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Kinetic determination of periodate based on its reaction with ferroin and its application to the indirect determination of ethylene glycol and glycerol
Microchemical Journal 68 Ž2001. 35᎐40
Kinetic determination of periodate based on its reaction with ferroin and its application to the indirect determination of ethylene glycol and glycerol Abbas AfkhamiU , Fatemeh Mosaed Department of Chemistry, Faculty of Sciences, Bu-Ali Sina Uni¨ ersity, Hamadan, Iran Received 6 July 2000; received in revised form 31 August 2000; accepted 1 September 2000
Abstract A simple, sensitive, rapid and reliable method has been developed for spectrophotometric determinations of periodate. The method is based on the redox reaction of periodate with ferroin in acidic media. Mn ŽII. accelerates the reaction. The reaction has been monitored spectrophotometrically by measuring the decrease in absorbance of reaction mixture at 510 nm by a fixed time method of 3 min. Periodate could be determined in the range of 0.40᎐5.0 grml. The relative standard deviation for 10 determinations of 1.0grml of periodate was 1.16% and the limit of detection, corresponding to a signal-to-noise ratio of 3, was 0.25grml. The proposed method was applied to the determination of ethylene glycol and glycerol via the Malaprade reaction with satisfactory results. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Kinetic determination; Spectrophotometry; Periodate; Ethylene glycol; Glycerol
1. Introduction Periodate is used for the oxidation of different inorganic and organic compounds and catalytic applications at trace levels w1᎐6x. Therefore, simple rapid, sensitive and accurate methods are
U
Corresponding author. Fax: q98-81-8272046. E-mail address: [email protected] ŽA. Afkhami..
required for determinations of trace amounts of periodate in different samples. Various methods have been reported for the determination of periodate, including thin layer chromatography w7x, electrophoresis w8x, pulse polarography w9x, chemiluminescence w3x, fluorimetry w10x and spectrophotometry w11᎐13x. Some of them are described below. Periodate in the range of 5.2= 10y5 ᎐3.1= 10y4 M has been determined by using its reaction with amodiaquine
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 0 . 0 0 1 6 9 - 7
36
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
dihydrochloride, which yields a chromogen in phosphate buffer w11x. The chromogen is determined spectrophotometrically at 442 nm after extraction with chloroform. The method is applied to indirect determination of ethylene glycol, glycerol and tartaric acid. The chemiluminescence reaction between luminol and periodate has been used for the determination of periodate and indirect determination of tartaric acid w3x. Periodate in the range of 1.0= 10y6 ᎐2.5= 10y4 M has been determined by the method. Jie et al. w10x used the reaction between thiamine and periodate for fluorimetric determination of periodate. The reaction was also applied to the indirect determination of ethylene glycol and glycerol. However, most of these methods are either not sensitive enough or require complicated and expensive instruments or are subject to interferences from other ions, suffer from small calibration range, or more or less are time consuming. In this work a kinetic spectrophotometric method is described for the determination of trace quantities of periodate based on the reaction with ferroin in the presence of Mn ŽII. as a catalyst. The proposed method is rapid, simple, precise, and accurate and is suitable for indirect determination of trace quantities of ethylene glycol and glycerol via Malaprade reaction with satisfactory results.
solution was prepared by dissolving 0.0570 g glycerol ŽMerck. in water and diluting to the mark with water in a 250-ml volumetric flask. A 4.0= 10y3 M ethylene glycol stock solution was prepared by dissolving 0.0627 g ethylene glycol ŽMerck. in water and diluting to the mark in a 250-ml volumetric flask. Working solutions were prepared by appropriate dilution of stock solutions as required. 2.2. Apparatus A Shimadzu model UV-120-01 spectrophotometer with a 1-cm glass cell was used for absorbance measurements. 2.3. Procedure
2.1. Reagents
2.3.1. Determination of periodate All the solutions were equilibrated at 25 " 0.1⬚C before the beginning of the reaction. A suitable aliquot of the sample solution containing 4᎐54 g periodate was transferred into a 10-ml volumetric flask. Then 0.4 ml of 0.1 M sulfuric acid solution and 1.0 ml of 40 grml Mn ŽII. solution was added. The solution wad diluted to approx. 9 ml with water then 1.0 ml of 1.241= 10y3 M ferroin solution was added to initiate the reaction. Time was measured from just after the addition of ferroin solution. The solution was diluted to the mark with water and a portion was transferred into a glass cell within 30 s for measuring the decrease in absorbance at 510 nm for the first 3.0 min after initiation of the reaction.
