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Indirect method for the determination of aluminium by atomic-absorption spectrometry using an air-acetylene flame
T8iants, 1972, Vol. 19, PP. 787 to 790. Pcmmon
PCCSJ. Printed in Northern
Inland
SHORT COMMUNICATIONS
Indirect method for the determination of aluminium by atomic-absorption spectrometry using an air-acetylene ffame (Received 9 November 1971. Accepted 13 December 1971) reported that serious intereIement interferences are observed in the atomic-absorption dete~ination of iron under certain operating conditions using an air-acetylene ffame.*Vs When flame conditions are optimized for maximum iron absorbance to be obtained for solutions prepared in sulphate or nitrate media, cobalt, nickel and copper cause depressive interferences.* Maximum absorption by iron occurs in a slightly fuel-lean air-acetylene flame, but in the stoichiometric or fuel-rich Same the iron absorption falls sharply when a purely sulphate or nitrate medium is used. Under these conditions many metals cause enhan~~nt of the iron absorbauce,s and of these, titanium, aluminium, calcium and zirconium are notable for the very low concentrations at which they interfere. A similar interference pattern has also been observed for nickel, cobalt and chromiuma** and may be explained on the basis of overexcitation phenomena.8*6 The enhancement effect has been used to provide a sensitive indirect method for the determination of titanium in the concentration range ~.~I-1Oppm .s This paper reports a simiir method for aluminium and includes a more detailed discussion of the choice of matrix eiement and the interAluminium cannot normally be determined directly by ference effects in this type of procedure. using an air-acetylene flame’ as the monoxide A10 is not suliiciently dissociated in this flame,* but a working range of SO-500 ppm aluminium has recently been attained in a direct procedure in the presence of ammonium fluoride.* Normally either a nitrous oxide-acetylene ffame or an oxy-acetylene flame must be used and a detection limit of about 1 ppm may be readily obtained.7@J1 Unfortunately, this is not sufficiently sensitive for many routine industrial applications and the method proposed here, which has a limit approximately two orders of magnitude lower than this, may be suitable in some cases. IT HAS BEEN
EXPERIMENTAL A Perkin-Elmer 290 Atomic Absorption Spectrometer and hollow-cathode lamps were used and operated at the manufacturer’s recommended settings. The standard 50-mm slot tubular burner head and the 50-mm slot high-temperature head were used for air-acetylene and nitrous oxide acetylene thunes respectively. Gas flow-rates were measured with Fischer and Porter &in. bore flowmeters and corrected to atmospheric pressure. The air flow-rate was maintained at 46 I./min (equivalent to a flowmeter reading of 14.0 at a feed pressure of 35 psig) and the acetylene flow-rate varied between 2.0 and 4.8 l./min (equivalent to a flowmeter reading of 13.7 at a feed pressure of 8 psig). The optimum height of the optical beam above the burner for maximum enhancement was found to be 15 mm, and absorbance readings were recorded on a Honeywell Electronik 19 chart recorder. Stock solutions of all the metals in sulphate medium were prepared as described previouslv.a A lOOO-ppm aluminium solution was prepa& from 1 g of alumin&m powder dissolved in sulphuric acid and diluted to 1 litre so that the iinal concentration of suluhuric acid was 10A8M. Distilled water from an all-quartz stiIP* was used throughout. The purity otwater from other types of still was not adequate, presumably because of the effect of very small amounts of calcium on the phenomena to be described. Care must be taken to avoid the addition of chloride ions. RESULTS
AND
DISCUSSION
The sharp drop in absorbance by iron (as sulphate or nitrate) which occurs in a fuel-rich airacetylene flame has been described previously. v The enhancement effect of small amounts of aluminium on this depression of the iron signal in the fuel-rich flame is shown in Fig. 1. The enhancement for a 25ppm iron solution gives a linear relationship between 0+25 and 1.2 ppm of aluminium, The signal became more or less constant from 2 up to at least 500 ppm ~umini~. The shape and characteristics of this calibration curve and the enhancement effect itself are dependent on the geometry of the burner and optical systems and their orientation relative to one another. This would be 187
Short communications
139 13.8 PE 290 Flowmeter reoding
13.7
136
FIG. L-Absorption of 25 p m iron(II1) in suiphate medium as a function of acetylene flow-rate. Concentration o P aluminium added: (A) none; (B) 0.5 ppm; (c) 1 ppm; (0 2 ppm; WI 10ppm; (F) 500ppm.
