-absorption spectrophotometry7 in place of direct flame atomic spectrometric methods8 the phosphorus content being determined indirectly by formation and extraction of phosphomolybdic acid and determination of the molybdenum in the organic phase by atomic absorption.g-‘2 An alternativer3*14 is to decompose the heteropolymolybdate in the separated extract and to strip the molybdenum into an aqueous phase for subsequent determination. The selectivity of the extraction, and the factors which affect it (principally the pH) have been widely investigated. Despite the difficulties related to the use of organic solvents in atomic-absorption work, many applications have been made of this method for determination of phosphorus, e.g., in biochemical,17-2’ agricultural,22 metallurgical,14*23-25 water10*26P2* and rockI analysis. This paper describes a method for the determination of phosphorus in aluminium-based alloys by indirect atomic-absorption, taking into account the possible interference of other ions. EXPERIMENTAL A double-beam Pye-Unicam SP-1900 atomic-absorption spectrometer was used in conjunction with a Unicam molybdenum hollow-cathode lamp. The instrument conditions used throughout were: slit-width 0.15 mm; wave* Taken in part from the Ph.D. work of A. J. Aller.
length 313.2 nm; lamp current 7 mA; a lo-cm burner was used with the air-acetylene flame and a 5-cm burner with the nitrous oxide-acetylene flame, both burners having a 0.7-mm wide slot. Reagents Ammonium molybdate solution. Dissolve 10.69 g of ammonium molybdate tetrahydrate, (NH4),Mo,02, .4H20, in distilled water and dilute to 1 litre. Standard phosphorus solution. Dissolve 0.1098 g of potassium dihydrogen phosphate, KH,PO,, in distilled water and dilute to 1 litre. Wash solution. Nitric acid diluted 1:1 with distilled water. Use analytical-reagent grade chemicals and store all solutions in polyethylene bottles to avoid contamination with silicon. Procedure
Dissolve the sample [large enough to give a final phosphorus concentration of 0.1-l pg/ml, but not less than 0.5 g (to avoid problems of inhomogeneity)] in 25 ml (more if necessary) of a 10: 10: 1 v/v mixture of nitric acid (1 + l), hydrochloric acid (1 + 1) and hydrogen peroxide (30%) by heating in a covered Teflon flask. For phosphorus contents >O.Ol’~, make this solution up to a suitable standard volume (e.g., 50 ml) and use a suitable aliquot for the subsequent analysis. Dilute this solution to about 50 ml with demineralized water, add 5 ml of ammonium molybdate solution and swirl the mixture for 3 min. Adjust to pH 1, transfer to a lOO-ml glass separating funnel, then shake with 20 ml of n-butyl acetate for 1 min. Discard the aqueous phase and wash the organic phase with three lo-ml portions of wash solution. Spray the organic phase directly into the flame and determine the molybdenum content. Use the organic solvent for determining the blank. RESULTS AND
DISCUSSION
Extraction of phosphomolybdic acid The extraction of molybdic acid into various solvents was investigated by shaking vigorously 20 ml of 469
470
SHORT COMMUNlCATIONS
PH Fig. 1. Relationship between degree of extraction and pH for the organic solvents used. -ODiethyl ether; --ct Acetophenone; -OMIBK; e b-butyl acetate; an-butanol; -w+ n-amyl alcohol; -A- isoamyl alcohol.
solvent with the aqueous molybdate solution (at various pH values) for 1 min and measuring the amount of molybdate in the organic phase. The same solvents were tested as extractants for phosphomolybdie acid, the solvent being added to a 0.5-ppm phosphorus solution obtained by mixing 10 ml of ammonium molydate reagent with potassium of dihydrogen phosphate solution and adjusted to the desired pH with buffer. Preliminary experiments showed that shaking for 1 min was sufficient. The results are shown in Fig. 1, where it can be seen that the amount extracted increases as the pH decreases and becomes quantitative over the pH range 0.4-1.6, depending on the solvent. For further work the pH was adjusted to 1 for convenience. From the results it was deduced that under the conditions used the best solvent is n-butyl acetate, because it is the most highly selective and gives virtually lOOo/, extraction in a single step. It is rec-
ommended that before aspiration into the flame the organic phase is washed to remove any molybdate reagent mechanically carried over from the aqueous phase. Calibration graph and optimum concentration ranges When the recommended procedure is used, the calibration graph for the determination of phosphorus is linear over the range 0.01-l ppm of phosphorus in the original aqueous solution. Precision A series of absorbance values was obtained by replicate (tenfold) analysis of 25 ml of aluminium alloy solution containing 0.8 ppm of phosphorus. The average absorbance was 0.48 and the relative standard deviation was 1.2% with the air-acetylene flame and 0.8% for the nitrous oxide-acetylene flame.
471
SHORT COMMUNICATIONS
Table 1. Concentrations (ppm) of Si, As, W and Ge which do not interfere in the phosphorus determination (0.5 ppm in aqueous solution) when different organic solvents are used Solvent n-Butyl acetate MIBK Acetophenone Diethyl ether n-Amy1 alcohol Isoamyl alcohol n-Butanol
Si
As
W
Ge
4000 10 200 2000 100 60 30
3000 20 300 300 60 50 20
4000
4000 20 200 2000 80 40 50
;: 3000 300 400 60
centration) interfered with the determination of phosphorus if n-butyl acetate was used as extractant. The use of other solvents increases the interference by these elements. Application of the method to metallurgical samples
The accuracy of the method was assessed by the determination of P in B.C.S. and other aluminium alloys. Table 2 shows the results, which confirm the validity and suitability of the proposed method.