All solutions were prepared using reagent grade substances and triple-distilled water. A stock solution of 1000 grml periodate was prepared by dissolving 0.1205 g KIO4 ŽMerck. in water and diluting to the mark in a 100-ml volumetric flask. A 1000 grml stock solution of Mn ŽII. was prepared by dissolving 0.3073 g MnSO4 ⭈ H 2 O ŽMerck. in water and diluting to the mark with water in a 100-ml volumetric flask. These solutions were diluted further as required. Ferroin solution Ž1.241= 10y3 M. was prepared by appropriate dilution of its 0.025 M solution ŽMerck. with water. A 2.17= 10y3 glycerol stock
2.3.2. Determination of ethylene glycol A suitable aliquot of sample solution containing 0.040᎐0.80 mol ethylene glycol was transferred into a 10-ml volumetric flask. Then 0.4 ml of 40.0 grml periodate solution was added and the flask was allowed to stand for 15.0 min at room temperature. The excess of periodate was determined as described in Section 2.3.1. The ethylene glycol concentration was calculated by substitution of the measured absorbance change for the first 3.0 min after initiation of the reaction in regression equation of ethylene glycol calibration graph Žsee Section 3.3..
2. Experimental
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
37
2.3.3. Determination of glycerol A suitable aliquot of sample solution containing 5.0= 10y3 ᎐0.1 mol glycerol was added to a 10-ml volumetric flask. Then 0.9 ml of 40 grml periodate solution was added and the flask was allowed to stand for 30 min at 30⬚C. The excess of periodate was determined as described in the Section 2.3.1. The glycerol concentration was calculated by substitution of the measured absorbance change for the first 3.0 min after initiation of the reaction in regression equation of glycerol calibration graph Žsee Section 3.3..
3. Results and discussion 3.1. Ferroin᎐periodate redox reaction The oxidation reaction of ferroin with periodate takes place in acidic media. Mn ŽII. catalyzes the reaction. The reaction could be monitored spectrophotometrically by measuring the decrease in absorbance of the reaction mixture at 510 nm
Fig. 2. Effect of ferroin concentration on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; Mn ŽII., 4 grml.
Ž max for absorption spectra of ferroin. w14,15x. The reaction could be used to the determination of trace quantities of periodate. 3.2. Effects of ¨ ariables
Fig. 1. Effect of temperature on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; Mn ŽII., 4 grml; ferroin, 1.24= 10y4 M.
To take full advantage of the procedure the reagent concentrations and reaction conditions must be optimized. Various experimental parameters were studied in order to obtain optimized systems. These parameters were optimized by setting all parameters to be constant and optimizing one each time. The effect of temperature was studied in the range of 5᎐45⬚C. The results are shown in Fig. 1. As Fig. 1 shows, the absorbance change increased by increasing temperature up to 25⬚C and decreased at higher temperatures. Therefore 25⬚C was selected as optimum temperature. The effect of ferroin concentration on the reaction rate was studied in the range of 1.12= 10y5 ᎐1.80= 10y4 M. As Fig. 2 shows, the absorbance change increased with increasing ferroin concentration up to 1.24= 10y4 M and decreased
38
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
3.3. Analytical parameters
Fig. 3. Effect of Mn ŽII. concentration on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; ferroin, 1.24= 10y4 M.
