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Use of acetohydroxamic acid in the direct spectrophotometric determination of iron(III) and iron(II) by flow injection analysis
Analytica Chimica Acta, 196 (1987) 333-336 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
USE OF ACETOHYDROXAMIC ACID IN THE DIRECT SPECTROPHOTOMETRIC DETERMINATION OF IRON(II1) AND IRON(I1) BY FLOW INJECTION ANALYSIS
A. T. SENIOR and J. D. GLENNON* Department
of Chemistry,
University
College, Cork (Ireland)
(Received 29th July 1986)
Summary. The direct spectrophotometric determination of iron(II1) and iron(I1) by flow injection analysis with acetohydroxamic acid and l,lO-phenanthroline as reagents is reported. The working ranges are 0.5-10 and 10-60 mg l-l, respectively. Results obtained for synthetic mixtures of Fe(II1) and Fe(I1) and for acid extracts of haematite samples were accurate. Interference studies indicate that the method is highly selective.
Flow injection analysis is increasingly recognised as an important tool for metal speciation [l]. The determination of iron(II1) in the presence of iron(II) has received particular attention. Methods have for the most part involved the calculation of the Fe(II1) concentration by a difference method after reduction of Fe(II1) to Fe(I1) [2, 31. A spectrophotometric method based on parallel flow-injection manifolds with simultaneous sample injection has also been described [4]. More recently, ion chromatography with postcolumn derivatization [5] and a flow-injection system with diode-array detection have been used [ 61. In the present work, the direct spectrophotometric determination of Fe(III) and Fe(I1) by FIA by means of l,lO-phenanthroline in conjunction with a hydroxamic acid chelating agent is reported. Hydroxamic acids from both naturally-occurring and synthetic sources have a strong affinity for Fe(II1) [7, 81 and a large variety of synthetic hydroxamic acids have been used for extractive-spectrophotometric determinations [9]. The simplest member of this class of reagent is acetohydroxamic acid, which undergoes a series of rapid stepwise reactions with Fe(II1) to yield a strong 1:3 complex at neutral pH. An investigation of the utility of acetohydroxamic acid for Fe(II1) determinations in the working range 0.5-100 mg 1-l and in the presence of Fe(I1) are reported here. Experimental Apparatus. A Tecator 5001 FIAstar system was used with a Shimadzu UV-260 spectrophotometer. The injection and flow-through cell volumes were 20 ~1 and 70 ~1, respectively. The total flow rate was 2 ml min-’ at 0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
334
0.2 bar pressure (nitrogen) with equal flow rates in each stream. Atomic absorption was measured with a Pye-Unicam SP191 spectrometer. Direct determination of Fe(III) and Fe(II). Separate reagent solutions of acetohydroxamic acid and l,lO-phenanthroline were prepared at concentrations of 2 X lo-* M and 2 X 10m3M, respectively, in acetate buffer. The acetate buffer solution was prepared by mixing 20 ml of 2.0 M ammonium acetate with 180 ml of 2.0 M acetic acid to yield a reagent pH of 3.7. The carrier stream consisted simply of triply-distilled water. The flow injection manifold is shown in Fig. 1. At a given time, only one reagent stream is allowed to merge with the carrier stream, while the other reagent stream is stopped. This necessitates two injections of each sample for the speciation of Fe(II1) and Fe(I1). The wavelengths of maximum absorption were 512 nm and 440 nm for the Fe(I1) and Fe(II1) determinations, respectively. Stock solutions of iron (200 pg ml-‘) were prepared from iron(II1) nitrate and iron(I1) ammonium sulphate in acid (2.0% v/v nitric acid and sulphuric acid, respectively). Synthetic Fe(II1) and Fe(I1) mixtures were prepared by appropriate dilutions of the two stock solutions. Analytical-grade salts were used for the interference studies, which involved alternating injections of a 20 Ergml-’ Fe(II1) standard with injections of the standard-plus-diverse ion solutions. Iron(II1) was also determined in a diluted acid extract of a haematite geological sample by the flow-injection method and also by flame atomic absorption spectrometry (a.a.s.). Results and discussion The flow-injection method outlined here takes advantage of the high affinity of the hydroxamic acid reagent for Fe(II1) and of the relatively strong but broad absorption band in the 420-440 nm region for Fe(II1) complexation. This direct determination of Fe(II1) in the presence of Fe(I1) has distinct advantages, particularly as the level of interference from other metals on the Fe(II1) determination is also reduced. Calibration graphs for Fe(II1) with acetohydroxamic acid and for Fe(I1) with l,lO-phenanthroline were constructed for the ranges l-10 mg 1-l and 10-60 mg 1-l with correlation coefficients greater than 0.999. The limit of detection for the Fe(II1) determination was calculated to be 0.2 mg 1-l for
Buffered Acetohydmxamlc Corrw
P”mpS(204 ,
Acid _ Stream
1
0.75
M W
Buffered I.10 Phenanthrolme
Detector
Fig. 1. Flow-injection manifold for the determination of Fe(II1) and Fe(I1). The reaction coil is 0.75 m long. All tubing was polyethylene (0.75 mm i.d.).
