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Differential electrolytic potentiometry, a detector in flow injection analysis for precipitaton reactions
Talanta 46 (1998) 639 – 646
Differential electrolytic potentiometry, a detector in flow injection analysis for precipitaton reactions A.M.S. Abdennabi *, M.E. Koken Chemistry Department, Box 5048, King Fahd Uni6ersity of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 14 May 1997; received in revised form 10 September 1997; accepted 11 September 1997
Abstract The application of differential electrolytic potentiometry as a detection system in flow injection analysis for precipitation reactions is described. Different combinations of electrodes were investigated. The optimum conditions for the current density and the flow rate were elucidated. In the case of chloride, an Ag/AgCl – Pt pair was found to be successful. For iodide a combination of Ag– Pt electrodes was found to give good results. The relation between the concentration of analyte and the measured signal was found to be linear. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Differential electrolytic potentiometry; Flow injection analysis; Chloride and iodide detection
1. Introduction Differential electrolytic potentiometry (dc DEP) consists of polarising two identical electrodes with a stabilised current and measuring the potential difference (DE) between them. This method has been applied to various types of titrimetric reactions in both aqueous [1 – 4] and non-aqueous [5–9] media using different types of electrodes. In this technique the polarized electrodes respond faster, the apparatus is simple and the salt bridge problems of the reference cell are eliminated. The determination of chloride by flow injection analysis has been based mainly on spectrophotometrical procedures where the reaction between thiocyanate and mercury complex leads to the * Corresponding author. Tel.: + 966 3 8602111; fax: + 966 3 8604227; e-mail: [email protected]
displacement of thiocyanate and the formation of an intensely colored complex of iron(III) [10–12]. This method has been found to suffer from poor detection limits, however, Slanina et al. [13] have improved the method and were able to detect chloride in the range of 0.2–15 ppm using both photometric and potentiometric detectors. Chloride was also determined by Cirelooegamina and Brindle [14] using a UV-photometric flow injection method. Taylor and Grate [15] determined chloride by means of a flow injection technique with a reflectance detector for the determination of chloride. The technique of differential electrolytic potentiometry has not as yet been applied as a detection system in flow injection analysis. This paper describes the application of this technique for certain precipitation reactions. The behaviours of different combinations of polarized electrodes
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A.M.S. Abdennabi, M.E. Koken / Talanta 46 (1998) 639–646
were investigated. Suitable combinations of electrodes for chloride and iodide determination is reported.
2. Experimental
2.1. Reagents Chloride ion solutions were prepared using Thorn Smith Standard sodium chloride. Supporting electrolyte solutions were prepared from AnalaR sodium nitrate and potassium nitrate. Hydrochloric acid (Fisher) and Baker Analyzed nitric acid were used for the required purposes. Fluka iron(III) chloride was used for the preparation of the solution that is required in the plating of silver electrodes with silver chloride. Fluka silver nitrate was used for the preparation of silver nitrate solution. Degussa mercury was used for the preparation of the amalgamated electrode. A stock solution of 0.01 M sodium chloride was prepared by dissolving 0.585 g of NaCl in distilled deionized water in 1000 ml volumetric flask. Working solutions were prepared by dilution. The same procedure was applied to prepare stock solutions of silver nitrate, sodium nitrate and potassium nitrate. A solution of 0.5 M iron(III) chloride was prepared by dissolving the required weight of iron(III) chloride in 8.06 ml of concentrated hydrochloric acid and 50 ml of distilled deionized water in a 100 ml flask then completing to the mark with distilled deionised water. Nitric acid solution (6 M) was prepared in the usual way from concentrated nitric acid. Fluka sodium iodide was used to prepare iodide solution. A standard solution of 0.01 M iodide was prepared in water.
Fig. 2. Current density – peak height relationship. Electrode pair, Ag – Ag; flow rate, 0.93 ml min − 1, volume, 140 ml of 3.55 ppm Cl − ; (a) 1.98 mA cm − 2; (b) 3.96 mA cm − 2; (c) 5.95 mA cm − 2; (d) 7.93 mA cm − 2; (e) 9.91 mA cm − 2.
Fig. 3. Flow rate – peak height relationship. Electrode pair, Ag – Ag; current, 5.95 mA cm − 2; vs, 140 ml 3.55 ppm Cl − ; (a) 0.55 ml min − 1; (b) 0.93 ml min − 1; (c) 1.17 ml min − 1; (d) 1.26 ml min − 1; (e) 1.45 ml min − 1.
2.2. Apparatus Fig. 1. Manifold used in the determination of chloride. I, silver nitrate carrier; II, supporting electrolyte solution; R, reactor; D, detector; W, waste.