at higher concentrations. Therefor, a final concentration of 1.24= 10y4 M ferroin was selected for the sake of high sensitivity. The effect of Mn ŽII. concentration on reaction rate was investigated in the range of 0.0᎐6.0 grml. The results are given in Fig. 3. As Fig. 3 shows, the absorbance change increased by increasing Mn ŽII. concentration up to 3.5 grml and remained nearly constant at higher concentrations. Thus 4.0 grml of Mn ŽII. was selected as optimum concentration. As mentioned before, the reaction proceeds in acidic media. Various acids were tested and sulfuric acid was found to be the best one. The effect of sulfuric acid on the rate of reaction in the range of 0.0᎐9.2= 10y3 M was also investigated. As Fig. 4 shows, the reaction rate increased by increasing sulfuric acid concentration up to 2.8= 10y3 M, remained nearly constant in the concentration range of 2.8= 10y3 ᎐6.0= 10y3 M and decreased at higher concentrations. A final concentration of 4.0= 10y3 M sulfuric acid was selected as optimum concentration. Ionic strength had no considerable effect on the determination of periodate up to 0.80 M.
3.3.1. Calibration graphs The calibration graphs were obtained by a fixed time method under the optimum conditions. For periodate, the calibration graph is linear under the following conditions: ferroin concentration of 1.24= 10y4 M, sulfuric acid concentration of 4.0 = 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C and periodate concentration of 0.4᎐5.4 grml. The regression equation is ⌬ A s 0.01326q 0.05577C, with a correlation coefficient of 0.9994, where ⌬ A is sample signal and C is preiodate concentration in grml. For ethylene glycol, calibration graph is linear under the following conditions: ferroin concentration of 1.24= 10y4 M, sulfuric acid concentration of 4 = 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C, periodate concentration of 3.6 grml and ethylene glycol concentration of 4.0= 10y6 ᎐8.0= 10y5 M. The regression equation is ⌬ A s 0.230᎐2.368= 10 3 C, with a correlation coefficient of 0.9991, where ⌬ A is sample signal and C is ethylene glycol concentration in molrl.
Fig. 4. Effect of sulfuric acid concentration on the reaction rate. Conditions: periodate, 1.7 grml; Mn ŽII., 4 grml; ferroin, 1.24= 10y4 M.
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40 Table 1 Accuracy and precision of the proposed methods Analyte
Taken
Found
Relative error Ž%.
R.S.D. Ž n s 10. Ž%.
IO4 ya
1.000 2.000 3.000
0.982 y1.8 2.023 1.15 3.020 0.65
1.15 0.85 0.55
Glycerolb
1.00 3.00 8.00
1.02 3.01 7.90
2.00 0.33 y1.25
1.58 1.35 0.96
8.00 10.00 30.00
8.15 9.90 29.00
1.88 y1.00 y3.3
2.1 1.15 0.96
Ethylene glycolb
a b
Concentration in grml. Concentration in M.
For glycerol, the calibration graph was linear under the following conditions: ferroin concentration of 1.24= 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C, periodate concentration of 3.6 grml and glycerol concentration of 5.0= 10y7 ᎐1.0= 10y5 M. The regression equation is ⌬ A s 0.228᎐1.966= 10 4 C, with a correlation coefficient of 0.9990, where ⌬ A is sample signal and C is glycerol concentration in molrl. 3.3.2. Limit of detection and precision The limit of detection, YLO D s YB q 3S B w16x, where YLO D , YB and S B are signal of detection limit, signal of blank and standard deviation of blank, respectively, was 0.25 grml, 2.62= 10y6 M and 2.8= 10y7 M for periodate, ethylene glycol and glycerol, respectively. To evaluate the precision and accuracy of the methods a series of independent synthetic samples was used. The results are given in Table 1.