O?
139
13.8 PE 290
13.7
136
Flowmeter reading
FIG. Z.-Absorption of 10 ppm chromium in suIphate medium as a function of acetylene flow-rate. Concentration of aluminium added: (A) none; (B) 0.5 ppm; (C) 2 ppm; (0) 20 ppm; (E) 100 ppm; (F) 500 ppm. expected to depend on the particular i~t~rn~nt used but with the PE 290 fitted with a standard air-acetylene burner the optimum conditions for maximum sensitivity are those given above. The working range for the determination of aluminium may be varied by using different concentrations of iron. Thus, for example, the most sensitive conditions with our apparatus required use of 10 ppm of iron when the working range was 0.01-0*09 ppm of aluminium. At higher concentrations of iron, higher levels of aIuminium may be detected but concentrations above 25 ppm wouid mean working in the range in which the direct atomi&-abso~tion dete~inatian of ai~ium is possible and the method would therefore not be particularly advantageous.
189
Short communications
log h13+l,
[A13+l,
ppm
3.-Effect of aluminium at a fuel-flow of 13.7 (meter reading) and?umer height of 15 mm, on (A) 10 ppm of iron; (B) 10 ppm of chromium; (C) 25 ppm of cobalt; (0) 25 ppm of nickel; all in sulphate medium.
FIG.
Aluminium gives a similar enhancement of the signals of cobalt, nickel and chromium and any of these may be used as the matrix element. The effect for chromium is shown in Fig. 2 and occurs only in the fuel-rich air-acetylene ilame. At high aluminium concentrations the chromium signal does not reach a constant level but begins to fall again (Fig. 2), the maximum in the calibration curve depending on the chromium concentration, being at 5 ppm of aluminium for 10 ppm of chromium. The effect of aluminium on the atomic-absorption signals of cobalt and nickel was very similar to that for iron, although the detection of aluminium was far less sensitive with nickel. The effects of increasing aluminium concentration are shown in Fig. 3 for all four elements, which suggests that iron is the most suitable matrix element. The operating conditions will varv from one instrument to another but thev may be summarized as follows. (i) Slightly fuel-rich a&acetylene flame such that yellow carbon~incandescence is just visible. (ii) Light-beam centred at 15-20 mm above the burner head. (iii1 Matrix element iron sulphate m 10-r, sulphuric acid at an iron concentration appropriate to the working range req&d~ (iv) Aluminium standards and samples prepared as sulphate and/or nitrate solutions. We have investigated some interference effects on this procedure and these are largely as expected from the known effects of the elements on the atomic-absorption signal of iron? Elements that behave in a similar manner to aluminium, e.g., calcium and titanium, interfere at an equivalent concentration to the ahuninium, whereas others that give an enhancement or depression of the iron signal at concentrations equivalent to the iron concentration, e.g., nickel and chromium, interfere at a similar level. Thus, this method is only suitable for relatively pure aluminium solutions or for analyses in which a separation process is also included. It does, however, offer a substantial improvement in detection limit over that normally obtainable by direct atomic-absorption spectrometry, using either a nitrous oxid*acetylene or oxy-acetylene flame. J. M. OTTAWAY D. T. C~KER* B. SINGLJXQN
Department of Pure and Applied Chemistry University of Strathclyde Cathedral Street Glasgow, C.l. Scotland
Summary-The enhancement of the atomic-absorption signals of iron, cobalt, nickel and chromium in a fuel-rich air-acetylene flame by small amounts of ahuninium makes possible the indirect determination of aluminimn in the concentration range O*Ol-10 ppm. The optimization of working conditions and the occurrence of interferences are reported. * Present address:
Esso Research Centre, Abingdon,
Berkshire.