REFERENCES
Detection limits and sensitivity
Khimia Fosfora, Nauka, MOSCOW, 1. Analiticheskaya 1974. 2. A. Syty and J. A. Dean, Appl. Optics, 1968, 7, 1331. 3. J. Paul and W. F. R. Pover, Anal. Chim. Acta, 1960, 22,
With a nitrous oxide-acetylene flame the detection limit is nearly 0.01 ppm of phosphorus in the aqueous solution, and the sensitivity (concentration for 1% absorption) is of the order of 0.02 ppm. The practical concentration range is 0.0&l ppm. The determination in the air-acetylene flame has a practical range of 0.1-l ppm. The detection limit of aqueous phosphorus solutions can be lowered to 0.03 ppm and the sensitivity is 0.05 ppm (for 1% absorption).
10. W. S. Zaugg and R. J. Knox, Anal. Chem., 1966. 38,
Effect of foreign ions
11. T. Kumamaru,
185.
4. 5. 6. 7.
J. Paul, Mikrochim. Acta, 1965, 830. Idem, ibid., 1965, 836.
Idem, Anal. Chim. Acta, 1966,35, 200. M. S. Cresser, Solvent Extraction in Flame Spectroscopic Analysis, Butterworths, London, 1978. 8. D. C. Manning and S. Slavin, Atom. Abs. News/., 1969,
8, 132. 9. G. F. Kirkbright, A. M. Smith and T. S. West, Analyst, 1967, 92, 411. 1759.
The effect of a number of ions on the determination of 0.5 ppm of phosphorus by the recommended -procedure has been investigated. An ion was considered not to interfere if it produced less than 5% change in the absorbances. No interference was observed from 5000 ppm of the following ions when tested individually: Bi3+, Ca2+ Cd2+, Ce3+, Cr3+, Cu’+, Fe3+, Hg*+, Mg2+, La’+, Mn2+, Nb5+, Pb2+, Ni2+, Sb3+, Se4*, Te4+, Ti4+, V’+, Zn2+, Zr4+, F-, Cl- and NO;. To study the effect of silicon, arsenic, tungsten and germanium, different quantities of these elements were added to samples of 1 g of high-purity aluminium alloy and the degree of extraction under various conditions was investigated. As seen from Table 1, none of these elements (over an appreciable range of con-
Y. Otani and Y. Yamamoto,
Chem. Sot. Japan, 1967,40.
12. T. V. Ramakrishna, J. W. Robinson and P. W. West, Anal. Chim. Acta, 1969, 45, 43.
13. T. R. Hurford and D. F. Boltz, Anal. Chem., 1968, 40. 379. 14. C. Riddle and A. Turek, Anal. Chim. Acta, 1977,92,49. 15. L. A. Trudell and D. F. Boltz, Mikrochim. Acra, 1970, 1220. 16. S. J. Simon and D. F. Boltz, Anal. Chem. 1975, 47, 1758.
17. W. S. Zaugg and R. J. Knox, Anal. Biochem.,
1967, 20, 282. 18. G. Linden, S. Turk and B. Tarodo de la Fuente, Chim. Anal. Paris, 1971, 53, 244. 19. G. Devoto., Boll. Sot. Ital. Biol. Speriment., 1968, 44,
424. 20. J. A. Parsons, B. Dawson, E. Callahan and J. T. Potts, Jr., Eiochem. J., 1970, 119, 791. 21. Y. Kidani, H. Takemura and H. Koike, Bunseki Kagaku,
1974,23,
212.
Table 2. Determination of phosphorus in aluminium alloys* Sample
Designation
Aluminium alloy II Si-Al Devarda’s alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy Aluminium alloy
BCS 181 BCS 182/l BDH L-2520 L-2521 L-2530 L-2550 L-2560 L-2561 L-2571
* Courtesy of E.N.D.A.S.A.
Bull.
429.
P found, % air-C,H, N20-C2H2 0.0012 0.0009 0.0119 0.0827 0.0763 0.0801 0.0912 0.0560 0.0360 0.0165
0.0010 0.0008
0.0115 0.0820 0.0760 0.0791 0.0911 0.0563 0.0358 0.0163
Certified P content, % 0.0012 0.0009 0.0116 0.0825 0.0760 0.0796
0.0910 0.0561 0.0355 0.0160
412 22. T. G. Carreno and 1971, 30, 501. 23. Y. Inokuma and 1026. 24. I. G. Yudelevich, and T. M. Korda.,
SHORT
F. D. Macias, J. Endo,
Anal Ed@.
Tetsu to Hagane,
COMhlUNlCATIONS
Agrobiol. 1977, 63,
L. M. Buyanova, N. F. Bakhturova Zh. Analyt. Khim., 1977, 32, 28.
25. A. M. Baialordo and A. Gomez-Coedo, Rev. Metal., 1973, 9, 35. 26. V. Rozenblum, Anal. Lett. 1975, 8, 549. 27. A. Sapek and B. Sapek, SCAN, 1976, 7, 10. 28. M. J. Fishman and D. E. Erdman, Anal. Chem., 197;. 49, 139R.