39
3.4. Selecti¨ ity To study the selectivity of the proposed method the effect of various cations and anions on the determination of preiodate was studied. An error of "3% was considered tolerable. The results showed that 500-fold Kq, Naq, Ca2q, Ba2q, NHq 4, 2y y y Ni 2q, Mg 2q, CNy, NOy 3 , SO4 , ClO 3 , ClO4 , CH 3 COOy, Cly; 100-fold of PO43y, CO 32y, Fy, Ce 3q, Fe 3q; fivefold tartrate, citrate and Fe 2q; 2y and 1-fold of Ce 4q, Bry, BrO3y, IOy 3 and Cr 2 O 7 did not interfere. 3.5. Applications To evaluate the analytical applicability of the proposed methods, the recommended procedures were applied to the determination of ethylene glycol in gasoline and glycerol in vegetable oils. 3.6. Determination of ethylene glycol in gasoline 10 ml gasoline was shaken with 10 ml water in a separating funnel. The aqueous phase was then extracted with three 10-ml portions of chloroform, which was discarded. The aqueous phase was diluted with water to the mark in a 100-ml volumetric flask and the ethylene glycol concentration was determined as described in Section 2.3 after appropriate dilution. The results are given in Table 2. 3.7. Determination of glycerol in ¨ egetable oil An accurately weighed amount Ž5᎐6 g. of vegetable oil was dissolved in 15 ml of chloroform. The solution was shaken with three 10-ml portions of distilled water in a separating funnel. The
Table 2 Analysis of real samples by proposed methods Sample
Gasoline Vegetable oil a
Analyte
Ethylene glycol Glycerol
Amount of analyte Proposed method
Standard methoda
2.82 mmolrl 0.48 mmolrkg
2.78 mmolrl 0.52 mmolrkg
See Welcher w17x for ethylene glycol and Horwitz w18x for glycerol.
40
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
aqueous phase was diluted to the mark with water in a 100-ml volumetric flask. The glycerol concentration was determined as described in Section 2.3 after appropriate dilution. The results are given in Table 2. References w1x A. Townshend, D.T. Burns, Talanta 39 Ž1992. 715. w2x M.S. El-Shahawi, A.B. Farag, Anal. Chim. Acta 307 Ž1995. 139. w3x A. Gaikwad, M. Silva, D. Perez-Bendito, Analyst 119 Ž1994. 1819. w4x P.S. Radhakrishnamurti, H.P. Panda, React. Kinet. Catal. Lett. 14 Ž1980. 193. w5x N.P. Evmiridis, Analyst 112 Ž1987. 825. w6x Z.A.K. Haree, Microchem. J. 31 Ž1985. 375. w7x A. Mohamad, S. Tiwari, J. Planar. Chromatogr. Mod. Tlc. 4 Ž1991. 485. w8x S. Honda, K. Suzuki, K. Kakehi, Anal. Biochem. 177 Ž1989. 62.
w9x Y.M. Termerk, M.M. Kamal, M.E. Ahmed, J. Electrochem. Soc. India 34 Ž1985. 177. w10x N. Jie, D. Yang, Q. Zhang, J. Yang, Z. Song, Anal. Chim. Acta 359 Ž1998. 87. w11x K.K. Verma, D. Gupta, S.K. Sanghi, A. Jain, Analyst 112 Ž1987. 1519. w12x M.S. El-Shahawi, F.A. Al-Hashemi, Talanta 43 Ž1996. 2037. w13x A.A. Ensafi, S. Zakeri, Anal. Lett. 32 Ž1999. 1643. w14x A. Afkhami, H. Bahrami, Iran. J. Chem. Eng. 14 Ž1995. 72. w15x A. Afkhami, H. Bahrami, Anal. Lett. 28 Ž1995. 1785. w16x J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Horwood, Chichester, 1984. w17x Welcher, F.J. ŽEd.., 1980. Standard Methods of Chemical Analysis, 6th ed., vol. 2, Part B. Krieger Publishing Company, New York, p. 1656. w18x Horwitz, W. ŽEd.., 1980. Official Methods of Analysis of the Association of Official Analytical Chemists, 13th ed. Association of Official Analytical Chemists, Washington, p. 667.