335
the 20-~1 injection volume. For each standard injected six times, the relative standard deviation did not exceed 2%. To verify the selectivity of the hydroxamic acid reagent for Fe(II1) and the accuracy of the method, synthetic mixtures of Fe(II1) and Fe(I1) were prepared and examined while fresh by the flow-injection method and by flame atomic absorption spectrometry. The results obtained at pH 3.7 for various mixtures are shown in Table 1. The Fe( III) and Fe(I1) levels found by the flow-injection method are in good agreement with the expected values and the total iron concentrations are in good agreement with the results obtained by a.a.s. The interference study was done with 20 mg 1-l Fe(II1); interference was considered to have occurred if the detector response differed by more than 2% relative from the interference-free standard [4], As shown in Table 2, Cr(III), Sn(I1) and Cu(I1) exhibit the lowest tolerance levels. However, the level for the interference of Cu(I1) is a considerable improvement over that attainable with the iron(II1) thiocyanate system. To test the accuracy of the Fe(II1) determination, aliquots of a diluted acid extract of a haematite geological sample were examined by the flowinjection method and by flame a.a.s. The results were in good agreement (13.0, 23.8 and 73.8 mg 1-l by the proposed method and 13.2, 22.9 and 75.0 mg 1-l by a.a.s.). Investigations into other applications of this approach to the speciation of Fe(II1) and Fe(I1) are continuing, and other watersoluble hydroxamic acids are under study with a view to improving the sensitivity and selectivity of the method.
TABLE
1
Levels of Fe(II1) and Fe(I1) found for synthetic mixtures by flow injection analysis and flame atomic absorption spectrometry Sample
Iron concentration
(mg l-l)*
Flow-injection
1 2 3 4 5 6
A.a.s.
Fe( II)
Fe( III)
Fe(Tota1)
Fe(Tota1)
35.5( 36.0) 31.0( 30.0) 30.0( 30.0) 20.6( 20.0) lO.O( 10.0) 1.8(2.0)
5.5(6.0) 10.2( 10.0) 30.0( 30.0) 20.7( 20.0) 30.0( 30.0) 40.0( 40.0)
41.0(41.0) 41.5( 40.0) 60.0( 60.0) 41.3(40.0) 40.0( 40.0) 42.0(42.0)
41.0 41.6 61.5 42.0 41.0 42.5
*Added levels are given in parentheses.
336 TABLE 2 Interference studies for the spectrophotometric Element
Al( III) Cd( II) Co( II) Cr(III) Cu( II)
Diverse ion
AI(NO,),* 9H,O Cd( NO,), 4qO CoCl, * 6H,O CrCI, * 6H,O Cu(NO,), .3H,O l
Tolerance level (mg 1-l) 500 > 1200 >800 200 200
Fe(II1) flow-injection Element
method
Diverse ion .
Mn( II) Ni( II) Pb( II) Sn( II) Zn( II)
MnSO,. 4HO NiCl, - 6H,O Pb(NO,), SnC1, . 2H,O ZnSO, - 7H,O
Tolerance level (me 1-l) > 1200 >1600 1500 100 >900
REFERENCES 1 G. E. Pacey and B. P. Bubnis, Am. Lab., 16 (1984) 17. 2 T. P. Lynch, N. J. Kernoghan and J. N. Wilson, Analyst, 109 (1984) 839. 3 J. Mortatti, F. J. Krug, L. C. R. Passenda, E. A. G. Zagatto and S. S. Jorgensen, Analyst, 107 (1982) 659. 4 T. P. Lynch, N. J. Kernoghan and J. N. Wilson, Analyst, 109 (1984) 843. 5 H. Saitoh and K. Oikawa, J. Chromatogr., 329 (1985) 247. 6 F. Lazaro, A. Rios, M. D. Luque de Castro and M. Val&cel, Anal. Chim. Acta, 179 (1986) 279. 7 D. A. Brown and M. V. Chidambaram, Bioinorg. Chem., 9 (1978) 255. 8 D. A. Brown, M. V. Chidambaram and J. D. Glennon, Inorg. Chem., 19 (1980) 3260. 9 Y. K. Agrawal and S. A. Patel, Rev. Anal. Chem., 4 (1980) 237.