790
Short communications Zusammenfassung-Die Verstarkung der Atomabsorptions-Signale von Eisen, Kobah, Nickel und Chrom in einer fetten Luft-AcetylenFlamme durch kleine Mengen Aluminium ermiiglicht die indirekte Bestimmung von Aluminium im Konzentrationsbereich O,Ol-10 ppm. Es wird tiber die Op~ier~g der Ar~its~dingungen und das Auftreten von Stiirungen berichtet. R&sum&--Le renforcement des signaux d’absorption atomique des fer, cobalt, nickel et chrome dans une flamme air-acetylene riche en combustible par de petites quantites ~alum~ium rend nossible le dosage indirect de i’alumini;m dans le domaine de concentration O,Ol-10 ppm. On decrit l’optimalisation des conditions de travail et la presence d’interferences. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
K. E. Curtis, Analyst, 1969, 94, 1068. J. M. Ottaway, D. T. Coker, W. B. Rowston and D. R. Bhattarai, ibid., 1970, 95, 567. D. T. Coker, N.&Y. Thesis, University of Strathclyde, 1970. J. A. Davies, MSc. Thesis, University of Strathclyde, 1970. D. T. Coker and J. M. Ottaway, Nature, 1970,227,831. J. M. Ottaway, D. T. Coker and J. A. Davies, Anal. Letr., 1970, 3, 385. R. J. Jaworowski. R. P. Weberling and D. J. Bracco. Anal. Chim. Acfa., 1967.,,37.284. D. T. Coker and J. M. Ottaway, kzture, 1971, 230,156. A. Hofer. 2. AnaL Chem.. 1971.253.206. M. 13. Amos and P. E. Thomas; An& Chim. Acta, 1965,32,139. T. V. Ramakrishna, P. W. West and J. W. Robinson, ibid., 1967,39,81. E. Bishop and J. R. B. Sutton, ibid., 1960, 22,590.
Tblmta,1972.Vol.19,pp.790to 793. PcrsamonPress.Printed in Northern Ireland
~e~ipi~~on of barium chromate in the presence of strontium and lead by complexation followed by volatilization of ammonia (Received 9 November 1971. Accepted 14 December 1971) A NEW PROCEDURE has been developed to separate barium from equal molar concentrations of strontium and lead. The multivalent cations are complexed with MEDTA (l,Z-diaminopropanetetra-acetic acid) or DCTA (1.2-diaminocyclohexane-N,N,N’,N’-tetra-acetic acid) at a pH of 10.3 or higher in ammoniacal solution. The addition of chromate does not bring about precipitation. The solution is then heated and agitated. Ammonia volatilizes from solution, causing a gradual increase in hydrogen ion concentration. Hydrogen ions compete for the chelating agent which results in a This causes a slow precipitation of barium gradual increase in the free barium ion concentration. chromate from homogeneous solution. Both the strontium and lead chelates are more stable than the barium chelate. Therefore, the concentration of free strontium ions and free lead ions remains low throughout the pr~ipitation of measurements show that less than @05 % of the strontium the barium chromate. Atomic-absorption and less than 0*06’% of the lead co-precipitate when MEDTA is used. About O-15‘A of the strontium and about 0.03 % of the lead co-precipitate when DCTA is used. Precipitation of barium chromate has been the classical method’ of separating barium fom strontium. The use of precipitation from homogeneous solutiona as well as complexations has improved the classical method. Complexation has also made a separation of barium from Iead possible, with chromate as the precipitant. Additional modifications based on these fundamental ideas have resulted in further improvements in the separation of barium from both strontium and lead. The method described co-precipitates only about a twelfth of the strontium co-precipitated by the earlier method,% and should be applicable to many other systems where cation release is required. Lower pH values can be attained by addition of acid by volatilization in a closed system.