Kinetic determination of periodate based on its reaction with ferroin and its application to the indirect determination of ethylene glycol and glycerol Abbas AfkhamiU , Fatemeh Mosaed Department of Chemistry, Faculty of Sciences, Bu-Ali Sina Uni¨ ersity, Hamadan, Iran Received 6 July 2000; received in revised form 31 August 2000; accepted 1 September 2000
Abstract A simple, sensitive, rapid and reliable method has been developed for spectrophotometric determinations of periodate. The method is based on the redox reaction of periodate with ferroin in acidic media. Mn ŽII. accelerates the reaction. The reaction has been monitored spectrophotometrically by measuring the decrease in absorbance of reaction mixture at 510 nm by a fixed time method of 3 min. Periodate could be determined in the range of 0.40᎐5.0 grml. The relative standard deviation for 10 determinations of 1.0grml of periodate was 1.16% and the limit of detection, corresponding to a signal-to-noise ratio of 3, was 0.25grml. The proposed method was applied to the determination of ethylene glycol and glycerol via the Malaprade reaction with satisfactory results. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Kinetic determination; Spectrophotometry; Periodate; Ethylene glycol; Glycerol
1. Introduction Periodate is used for the oxidation of different inorganic and organic compounds and catalytic applications at trace levels w1᎐6x. Therefore, simple rapid, sensitive and accurate methods are
U
Corresponding author. Fax: q98-81-8272046. E-mail address: [email protected] ŽA. Afkhami..
required for determinations of trace amounts of periodate in different samples. Various methods have been reported for the determination of periodate, including thin layer chromatography w7x, electrophoresis w8x, pulse polarography w9x, chemiluminescence w3x, fluorimetry w10x and spectrophotometry w11᎐13x. Some of them are described below. Periodate in the range of 5.2= 10y5 ᎐3.1= 10y4 M has been determined by using its reaction with amodiaquine
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 0 . 0 0 1 6 9 - 7
36
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
dihydrochloride, which yields a chromogen in phosphate buffer w11x. The chromogen is determined spectrophotometrically at 442 nm after extraction with chloroform. The method is applied to indirect determination of ethylene glycol, glycerol and tartaric acid. The chemiluminescence reaction between luminol and periodate has been used for the determination of periodate and indirect determination of tartaric acid w3x. Periodate in the range of 1.0= 10y6 ᎐2.5= 10y4 M has been determined by the method. Jie et al. w10x used the reaction between thiamine and periodate for fluorimetric determination of periodate. The reaction was also applied to the indirect determination of ethylene glycol and glycerol. However, most of these methods are either not sensitive enough or require complicated and expensive instruments or are subject to interferences from other ions, suffer from small calibration range, or more or less are time consuming. In this work a kinetic spectrophotometric method is described for the determination of trace quantities of periodate based on the reaction with ferroin in the presence of Mn ŽII. as a catalyst. The proposed method is rapid, simple, precise, and accurate and is suitable for indirect determination of trace quantities of ethylene glycol and glycerol via Malaprade reaction with satisfactory results.
solution was prepared by dissolving 0.0570 g glycerol ŽMerck. in water and diluting to the mark with water in a 250-ml volumetric flask. A 4.0= 10y3 M ethylene glycol stock solution was prepared by dissolving 0.0627 g ethylene glycol ŽMerck. in water and diluting to the mark in a 250-ml volumetric flask. Working solutions were prepared by appropriate dilution of stock solutions as required. 2.2. Apparatus A Shimadzu model UV-120-01 spectrophotometer with a 1-cm glass cell was used for absorbance measurements. 2.3. Procedure
2.1. Reagents
2.3.1. Determination of periodate All the solutions were equilibrated at 25 " 0.1⬚C before the beginning of the reaction. A suitable aliquot of the sample solution containing 4᎐54 g periodate was transferred into a 10-ml volumetric flask. Then 0.4 ml of 0.1 M sulfuric acid solution and 1.0 ml of 40 grml Mn ŽII. solution was added. The solution wad diluted to approx. 9 ml with water then 1.0 ml of 1.241= 10y3 M ferroin solution was added to initiate the reaction. Time was measured from just after the addition of ferroin solution. The solution was diluted to the mark with water and a portion was transferred into a glass cell within 30 s for measuring the decrease in absorbance at 510 nm for the first 3.0 min after initiation of the reaction.