Keithley Instruments 225 Constant Current Source was used to polarize the electrodes.
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The potentiometer used is Corning Scientific Instruments Model 12 research pH meter for oxidation reactions and changed to digital Fisher
Fig. 6. Calibration curve. Electrode pair Ag/AgCl – Pt, current density, 19.82 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 100 ml of Cl − ; (a) 3.0 ppm; (b) 4.8 ppm; (c) 29 ppm; (d) 48 ppm; (e) 68 ppm; (f) signal of sample 1; (g) signal of sample 2.
Fig. 4. Calibration curve. Electrode pair, Ag–Ag; current, 5.95 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 140 ml; (a) 3.55 ppm; (b) 10.7 ppm; (c) 17.8 ppm; (d) 24.8 ppm; (e) 32.0 ppm.*
Fig. 5. Current density–peak height relationship. Electrode pair, Ag/AgCl –Pt; flow rate, 0.93 ml min − 1, volume, 100 ml of 6.4 ppm Cl; (a) 1.98 mA cm − 2; (b) 9.91 mA cm − 2; (c) 19.8 mA cm − 2; (d) 29.7 mA cm − 2; (e) 39.6 mA cm − 2.
Accumet Selective Ion Analyzer Model 750 for the other experiments. A Cole Parmer chart recorder was used and the data was recorded with a speed of 0.3 cm/min. The test solutions were propelled by a peristaltic pump of Alitea USA/ FIA laboratory for all experiments. Gateway 2000 computer with RS 332-098 4 phase unipolar stepper motor drive board introduced into the hard disk was used for the injection of samples into carrier streams to provide highly precise and reproducible volume. The injector used was a plastic syringe with the metallic needle replaced by a plastic one to prevent corrosion. A two line manifold system was used as shown in Fig. 1. Silver nitrate solution was passed from one line and the supporting electrolyte from the other. The reactor was made of a coiled tube formed from a plastic tube of 25 cm length and 1.14 mm inner diameter. This tube was mounted on a glass cylinder of diameter 0.7 cm. The detector was made of Teflon in the form of a cylinder 3 cm in length where the two electrodes were placed and this detector was connected to the reactor.
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Table 1 Results for the determination of chloride in real samples by precipitation and FIA-DEP methods Sample No.
Cl− (found by pptn method) in ppm
Cl− (found by FIA-DEP) in ppm
% Recovery
1 2
3.50 32.0
3.58 32.1
102.3 100.3
rsd, 0.2%
2.3. Electrodes Silver wire electrodes were used in the first part of the chloride and iodide analysis. They were cleaned with 6 M nitric acid as a pretreatment before each experiment. The electrodes were then rinsed with distilled deionized water and placed in the cell. The platinum electrodes were SargentWelch type 30-415 and 30-415 respectively. They were cleaned with concentrated sulfuric and/or nitric acid before each experiment as a pretreatment, rinsed with distilled deionized water and placed in the cell. Platinum electrode were cleaned by dipping in boiling aqua regia and then rinsed with distilled deionized water. It was then immediately immersed into pure mercury for 30 s to prepare a Pt/Hg electrode. Silver/silver chloride electrodes were prepared by surface oxidation of silver electrodes in a solution of 0.5 M Fe(III) chloride solution which was found to be more effective than the traditional anodic oxidation process [16]. The electrode was immersed in the solution, left overnight then rinsed with distilled deionized water and placed in the cell.
3. Results and discussion Fig. 1 shows the manifold used in this study where silver nitrate was delivered as reactant from line I. The supporting electrolyte solution was passed through line II. At the start of this work, the proper current density was determined by injecting a volume of 140 ml of 3.55 ppm chloride solution into a stream of 0.01 M KNO3 and 10 − 3 M AgNO3 as shown in Fig. 1. Different current
densities were employed to polarise two identical silver electrodes. Fig. 2a shows that the heights of the resulting peaks increased with the current densities employed. It was noted that at lower current densities, for instance 1.98 mA cm − 2, the electrodes were found to have a slow response. This is reflected in the small heights of the resulting peaks. However, at current densities greater than 7.93 mA cm − 2, the electrodes response becomes abnormal. Consequently the resulting peaks will show poor reproducibility as seen from peaks at d and f of Fig. 2. This abnormal behaviour is probably due to the fact that higher currents will increase the concentration overpotentials of both the anode and the cathode. Consequently the measured signal which is the difference between the potentials of these two electrodes will be affected. Fig. 2 shows that the three peaks at point (c) are symmetrical, reproducible and of a considerable height. These peaks were obtained at a current density of 5.95 mA cm − 2, hence, this value is considered to be an optimum under the employed conditions. The relation between peak height and current density employed to polarise the silver electrodes is shown in table. 2. The effect of flow rate on the produced signal was also studied and the results are depicted in Fig. 3. It is obvious from this figure that the peak height decreases with an increase in flow rate. The peaks of point b are symmetrical, having a reasonable width and they are considerably high, therefore, the flow rate at this point which is 0.93 ml min − 1 was considered to be appropriate for the experimental measurements. A calibration curve was constructed by injecting standard chloride solutions of concentrations that range from 3.55–35.5 ppm. This calibration curve is shown in Fig. 4 and it has a regression
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coefficient of 0.991. During the course of this work it was observed that the response of the silver electrodes became sluggish. This can be attributed to the fact that the resulting silver
Fig. 8. Calibration curve. Electrode pair, Ag/AgCl – Pt/Hg; current density, 19.8 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 100 ml of Cl − ; (a) 1.8 ppm; (b) 3.0 ppm; (c) 18.2 ppm; (d) 29.8 ppm; (e)42.7 ppm.