Short Communication
USE OF ACETOHYDROXAMIC ACID IN THE DIRECT SPECTROPHOTOMETRIC DETERMINATION OF IRON(II1) AND IRON(I1) BY FLOW INJECTION ANALYSIS
A. T. SENIOR and J. D. GLENNON* Department
of Chemistry,
University
College, Cork (Ireland)
(Received 29th July 1986)
Summary. The direct spectrophotometric determination of iron(II1) and iron(I1) by flow injection analysis with acetohydroxamic acid and l,lO-phenanthroline as reagents is reported. The working ranges are 0.5-10 and 10-60 mg l-l, respectively. Results obtained for synthetic mixtures of Fe(II1) and Fe(I1) and for acid extracts of haematite samples were accurate. Interference studies indicate that the method is highly selective.
Flow injection analysis is increasingly recognised as an important tool for metal speciation [l]. The determination of iron(II1) in the presence of iron(II) has received particular attention. Methods have for the most part involved the calculation of the Fe(II1) concentration by a difference method after reduction of Fe(II1) to Fe(I1) [2, 31. A spectrophotometric method based on parallel flow-injection manifolds with simultaneous sample injection has also been described [4]. More recently, ion chromatography with postcolumn derivatization [5] and a flow-injection system with diode-array detection have been used [ 61. In the present work, the direct spectrophotometric determination of Fe(III) and Fe(I1) by FIA by means of l,lO-phenanthroline in conjunction with a hydroxamic acid chelating agent is reported. Hydroxamic acids from both naturally-occurring and synthetic sources have a strong affinity for Fe(II1) [7, 81 and a large variety of synthetic hydroxamic acids have been used for extractive-spectrophotometric determinations [9]. The simplest member of this class of reagent is acetohydroxamic acid, which undergoes a series of rapid stepwise reactions with Fe(II1) to yield a strong 1:3 complex at neutral pH. An investigation of the utility of acetohydroxamic acid for Fe(II1) determinations in the working range 0.5-100 mg 1-l and in the presence of Fe(I1) are reported here. Experimental Apparatus. A Tecator 5001 FIAstar system was used with a Shimadzu UV-260 spectrophotometer. The injection and flow-through cell volumes were 20 ~1 and 70 ~1, respectively. The total flow rate was 2 ml min-’ at 0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
334
0.2 bar pressure (nitrogen) with equal flow rates in each stream. Atomic absorption was measured with a Pye-Unicam SP191 spectrometer. Direct determination of Fe(III) and Fe(II). Separate reagent solutions of acetohydroxamic acid and l,lO-phenanthroline were prepared at concentrations of 2 X lo-* M and 2 X 10m3M, respectively, in acetate buffer. The acetate buffer solution was prepared by mixing 20 ml of 2.0 M ammonium acetate with 180 ml of 2.0 M acetic acid to yield a reagent pH of 3.7. The carrier stream consisted simply of triply-distilled water. The flow injection manifold is shown in Fig. 1. At a given time, only one reagent stream is allowed to merge with the carrier stream, while the other reagent stream is stopped. This necessitates two injections of each sample for the speciation of Fe(II1) and Fe(I1). The wavelengths of maximum absorption were 512 nm and 440 nm for the Fe(I1) and Fe(II1) determinations, respectively. Stock solutions of iron (200 pg ml-‘) were prepared from iron(II1) nitrate and iron(I1) ammonium sulphate in acid (2.0% v/v nitric acid and sulphuric acid, respectively). Synthetic Fe(II1) and Fe(I1) mixtures were prepared by appropriate dilutions of the two stock solutions. Analytical-grade salts were used for the interference studies, which involved alternating injections of a 20 Ergml-’ Fe(II1) standard with injections of the standard-plus-diverse ion solutions. Iron(II1) was also determined in a diluted acid extract of a haematite geological sample by the flow-injection method and also by flame atomic absorption spectrometry (a.a.s.). Results and discussion The flow-injection method outlined here takes advantage of the high affinity of the hydroxamic acid reagent for Fe(II1) and of the relatively strong but broad absorption band in the 420-440 nm region for Fe(II1) complexation. This direct determination of Fe(II1) in the presence of Fe(I1) has distinct advantages, particularly as the level of interference from other metals on the Fe(II1) determination is also reduced. Calibration graphs for Fe(II1) with acetohydroxamic acid and for Fe(I1) with l,lO-phenanthroline were constructed for the ranges l-10 mg 1-l and 10-60 mg 1-l with correlation coefficients greater than 0.999. The limit of detection for the Fe(II1) determination was calculated to be 0.2 mg 1-l for
Buffered Acetohydmxamlc Corrw
P”mpS(204 ,
Acid _ Stream
1
0.75
M W
Buffered I.10 Phenanthrolme
Detector
Fig. 1. Flow-injection manifold for the determination of Fe(II1) and Fe(I1). The reaction coil is 0.75 m long. All tubing was polyethylene (0.75 mm i.d.).