All solutions were prepared using reagent grade substances and triple-distilled water. A stock solution of 1000 grml periodate was prepared by dissolving 0.1205 g KIO4 ŽMerck. in water and diluting to the mark in a 100-ml volumetric flask. A 1000 grml stock solution of Mn ŽII. was prepared by dissolving 0.3073 g MnSO4 ⭈ H 2 O ŽMerck. in water and diluting to the mark with water in a 100-ml volumetric flask. These solutions were diluted further as required. Ferroin solution Ž1.241= 10y3 M. was prepared by appropriate dilution of its 0.025 M solution ŽMerck. with water. A 2.17= 10y3 glycerol stock
2.3.2. Determination of ethylene glycol A suitable aliquot of sample solution containing 0.040᎐0.80 mol ethylene glycol was transferred into a 10-ml volumetric flask. Then 0.4 ml of 40.0 grml periodate solution was added and the flask was allowed to stand for 15.0 min at room temperature. The excess of periodate was determined as described in Section 2.3.1. The ethylene glycol concentration was calculated by substitution of the measured absorbance change for the first 3.0 min after initiation of the reaction in regression equation of ethylene glycol calibration graph Žsee Section 3.3..
2. Experimental
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
37
2.3.3. Determination of glycerol A suitable aliquot of sample solution containing 5.0= 10y3 ᎐0.1 mol glycerol was added to a 10-ml volumetric flask. Then 0.9 ml of 40 grml periodate solution was added and the flask was allowed to stand for 30 min at 30⬚C. The excess of periodate was determined as described in the Section 2.3.1. The glycerol concentration was calculated by substitution of the measured absorbance change for the first 3.0 min after initiation of the reaction in regression equation of glycerol calibration graph Žsee Section 3.3..
3. Results and discussion 3.1. Ferroin᎐periodate redox reaction The oxidation reaction of ferroin with periodate takes place in acidic media. Mn ŽII. catalyzes the reaction. The reaction could be monitored spectrophotometrically by measuring the decrease in absorbance of the reaction mixture at 510 nm
Fig. 2. Effect of ferroin concentration on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; Mn ŽII., 4 grml.
Ž max for absorption spectra of ferroin. w14,15x. The reaction could be used to the determination of trace quantities of periodate. 3.2. Effects of ¨ ariables
Fig. 1. Effect of temperature on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; Mn ŽII., 4 grml; ferroin, 1.24= 10y4 M.
To take full advantage of the procedure the reagent concentrations and reaction conditions must be optimized. Various experimental parameters were studied in order to obtain optimized systems. These parameters were optimized by setting all parameters to be constant and optimizing one each time. The effect of temperature was studied in the range of 5᎐45⬚C. The results are shown in Fig. 1. As Fig. 1 shows, the absorbance change increased by increasing temperature up to 25⬚C and decreased at higher temperatures. Therefore 25⬚C was selected as optimum temperature. The effect of ferroin concentration on the reaction rate was studied in the range of 1.12= 10y5 ᎐1.80= 10y4 M. As Fig. 2 shows, the absorbance change increased with increasing ferroin concentration up to 1.24= 10y4 M and decreased
38
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
3.3. Analytical parameters
Fig. 3. Effect of Mn ŽII. concentration on the reaction rate. Conditions: periodate, 1.7 grml; sulfuric acid, 4.0= 10y3 M; ferroin, 1.24= 10y4 M.