Fig. 7. Current density–peak height relationship. Electrode pair, Ag/AgCl–Pt/Hg; flow rate, 0.93 ml min − 1, volume 100 ml of 4.5 ppm Cl − ; (a) 0.99 mA cm − 2; (b) 2.97 mA cm − 2; (c) 11.89 mA cm − 2; (d)14.9 mA cm − 2; (e) 19.8 mA cm − 2. Table 2 The relation between the peak heights and the current densities employed for different combinations of electrodes used for chloride determination Type of electrodes
Current density (mA cm−2)
Height of peak (mV)
Ag – Ag
1.98 3.96 5.95 7.93 9.91
183 225 283 333 400
Ag/AgCl – Pt
1.98 9.91 19.8 29.7 39.6
58 100 225 258 400
Pt/Hg – Ag/AgCl
0.99 2.97 11.89 14.9 19.8
58 75 141 225 500
chloride dissociates and the ions formed will affect the behaviour of the polarised electrodes. A pair of Ag/AgCl electrodes has been used for the determination of chloride [17]. However, the use of Ag/AgCl electrode as a cathode in DEP will lead to the dissolution of AgCl coating and the electrode will eventually become a silver electrode. Consequently, the resulting signal will differ from the previous ones and abnormal behaviour becomes inevitable. Polarised platinum electrodes have also been used for chloride analysis using silver nitrate as a titrant [18]. The response of the platinum electrode was attributed to the adsorption of silver chloride precipitate on the surface of the electrode. It was decided to make a combination of a platinum electrode with a Ag/AgCl electrode where the platinum electrode would act as cathode. This combination was employed for the determination of chloride by injecting the latter into a stream containing silver nitrate. When the sample is injected, a potential difference develops between the anode and the cathode, consequently, a peak is formed. The relation between the height of the peak and the current density was studied and the results are shown in Fig. 5 and Table 2. It
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Fig. 9. Effect of change in the current density employed. Electrode pair, Ag – Ag; flow rate, 0.93 ml min − 1; volume, 140 ml of 12.7 ppm I − ; (a) 1.98 mA cm − 2; (b) 3.96 mA cm − 2; (c) 5.95 mA cm − 2; (d) 9.91 mA cm − 2.
is evident from this figure that the peak height increases with current density and the value of 19.82 mA cm − 2 was chosen for experimental measurements because the resulting peaks are symmetrical and have similar heights. A calibration curve was established by applying the above mentioned conditions. Standard chloride solutions of concentrations that range from 3.0–68 ppm were injected. The resulting calibration curve is depicted in Fig. 6. This curve was found to be linear with r =0.997 and can be applied to determine chloride in real water samples where the sulphate ion if exists is expected to pose no effect on the response of the indicating system. Fig. 6 shows the results of applying DEP in combination with Ag/AgCl – Pt as a detection system for the analysis of chloride in two samples of drinking water. Peaks (f) and (g) shown in Fig. 6 represent the signals for these two samples. Table 1 gives a comparison between the results obtained by this technique and that of standard precipitation titration. It is apparent from this table that DEP can be applied as a detector in FIA and better sensitivity is expected compared
with direct titration methods including the classical DEP technique. In this technique two identical electrodes are employed to obtain a symmetrical peak which gives the end point. The morphology of the peak depends on the completion of both anodic and cathodic reactions that take place on the two electrodes. However, the use of the DEP technique as a detector in flow injection analysis does not require the completion of the reaction. Therefore different combinations of electrodes can be employed to obtain a better signal. In this regard, a combination of Pt/Hg–Ag/AgCl electrodes was used for the determination of chloride. The proper conditions were found using the univariant method. Fig. 7 shows the effects of changing current density on the height of the signal and the optimum current density that gives the highest reproducible signal was found to be 19.82 mA cm − 2. The relation between the peak height and the current density employed is shown in Table 2. On applying these conditions, a calibration curve that covers a chloride concentration range between 1 and 45 ppm was constructed and is depicted in Fig. 8. It is apparent that this curve is
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linear with r =0.996. However when the combination of Pt/Hg – Ag/AgCl electrodes were used for the determination of chloride in real water samples, higher signals resulted. A similar observation Table 3 The relation between the peak heights and the current densities employed for combinations of electrodes used for iodide determination Type of electrodes
Current density (mA cm−2)
Height of peak (mV)
Ag – Ag
1.98 3.96 5.95 9.91
158 266 316 416
Ag – Pt
4.95 9.91 29.73 39.4
17 41 166 383
0.99 2.97 11.89 14.9 19.8
58 75 141 225 500
Pt/Hg – Ag/AgCl
Fig. 11. Effect of the change in the current density employed. Electrode pair, Ag – Pt; flow rate, 0.93 ml min − 1; volume 100 ml of 1.27 ppm I −; (a) 4.95 mA cm − 2; (b) 9.91 mA cm − 2; (c) 29.73 mA cm − 2; (d) 39.64 mA cm − 2.