335
the 20-~1 injection volume. For each standard injected six times, the relative standard deviation did not exceed 2%. To verify the selectivity of the hydroxamic acid reagent for Fe(II1) and the accuracy of the method, synthetic mixtures of Fe(II1) and Fe(I1) were prepared and examined while fresh by the flow-injection method and by flame atomic absorption spectrometry. The results obtained at pH 3.7 for various mixtures are shown in Table 1. The Fe( III) and Fe(I1) levels found by the flow-injection method are in good agreement with the expected values and the total iron concentrations are in good agreement with the results obtained by a.a.s. The interference study was done with 20 mg 1-l Fe(II1); interference was considered to have occurred if the detector response differed by more than 2% relative from the interference-free standard [4], As shown in Table 2, Cr(III), Sn(I1) and Cu(I1) exhibit the lowest tolerance levels. However, the level for the interference of Cu(I1) is a considerable improvement over that attainable with the iron(II1) thiocyanate system. To test the accuracy of the Fe(II1) determination, aliquots of a diluted acid extract of a haematite geological sample were examined by the flowinjection method and by flame a.a.s. The results were in good agreement (13.0, 23.8 and 73.8 mg 1-l by the proposed method and 13.2, 22.9 and 75.0 mg 1-l by a.a.s.). Investigations into other applications of this approach to the speciation of Fe(II1) and Fe(I1) are continuing, and other watersoluble hydroxamic acids are under study with a view to improving the sensitivity and selectivity of the method.
TABLE
1
Levels of Fe(II1) and Fe(I1) found for synthetic mixtures by flow injection analysis and flame atomic absorption spectrometry Sample
Iron concentration
(mg l-l)*
Flow-injection
1 2 3 4 5 6
A.a.s.
Fe( II)
Fe( III)
Fe(Tota1)
Fe(Tota1)
35.5( 36.0) 31.0( 30.0) 30.0( 30.0) 20.6( 20.0) lO.O( 10.0) 1.8(2.0)
5.5(6.0) 10.2( 10.0) 30.0( 30.0) 20.7( 20.0) 30.0( 30.0) 40.0( 40.0)
41.0(41.0) 41.5( 40.0) 60.0( 60.0) 41.3(40.0) 40.0( 40.0) 42.0(42.0)
41.0 41.6 61.5 42.0 41.0 42.5
*Added levels are given in parentheses.
336 TABLE 2 Interference studies for the spectrophotometric Element
Al( III) Cd( II) Co( II) Cr(III) Cu( II)
Diverse ion
AI(NO,),* 9H,O Cd( NO,), 4qO CoCl, * 6H,O CrCI, * 6H,O Cu(NO,), .3H,O l
Tolerance level (mg 1-l) 500 > 1200 >800 200 200
Fe(II1) flow-injection Element
method
Diverse ion .
Mn( II) Ni( II) Pb( II) Sn( II) Zn( II)
MnSO,. 4HO NiCl, - 6H,O Pb(NO,), SnC1, . 2H,O ZnSO, - 7H,O
Tolerance level (me 1-l) > 1200 >1600 1500 100 >900
REFERENCES 1 G. E. Pacey and B. P. Bubnis, Am. Lab., 16 (1984) 17. 2 T. P. Lynch, N. J. Kernoghan and J. N. Wilson, Analyst, 109 (1984) 839. 3 J. Mortatti, F. J. Krug, L. C. R. Passenda, E. A. G. Zagatto and S. S. Jorgensen, Analyst, 107 (1982) 659. 4 T. P. Lynch, N. J. Kernoghan and J. N. Wilson, Analyst, 109 (1984) 843. 5 H. Saitoh and K. Oikawa, J. Chromatogr., 329 (1985) 247. 6 F. Lazaro, A. Rios, M. D. Luque de Castro and M. Val&cel, Anal. Chim. Acta, 179 (1986) 279. 7 D. A. Brown and M. V. Chidambaram, Bioinorg. Chem., 9 (1978) 255. 8 D. A. Brown, M. V. Chidambaram and J. D. Glennon, Inorg. Chem., 19 (1980) 3260. 9 Y. K. Agrawal and S. A. Patel, Rev. Anal. Chem., 4 (1980) 237.