PCCSJ. Printed in Northern
Inland
SHORT COMMUNICATIONS
Indirect method for the determination of aluminium by atomic-absorption spectrometry using an air-acetylene ffame (Received 9 November 1971. Accepted 13 December 1971) reported that serious intereIement interferences are observed in the atomic-absorption dete~ination of iron under certain operating conditions using an air-acetylene ffame.*Vs When flame conditions are optimized for maximum iron absorbance to be obtained for solutions prepared in sulphate or nitrate media, cobalt, nickel and copper cause depressive interferences.* Maximum absorption by iron occurs in a slightly fuel-lean air-acetylene flame, but in the stoichiometric or fuel-rich Same the iron absorption falls sharply when a purely sulphate or nitrate medium is used. Under these conditions many metals cause enhan~~nt of the iron absorbauce,s and of these, titanium, aluminium, calcium and zirconium are notable for the very low concentrations at which they interfere. A similar interference pattern has also been observed for nickel, cobalt and chromiuma** and may be explained on the basis of overexcitation phenomena.8*6 The enhancement effect has been used to provide a sensitive indirect method for the determination of titanium in the concentration range ~.~I-1Oppm .s This paper reports a simiir method for aluminium and includes a more detailed discussion of the choice of matrix eiement and the interAluminium cannot normally be determined directly by ference effects in this type of procedure. using an air-acetylene flame’ as the monoxide A10 is not suliiciently dissociated in this flame,* but a working range of SO-500 ppm aluminium has recently been attained in a direct procedure in the presence of ammonium fluoride.* Normally either a nitrous oxide-acetylene ffame or an oxy-acetylene flame must be used and a detection limit of about 1 ppm may be readily obtained.7@J1 Unfortunately, this is not sufficiently sensitive for many routine industrial applications and the method proposed here, which has a limit approximately two orders of magnitude lower than this, may be suitable in some cases. IT HAS BEEN
EXPERIMENTAL A Perkin-Elmer 290 Atomic Absorption Spectrometer and hollow-cathode lamps were used and operated at the manufacturer’s recommended settings. The standard 50-mm slot tubular burner head and the 50-mm slot high-temperature head were used for air-acetylene and nitrous oxide acetylene thunes respectively. Gas flow-rates were measured with Fischer and Porter &in. bore flowmeters and corrected to atmospheric pressure. The air flow-rate was maintained at 46 I./min (equivalent to a flowmeter reading of 14.0 at a feed pressure of 35 psig) and the acetylene flow-rate varied between 2.0 and 4.8 l./min (equivalent to a flowmeter reading of 13.7 at a feed pressure of 8 psig). The optimum height of the optical beam above the burner for maximum enhancement was found to be 15 mm, and absorbance readings were recorded on a Honeywell Electronik 19 chart recorder. Stock solutions of all the metals in sulphate medium were prepared as described previouslv.a A lOOO-ppm aluminium solution was prepa& from 1 g of alumin&m powder dissolved in sulphuric acid and diluted to 1 litre so that the iinal concentration of suluhuric acid was 10A8M. Distilled water from an all-quartz stiIP* was used throughout. The purity otwater from other types of still was not adequate, presumably because of the effect of very small amounts of calcium on the phenomena to be described. Care must be taken to avoid the addition of chloride ions. RESULTS
AND
DISCUSSION
The sharp drop in absorbance by iron (as sulphate or nitrate) which occurs in a fuel-rich airacetylene flame has been described previously. v The enhancement effect of small amounts of aluminium on this depression of the iron signal in the fuel-rich flame is shown in Fig. 1. The enhancement for a 25ppm iron solution gives a linear relationship between 0+25 and 1.2 ppm of aluminium, The signal became more or less constant from 2 up to at least 500 ppm ~umini~. The shape and characteristics of this calibration curve and the enhancement effect itself are dependent on the geometry of the burner and optical systems and their orientation relative to one another. This would be 187
Short communications
139 13.8 PE 290 Flowmeter reoding
13.7
136
FIG. L-Absorption of 25 p m iron(II1) in suiphate medium as a function of acetylene flow-rate. Concentration o P aluminium added: (A) none; (B) 0.5 ppm; (c) 1 ppm; (0 2 ppm; WI 10ppm; (F) 500ppm.