Fig. 12. Calibration curve. Electrode pair, Ag – Pt, current density, 35 mA cm − 2, flow rate: 0.93 ml min − 1; volume of I− , 140 ml; (a) 1.27 ppm; (b)3.81 ppm; (c) 6.35 ppm; (d) 7.62 ppm; (e) 8.89 ppm; (f)11.4 ppm; (g) 19.0 ppm; (h) 31.8 ppm.
Fig. 10. Calibration curve. Electrode pair, Ag–Ag; current density, 5.95 mA cm − 2; flow rate, 0.93 ml min − 1; volume, 140 ml; (a) 12.7 ppm; (b) 38.1 ppm; (c) 63.5 ppm; (d) 88.9 ppm; (e) 114 ppm.
was noted by others [19] where sulfate was found to interfere. Therefore the combination of Pt/Hg– Ag/AgCl is recommended for the determination of chloride in the absence of interfering ions such as sulphate.
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The applicability of the DEP technique as a detector in FIA for precipitation reactions was extended to include iodide. Since the solubility of silver iodide is very low, a combination of two identical silver electrodes were expected to give satisfactory results. The appropriate current density gives symmetrical and reproducible peaks of considerable height which was found to be 5.95 mA cm − 2 as shown in Fig. 9. The relation between peak height and current densities employed is shown in Table 3. A calibration curve was constructed for standard iodide solutions of concentrations from 12.7 to 127 ppm. Fig. 10 depicts this curve which has an excellent correlation coefficient of 0.998. In order to examine the applicability of other combinations of electrodes, one silver electrode was replaced by a platinum electrode in iodide determination. The height of the signal was found to increase with an increase in current density employed as shown in Fig. 11. Its optimum value was chosen as 34.7 mA cm − 2 and Table 3 shows the relation between the peak height and the current densities employed. A calibration curve was established for a concentration range of 1.27–31.8 ppm. This curve is shown in Fig. 12 and it has a correlation coefficient of 0.997 which indicates the applicability of the DEP technique as a detector in flow injection analysis.
4. Conclusion The technique of DEP which depends on polarising two identical electrodes using a small current and measuring the potential difference between them can be used as a detector in FIA. A combi-
.
nation of Ag/AgCl–Pt electrodes can be applied for the determination of chloride. For iodide determination Ag–Pt combination was found to give acceptable results. The appropriate conditions of flow rate and current density were investigated.
Acknowledgements M. Koken wishes to thank KFUPM for donating the scholarship to read for MS degree.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
E. Bishop, G.D. Short, Analyst 87 (1962) 467. G.D. Short, E. Bishop, Analyst 87 (1962) 724. G.D. Short, E. Bishop, Analyst 89 (1964) 415. E. Bishop, R.B. Dhaneshwer, Analyst 87 (1962) 207. A.M.S. Abdennabi, E. Bishop, Analyst 107 (1982) 1032. E. Bishop, A.M.S. Abdennabi, Analyst 108 (1983) 1349. A.M.S Abdennabi, E. Bishop, Analyst 108 (1983) 71. A.M.S. Abdennabi, M. Rashid, A.J.S.E 12 (1986) 82. A.M.S. Abdennabi, E. Bishop, Analyst 108 (1983) 1227. J. Ruzicka, J.W.B. Stewart, E.A. Zagatto, Anal. Chim. Acta. 81 (1976) 387. E.H. Hansen, J. Ruzicka, Anal. Chim. Acta. 87 (1976) 353. J. Ruzicka, E.H. Hansen, H. Mosbaek, Anal. Chim. Acta. 49 (1977) 1958. J. Slanina, F. Bakker, A. Bruyn-Hes, J.J. Mols, Anal. Chim. Acta. 33 (1980) 33. J. Cirelloegamino, I.D. Brindle, Analyst 120 (1995) 183. R.H. Taylor, J.N. Grate, Talanta 42 (1995) 257. M. Trojanowickz, W. Matuszewski, Anal. Chim. Acta. 151 (1983) 77. E. Bishop, R.G. Dhaneshwar, Anal.Chem. 36 (1964) 726. R.W. Freedman, Anal. Chem. 31 (1959) 214. E.P. Serjeant, Potentiometry and Potentiometric Titrations, Chemical Analysis vol.69, Wiley, Canada, 1984.