at higher concentrations. Therefor, a final concentration of 1.24= 10y4 M ferroin was selected for the sake of high sensitivity. The effect of Mn ŽII. concentration on reaction rate was investigated in the range of 0.0᎐6.0 grml. The results are given in Fig. 3. As Fig. 3 shows, the absorbance change increased by increasing Mn ŽII. concentration up to 3.5 grml and remained nearly constant at higher concentrations. Thus 4.0 grml of Mn ŽII. was selected as optimum concentration. As mentioned before, the reaction proceeds in acidic media. Various acids were tested and sulfuric acid was found to be the best one. The effect of sulfuric acid on the rate of reaction in the range of 0.0᎐9.2= 10y3 M was also investigated. As Fig. 4 shows, the reaction rate increased by increasing sulfuric acid concentration up to 2.8= 10y3 M, remained nearly constant in the concentration range of 2.8= 10y3 ᎐6.0= 10y3 M and decreased at higher concentrations. A final concentration of 4.0= 10y3 M sulfuric acid was selected as optimum concentration. Ionic strength had no considerable effect on the determination of periodate up to 0.80 M.
3.3.1. Calibration graphs The calibration graphs were obtained by a fixed time method under the optimum conditions. For periodate, the calibration graph is linear under the following conditions: ferroin concentration of 1.24= 10y4 M, sulfuric acid concentration of 4.0 = 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C and periodate concentration of 0.4᎐5.4 grml. The regression equation is ⌬ A s 0.01326q 0.05577C, with a correlation coefficient of 0.9994, where ⌬ A is sample signal and C is preiodate concentration in grml. For ethylene glycol, calibration graph is linear under the following conditions: ferroin concentration of 1.24= 10y4 M, sulfuric acid concentration of 4 = 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C, periodate concentration of 3.6 grml and ethylene glycol concentration of 4.0= 10y6 ᎐8.0= 10y5 M. The regression equation is ⌬ A s 0.230᎐2.368= 10 3 C, with a correlation coefficient of 0.9991, where ⌬ A is sample signal and C is ethylene glycol concentration in molrl.
Fig. 4. Effect of sulfuric acid concentration on the reaction rate. Conditions: periodate, 1.7 grml; Mn ŽII., 4 grml; ferroin, 1.24= 10y4 M.
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40 Table 1 Accuracy and precision of the proposed methods Analyte
Taken
Found
Relative error Ž%.
R.S.D. Ž n s 10. Ž%.
IO4 ya
1.000 2.000 3.000
0.982 y1.8 2.023 1.15 3.020 0.65
1.15 0.85 0.55
Glycerolb
1.00 3.00 8.00
1.02 3.01 7.90
2.00 0.33 y1.25
1.58 1.35 0.96
8.00 10.00 30.00
8.15 9.90 29.00
1.88 y1.00 y3.3
2.1 1.15 0.96
Ethylene glycolb
a b
Concentration in grml. Concentration in M.
For glycerol, the calibration graph was linear under the following conditions: ferroin concentration of 1.24= 10y3 M, Mn ŽII. concentration of 4.0 grml, temperature of 25⬚C, periodate concentration of 3.6 grml and glycerol concentration of 5.0= 10y7 ᎐1.0= 10y5 M. The regression equation is ⌬ A s 0.228᎐1.966= 10 4 C, with a correlation coefficient of 0.9990, where ⌬ A is sample signal and C is glycerol concentration in molrl. 3.3.2. Limit of detection and precision The limit of detection, YLO D s YB q 3S B w16x, where YLO D , YB and S B are signal of detection limit, signal of blank and standard deviation of blank, respectively, was 0.25 grml, 2.62= 10y6 M and 2.8= 10y7 M for periodate, ethylene glycol and glycerol, respectively. To evaluate the precision and accuracy of the methods a series of independent synthetic samples was used. The results are given in Table 1.