O?
139
13.8 PE 290
13.7
136
Flowmeter reading
FIG. Z.-Absorption of 10 ppm chromium in suIphate medium as a function of acetylene flow-rate. Concentration of aluminium added: (A) none; (B) 0.5 ppm; (C) 2 ppm; (0) 20 ppm; (E) 100 ppm; (F) 500 ppm. expected to depend on the particular i~t~rn~nt used but with the PE 290 fitted with a standard air-acetylene burner the optimum conditions for maximum sensitivity are those given above. The working range for the determination of aluminium may be varied by using different concentrations of iron. Thus, for example, the most sensitive conditions with our apparatus required use of 10 ppm of iron when the working range was 0.01-0*09 ppm of aluminium. At higher concentrations of iron, higher levels of aIuminium may be detected but concentrations above 25 ppm wouid mean working in the range in which the direct atomi&-abso~tion dete~inatian of ai~ium is possible and the method would therefore not be particularly advantageous.
189
Short communications
log h13+l,
[A13+l,
ppm
3.-Effect of aluminium at a fuel-flow of 13.7 (meter reading) and?umer height of 15 mm, on (A) 10 ppm of iron; (B) 10 ppm of chromium; (C) 25 ppm of cobalt; (0) 25 ppm of nickel; all in sulphate medium.
FIG.
Aluminium gives a similar enhancement of the signals of cobalt, nickel and chromium and any of these may be used as the matrix element. The effect for chromium is shown in Fig. 2 and occurs only in the fuel-rich air-acetylene ilame. At high aluminium concentrations the chromium signal does not reach a constant level but begins to fall again (Fig. 2), the maximum in the calibration curve depending on the chromium concentration, being at 5 ppm of aluminium for 10 ppm of chromium. The effect of aluminium on the atomic-absorption signals of cobalt and nickel was very similar to that for iron, although the detection of aluminium was far less sensitive with nickel. The effects of increasing aluminium concentration are shown in Fig. 3 for all four elements, which suggests that iron is the most suitable matrix element. The operating conditions will varv from one instrument to another but thev may be summarized as follows. (i) Slightly fuel-rich a&acetylene flame such that yellow carbon~incandescence is just visible. (ii) Light-beam centred at 15-20 mm above the burner head. (iii1 Matrix element iron sulphate m 10-r, sulphuric acid at an iron concentration appropriate to the working range req&d~ (iv) Aluminium standards and samples prepared as sulphate and/or nitrate solutions. We have investigated some interference effects on this procedure and these are largely as expected from the known effects of the elements on the atomic-absorption signal of iron? Elements that behave in a similar manner to aluminium, e.g., calcium and titanium, interfere at an equivalent concentration to the ahuninium, whereas others that give an enhancement or depression of the iron signal at concentrations equivalent to the iron concentration, e.g., nickel and chromium, interfere at a similar level. Thus, this method is only suitable for relatively pure aluminium solutions or for analyses in which a separation process is also included. It does, however, offer a substantial improvement in detection limit over that normally obtainable by direct atomic-absorption spectrometry, using either a nitrous oxid*acetylene or oxy-acetylene flame. J. M. OTTAWAY D. T. C~KER* B. SINGLJXQN
Department of Pure and Applied Chemistry University of Strathclyde Cathedral Street Glasgow, C.l. Scotland
Summary-The enhancement of the atomic-absorption signals of iron, cobalt, nickel and chromium in a fuel-rich air-acetylene flame by small amounts of ahuninium makes possible the indirect determination of aluminimn in the concentration range O*Ol-10 ppm. The optimization of working conditions and the occurrence of interferences are reported. * Present address:
Esso Research Centre, Abingdon,
Berkshire.