Differential electrolytic potentiometry, a detector in flow injection analysis for precipitaton reactions A.M.S. Abdennabi *, M.E. Koken Chemistry Department, Box 5048, King Fahd Uni6ersity of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 14 May 1997; received in revised form 10 September 1997; accepted 11 September 1997
Abstract The application of differential electrolytic potentiometry as a detection system in flow injection analysis for precipitation reactions is described. Different combinations of electrodes were investigated. The optimum conditions for the current density and the flow rate were elucidated. In the case of chloride, an Ag/AgCl – Pt pair was found to be successful. For iodide a combination of Ag– Pt electrodes was found to give good results. The relation between the concentration of analyte and the measured signal was found to be linear. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Differential electrolytic potentiometry; Flow injection analysis; Chloride and iodide detection
1. Introduction Differential electrolytic potentiometry (dc DEP) consists of polarising two identical electrodes with a stabilised current and measuring the potential difference (DE) between them. This method has been applied to various types of titrimetric reactions in both aqueous [1 – 4] and non-aqueous [5–9] media using different types of electrodes. In this technique the polarized electrodes respond faster, the apparatus is simple and the salt bridge problems of the reference cell are eliminated. The determination of chloride by flow injection analysis has been based mainly on spectrophotometrical procedures where the reaction between thiocyanate and mercury complex leads to the * Corresponding author. Tel.: + 966 3 8602111; fax: + 966 3 8604227; e-mail: [email protected]
displacement of thiocyanate and the formation of an intensely colored complex of iron(III) [10–12]. This method has been found to suffer from poor detection limits, however, Slanina et al. [13] have improved the method and were able to detect chloride in the range of 0.2–15 ppm using both photometric and potentiometric detectors. Chloride was also determined by Cirelooegamina and Brindle [14] using a UV-photometric flow injection method. Taylor and Grate [15] determined chloride by means of a flow injection technique with a reflectance detector for the determination of chloride. The technique of differential electrolytic potentiometry has not as yet been applied as a detection system in flow injection analysis. This paper describes the application of this technique for certain precipitation reactions. The behaviours of different combinations of polarized electrodes
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 3 0 4 - 4
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A.M.S. Abdennabi, M.E. Koken / Talanta 46 (1998) 639–646
were investigated. Suitable combinations of electrodes for chloride and iodide determination is reported.
2. Experimental
2.1. Reagents Chloride ion solutions were prepared using Thorn Smith Standard sodium chloride. Supporting electrolyte solutions were prepared from AnalaR sodium nitrate and potassium nitrate. Hydrochloric acid (Fisher) and Baker Analyzed nitric acid were used for the required purposes. Fluka iron(III) chloride was used for the preparation of the solution that is required in the plating of silver electrodes with silver chloride. Fluka silver nitrate was used for the preparation of silver nitrate solution. Degussa mercury was used for the preparation of the amalgamated electrode. A stock solution of 0.01 M sodium chloride was prepared by dissolving 0.585 g of NaCl in distilled deionized water in 1000 ml volumetric flask. Working solutions were prepared by dilution. The same procedure was applied to prepare stock solutions of silver nitrate, sodium nitrate and potassium nitrate. A solution of 0.5 M iron(III) chloride was prepared by dissolving the required weight of iron(III) chloride in 8.06 ml of concentrated hydrochloric acid and 50 ml of distilled deionized water in a 100 ml flask then completing to the mark with distilled deionised water. Nitric acid solution (6 M) was prepared in the usual way from concentrated nitric acid. Fluka sodium iodide was used to prepare iodide solution. A standard solution of 0.01 M iodide was prepared in water.