39
3.4. Selecti¨ ity To study the selectivity of the proposed method the effect of various cations and anions on the determination of preiodate was studied. An error of "3% was considered tolerable. The results showed that 500-fold Kq, Naq, Ca2q, Ba2q, NHq 4, 2y y y Ni 2q, Mg 2q, CNy, NOy 3 , SO4 , ClO 3 , ClO4 , CH 3 COOy, Cly; 100-fold of PO43y, CO 32y, Fy, Ce 3q, Fe 3q; fivefold tartrate, citrate and Fe 2q; 2y and 1-fold of Ce 4q, Bry, BrO3y, IOy 3 and Cr 2 O 7 did not interfere. 3.5. Applications To evaluate the analytical applicability of the proposed methods, the recommended procedures were applied to the determination of ethylene glycol in gasoline and glycerol in vegetable oils. 3.6. Determination of ethylene glycol in gasoline 10 ml gasoline was shaken with 10 ml water in a separating funnel. The aqueous phase was then extracted with three 10-ml portions of chloroform, which was discarded. The aqueous phase was diluted with water to the mark in a 100-ml volumetric flask and the ethylene glycol concentration was determined as described in Section 2.3 after appropriate dilution. The results are given in Table 2. 3.7. Determination of glycerol in ¨ egetable oil An accurately weighed amount Ž5᎐6 g. of vegetable oil was dissolved in 15 ml of chloroform. The solution was shaken with three 10-ml portions of distilled water in a separating funnel. The
Table 2 Analysis of real samples by proposed methods Sample
Gasoline Vegetable oil a
Analyte
Ethylene glycol Glycerol
Amount of analyte Proposed method
Standard methoda
2.82 mmolrl 0.48 mmolrkg
2.78 mmolrl 0.52 mmolrkg
See Welcher w17x for ethylene glycol and Horwitz w18x for glycerol.
40
A. Afkhami, F. Mosaed r Microchemical Journal 68 (2001) 35᎐40
aqueous phase was diluted to the mark with water in a 100-ml volumetric flask. The glycerol concentration was determined as described in Section 2.3 after appropriate dilution. The results are given in Table 2. References w1x A. Townshend, D.T. Burns, Talanta 39 Ž1992. 715. w2x M.S. El-Shahawi, A.B. Farag, Anal. Chim. Acta 307 Ž1995. 139. w3x A. Gaikwad, M. Silva, D. Perez-Bendito, Analyst 119 Ž1994. 1819. w4x P.S. Radhakrishnamurti, H.P. Panda, React. Kinet. Catal. Lett. 14 Ž1980. 193. w5x N.P. Evmiridis, Analyst 112 Ž1987. 825. w6x Z.A.K. Haree, Microchem. J. 31 Ž1985. 375. w7x A. Mohamad, S. Tiwari, J. Planar. Chromatogr. Mod. Tlc. 4 Ž1991. 485. w8x S. Honda, K. Suzuki, K. Kakehi, Anal. Biochem. 177 Ž1989. 62.
w9x Y.M. Termerk, M.M. Kamal, M.E. Ahmed, J. Electrochem. Soc. India 34 Ž1985. 177. w10x N. Jie, D. Yang, Q. Zhang, J. Yang, Z. Song, Anal. Chim. Acta 359 Ž1998. 87. w11x K.K. Verma, D. Gupta, S.K. Sanghi, A. Jain, Analyst 112 Ž1987. 1519. w12x M.S. El-Shahawi, F.A. Al-Hashemi, Talanta 43 Ž1996. 2037. w13x A.A. Ensafi, S. Zakeri, Anal. Lett. 32 Ž1999. 1643. w14x A. Afkhami, H. Bahrami, Iran. J. Chem. Eng. 14 Ž1995. 72. w15x A. Afkhami, H. Bahrami, Anal. Lett. 28 Ž1995. 1785. w16x J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Horwood, Chichester, 1984. w17x Welcher, F.J. ŽEd.., 1980. Standard Methods of Chemical Analysis, 6th ed., vol. 2, Part B. Krieger Publishing Company, New York, p. 1656. w18x Horwitz, W. ŽEd.., 1980. Official Methods of Analysis of the Association of Official Analytical Chemists, 13th ed. Association of Official Analytical Chemists, Washington, p. 667.