790
Short communications Zusammenfassung-Die Verstarkung der Atomabsorptions-Signale von Eisen, Kobah, Nickel und Chrom in einer fetten Luft-AcetylenFlamme durch kleine Mengen Aluminium ermiiglicht die indirekte Bestimmung von Aluminium im Konzentrationsbereich O,Ol-10 ppm. Es wird tiber die Op~ier~g der Ar~its~dingungen und das Auftreten von Stiirungen berichtet. R&sum&--Le renforcement des signaux d’absorption atomique des fer, cobalt, nickel et chrome dans une flamme air-acetylene riche en combustible par de petites quantites ~alum~ium rend nossible le dosage indirect de i’alumini;m dans le domaine de concentration O,Ol-10 ppm. On decrit l’optimalisation des conditions de travail et la presence d’interferences. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
K. E. Curtis, Analyst, 1969, 94, 1068. J. M. Ottaway, D. T. Coker, W. B. Rowston and D. R. Bhattarai, ibid., 1970, 95, 567. D. T. Coker, N.&Y. Thesis, University of Strathclyde, 1970. J. A. Davies, MSc. Thesis, University of Strathclyde, 1970. D. T. Coker and J. M. Ottaway, Nature, 1970,227,831. J. M. Ottaway, D. T. Coker and J. A. Davies, Anal. Letr., 1970, 3, 385. R. J. Jaworowski. R. P. Weberling and D. J. Bracco. Anal. Chim. Acfa., 1967.,,37.284. D. T. Coker and J. M. Ottaway, kzture, 1971, 230,156. A. Hofer. 2. AnaL Chem.. 1971.253.206. M. 13. Amos and P. E. Thomas; An& Chim. Acta, 1965,32,139. T. V. Ramakrishna, P. W. West and J. W. Robinson, ibid., 1967,39,81. E. Bishop and J. R. B. Sutton, ibid., 1960, 22,590.
Tblmta,1972.Vol.19,pp.790to 793. PcrsamonPress.Printed in Northern Ireland
~e~ipi~~on of barium chromate in the presence of strontium and lead by complexation followed by volatilization of ammonia (Received 9 November 1971. Accepted 14 December 1971) A NEW PROCEDURE has been developed to separate barium from equal molar concentrations of strontium and lead. The multivalent cations are complexed with MEDTA (l,Z-diaminopropanetetra-acetic acid) or DCTA (1.2-diaminocyclohexane-N,N,N’,N’-tetra-acetic acid) at a pH of 10.3 or higher in ammoniacal solution. The addition of chromate does not bring about precipitation. The solution is then heated and agitated. Ammonia volatilizes from solution, causing a gradual increase in hydrogen ion concentration. Hydrogen ions compete for the chelating agent which results in a This causes a slow precipitation of barium gradual increase in the free barium ion concentration. chromate from homogeneous solution. Both the strontium and lead chelates are more stable than the barium chelate. Therefore, the concentration of free strontium ions and free lead ions remains low throughout the pr~ipitation of measurements show that less than @05 % of the strontium the barium chromate. Atomic-absorption and less than 0*06’% of the lead co-precipitate when MEDTA is used. About O-15‘A of the strontium and about 0.03 % of the lead co-precipitate when DCTA is used. Precipitation of barium chromate has been the classical method’ of separating barium fom strontium. The use of precipitation from homogeneous solutiona as well as complexations has improved the classical method. Complexation has also made a separation of barium from Iead possible, with chromate as the precipitant. Additional modifications based on these fundamental ideas have resulted in further improvements in the separation of barium from both strontium and lead. The method described co-precipitates only about a twelfth of the strontium co-precipitated by the earlier method,% and should be applicable to many other systems where cation release is required. Lower pH values can be attained by addition of acid by volatilization in a closed system.