Fig. 2. Current density – peak height relationship. Electrode pair, Ag – Ag; flow rate, 0.93 ml min − 1, volume, 140 ml of 3.55 ppm Cl − ; (a) 1.98 mA cm − 2; (b) 3.96 mA cm − 2; (c) 5.95 mA cm − 2; (d) 7.93 mA cm − 2; (e) 9.91 mA cm − 2.
Fig. 3. Flow rate – peak height relationship. Electrode pair, Ag – Ag; current, 5.95 mA cm − 2; vs, 140 ml 3.55 ppm Cl − ; (a) 0.55 ml min − 1; (b) 0.93 ml min − 1; (c) 1.17 ml min − 1; (d) 1.26 ml min − 1; (e) 1.45 ml min − 1.
2.2. Apparatus Fig. 1. Manifold used in the determination of chloride. I, silver nitrate carrier; II, supporting electrolyte solution; R, reactor; D, detector; W, waste.
Keithley Instruments 225 Constant Current Source was used to polarize the electrodes.
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The potentiometer used is Corning Scientific Instruments Model 12 research pH meter for oxidation reactions and changed to digital Fisher
Fig. 6. Calibration curve. Electrode pair Ag/AgCl – Pt, current density, 19.82 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 100 ml of Cl − ; (a) 3.0 ppm; (b) 4.8 ppm; (c) 29 ppm; (d) 48 ppm; (e) 68 ppm; (f) signal of sample 1; (g) signal of sample 2.
Fig. 4. Calibration curve. Electrode pair, Ag–Ag; current, 5.95 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 140 ml; (a) 3.55 ppm; (b) 10.7 ppm; (c) 17.8 ppm; (d) 24.8 ppm; (e) 32.0 ppm.*
Fig. 5. Current density–peak height relationship. Electrode pair, Ag/AgCl –Pt; flow rate, 0.93 ml min − 1, volume, 100 ml of 6.4 ppm Cl; (a) 1.98 mA cm − 2; (b) 9.91 mA cm − 2; (c) 19.8 mA cm − 2; (d) 29.7 mA cm − 2; (e) 39.6 mA cm − 2.
Accumet Selective Ion Analyzer Model 750 for the other experiments. A Cole Parmer chart recorder was used and the data was recorded with a speed of 0.3 cm/min. The test solutions were propelled by a peristaltic pump of Alitea USA/ FIA laboratory for all experiments. Gateway 2000 computer with RS 332-098 4 phase unipolar stepper motor drive board introduced into the hard disk was used for the injection of samples into carrier streams to provide highly precise and reproducible volume. The injector used was a plastic syringe with the metallic needle replaced by a plastic one to prevent corrosion. A two line manifold system was used as shown in Fig. 1. Silver nitrate solution was passed from one line and the supporting electrolyte from the other. The reactor was made of a coiled tube formed from a plastic tube of 25 cm length and 1.14 mm inner diameter. This tube was mounted on a glass cylinder of diameter 0.7 cm. The detector was made of Teflon in the form of a cylinder 3 cm in length where the two electrodes were placed and this detector was connected to the reactor.
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Table 1 Results for the determination of chloride in real samples by precipitation and FIA-DEP methods Sample No.
Cl− (found by pptn method) in ppm
Cl− (found by FIA-DEP) in ppm
% Recovery
1 2
3.50 32.0
3.58 32.1
102.3 100.3
rsd, 0.2%
2.3. Electrodes Silver wire electrodes were used in the first part of the chloride and iodide analysis. They were cleaned with 6 M nitric acid as a pretreatment before each experiment. The electrodes were then rinsed with distilled deionized water and placed in the cell. The platinum electrodes were SargentWelch type 30-415 and 30-415 respectively. They were cleaned with concentrated sulfuric and/or nitric acid before each experiment as a pretreatment, rinsed with distilled deionized water and placed in the cell. Platinum electrode were cleaned by dipping in boiling aqua regia and then rinsed with distilled deionized water. It was then immediately immersed into pure mercury for 30 s to prepare a Pt/Hg electrode. Silver/silver chloride electrodes were prepared by surface oxidation of silver electrodes in a solution of 0.5 M Fe(III) chloride solution which was found to be more effective than the traditional anodic oxidation process [16]. The electrode was immersed in the solution, left overnight then rinsed with distilled deionized water and placed in the cell.
3. Results and discussion Fig. 1 shows the manifold used in this study where silver nitrate was delivered as reactant from line I. The supporting electrolyte solution was passed through line II. At the start of this work, the proper current density was determined by injecting a volume of 140 ml of 3.55 ppm chloride solution into a stream of 0.01 M KNO3 and 10 − 3 M AgNO3 as shown in Fig. 1. Different current
densities were employed to polarise two identical silver electrodes. Fig. 2a shows that the heights of the resulting peaks increased with the current densities employed. It was noted that at lower current densities, for instance 1.98 mA cm − 2, the electrodes were found to have a slow response. This is reflected in the small heights of the resulting peaks. However, at current densities greater than 7.93 mA cm − 2, the electrodes response becomes abnormal. Consequently the resulting peaks will show poor reproducibility as seen from peaks at d and f of Fig. 2. This abnormal behaviour is probably due to the fact that higher currents will increase the concentration overpotentials of both the anode and the cathode. Consequently the measured signal which is the difference between the potentials of these two electrodes will be affected. Fig. 2 shows that the three peaks at point (c) are symmetrical, reproducible and of a considerable height. These peaks were obtained at a current density of 5.95 mA cm − 2, hence, this value is considered to be an optimum under the employed conditions. The relation between peak height and current density employed to polarise the silver electrodes is shown in table. 2. The effect of flow rate on the produced signal was also studied and the results are depicted in Fig. 3. It is obvious from this figure that the peak height decreases with an increase in flow rate. The peaks of point b are symmetrical, having a reasonable width and they are considerably high, therefore, the flow rate at this point which is 0.93 ml min − 1 was considered to be appropriate for the experimental measurements. A calibration curve was constructed by injecting standard chloride solutions of concentrations that range from 3.55–35.5 ppm. This calibration curve is shown in Fig. 4 and it has a regression
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coefficient of 0.991. During the course of this work it was observed that the response of the silver electrodes became sluggish. This can be attributed to the fact that the resulting silver
Fig. 8. Calibration curve. Electrode pair, Ag/AgCl – Pt/Hg; current density, 19.8 mA cm − 2; flow rate, 0.93 ml min − 1, volume, 100 ml of Cl − ; (a) 1.8 ppm; (b) 3.0 ppm; (c) 18.2 ppm; (d) 29.8 ppm; (e)42.7 ppm.
Fig. 7. Current density–peak height relationship. Electrode pair, Ag/AgCl–Pt/Hg; flow rate, 0.93 ml min − 1, volume 100 ml of 4.5 ppm Cl − ; (a) 0.99 mA cm − 2; (b) 2.97 mA cm − 2; (c) 11.89 mA cm − 2; (d)14.9 mA cm − 2; (e) 19.8 mA cm − 2. Table 2 The relation between the peak heights and the current densities employed for different combinations of electrodes used for chloride determination Type of electrodes
Current density (mA cm−2)
Height of peak (mV)
Ag – Ag
1.98 3.96 5.95 7.93 9.91
183 225 283 333 400
Ag/AgCl – Pt
1.98 9.91 19.8 29.7 39.6
58 100 225 258 400
Pt/Hg – Ag/AgCl
0.99 2.97 11.89 14.9 19.8
58 75 141 225 500
chloride dissociates and the ions formed will affect the behaviour of the polarised electrodes. A pair of Ag/AgCl electrodes has been used for the determination of chloride [17]. However, the use of Ag/AgCl electrode as a cathode in DEP will lead to the dissolution of AgCl coating and the electrode will eventually become a silver electrode. Consequently, the resulting signal will differ from the previous ones and abnormal behaviour becomes inevitable. Polarised platinum electrodes have also been used for chloride analysis using silver nitrate as a titrant [18]. The response of the platinum electrode was attributed to the adsorption of silver chloride precipitate on the surface of the electrode. It was decided to make a combination of a platinum electrode with a Ag/AgCl electrode where the platinum electrode would act as cathode. This combination was employed for the determination of chloride by injecting the latter into a stream containing silver nitrate. When the sample is injected, a potential difference develops between the anode and the cathode, consequently, a peak is formed. The relation between the height of the peak and the current density was studied and the results are shown in Fig. 5 and Table 2. It
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Fig. 9. Effect of change in the current density employed. Electrode pair, Ag – Ag; flow rate, 0.93 ml min − 1; volume, 140 ml of 12.7 ppm I − ; (a) 1.98 mA cm − 2; (b) 3.96 mA cm − 2; (c) 5.95 mA cm − 2; (d) 9.91 mA cm − 2.
is evident from this figure that the peak height increases with current density and the value of 19.82 mA cm − 2 was chosen for experimental measurements because the resulting peaks are symmetrical and have similar heights. A calibration curve was established by applying the above mentioned conditions. Standard chloride solutions of concentrations that range from 3.0–68 ppm were injected. The resulting calibration curve is depicted in Fig. 6. This curve was found to be linear with r =0.997 and can be applied to determine chloride in real water samples where the sulphate ion if exists is expected to pose no effect on the response of the indicating system. Fig. 6 shows the results of applying DEP in combination with Ag/AgCl – Pt as a detection system for the analysis of chloride in two samples of drinking water. Peaks (f) and (g) shown in Fig. 6 represent the signals for these two samples. Table 1 gives a comparison between the results obtained by this technique and that of standard precipitation titration. It is apparent from this table that DEP can be applied as a detector in FIA and better sensitivity is expected compared
with direct titration methods including the classical DEP technique. In this technique two identical electrodes are employed to obtain a symmetrical peak which gives the end point. The morphology of the peak depends on the completion of both anodic and cathodic reactions that take place on the two electrodes. However, the use of the DEP technique as a detector in flow injection analysis does not require the completion of the reaction. Therefore different combinations of electrodes can be employed to obtain a better signal. In this regard, a combination of Pt/Hg–Ag/AgCl electrodes was used for the determination of chloride. The proper conditions were found using the univariant method. Fig. 7 shows the effects of changing current density on the height of the signal and the optimum current density that gives the highest reproducible signal was found to be 19.82 mA cm − 2. The relation between the peak height and the current density employed is shown in Table 2. On applying these conditions, a calibration curve that covers a chloride concentration range between 1 and 45 ppm was constructed and is depicted in Fig. 8. It is apparent that this curve is
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linear with r =0.996. However when the combination of Pt/Hg – Ag/AgCl electrodes were used for the determination of chloride in real water samples, higher signals resulted. A similar observation Table 3 The relation between the peak heights and the current densities employed for combinations of electrodes used for iodide determination Type of electrodes
Current density (mA cm−2)
Height of peak (mV)
Ag – Ag
1.98 3.96 5.95 9.91
158 266 316 416
Ag – Pt
4.95 9.91 29.73 39.4
17 41 166 383
0.99 2.97 11.89 14.9 19.8
58 75 141 225 500
Pt/Hg – Ag/AgCl
Fig. 11. Effect of the change in the current density employed. Electrode pair, Ag – Pt; flow rate, 0.93 ml min − 1; volume 100 ml of 1.27 ppm I −; (a) 4.95 mA cm − 2; (b) 9.91 mA cm − 2; (c) 29.73 mA cm − 2; (d) 39.64 mA cm − 2.
Fig. 12. Calibration curve. Electrode pair, Ag – Pt, current density, 35 mA cm − 2, flow rate: 0.93 ml min − 1; volume of I− , 140 ml; (a) 1.27 ppm; (b)3.81 ppm; (c) 6.35 ppm; (d) 7.62 ppm; (e) 8.89 ppm; (f)11.4 ppm; (g) 19.0 ppm; (h) 31.8 ppm.
Fig. 10. Calibration curve. Electrode pair, Ag–Ag; current density, 5.95 mA cm − 2; flow rate, 0.93 ml min − 1; volume, 140 ml; (a) 12.7 ppm; (b) 38.1 ppm; (c) 63.5 ppm; (d) 88.9 ppm; (e) 114 ppm.
was noted by others [19] where sulfate was found to interfere. Therefore the combination of Pt/Hg– Ag/AgCl is recommended for the determination of chloride in the absence of interfering ions such as sulphate.
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The applicability of the DEP technique as a detector in FIA for precipitation reactions was extended to include iodide. Since the solubility of silver iodide is very low, a combination of two identical silver electrodes were expected to give satisfactory results. The appropriate current density gives symmetrical and reproducible peaks of considerable height which was found to be 5.95 mA cm − 2 as shown in Fig. 9. The relation between peak height and current densities employed is shown in Table 3. A calibration curve was constructed for standard iodide solutions of concentrations from 12.7 to 127 ppm. Fig. 10 depicts this curve which has an excellent correlation coefficient of 0.998. In order to examine the applicability of other combinations of electrodes, one silver electrode was replaced by a platinum electrode in iodide determination. The height of the signal was found to increase with an increase in current density employed as shown in Fig. 11. Its optimum value was chosen as 34.7 mA cm − 2 and Table 3 shows the relation between the peak height and the current densities employed. A calibration curve was established for a concentration range of 1.27–31.8 ppm. This curve is shown in Fig. 12 and it has a correlation coefficient of 0.997 which indicates the applicability of the DEP technique as a detector in flow injection analysis.
4. Conclusion The technique of DEP which depends on polarising two identical electrodes using a small current and measuring the potential difference between them can be used as a detector in FIA. A combi-
.
nation of Ag/AgCl–Pt electrodes can be applied for the determination of chloride. For iodide determination Ag–Pt combination was found to give acceptable results. The appropriate conditions of flow rate and current density were investigated.
Acknowledgements M. Koken wishes to thank KFUPM for donating the scholarship to read for MS degree.
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