A potentiometric micro titration method was applied for the determination of potassium tetraphe nylborate in acetone/water mixtures using a silver nitrate solution ...
ISSN 10619348, Journal of Analytical Chemistry, 2013, Vol. 68, No. 1, pp. 57–60. © Pleiades Publishing, Ltd., 2013.
ARTICLES
Microtitrimetry by Controlled Current Potentiometric Titration1 Abdallah M. Abulkibasha, Munir AlAbsib, and Abdul aziz Nabil Amroa a
b
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, 31261 Saudi Arabia Electrical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261 Saudi Arabia Received December 15, 2011; in final form June 5, 2012
Abstract—The classical methods of titration require large volumes of chemicals. Microtritimetry is the method of choice since it utilizes small quantities of chemicals and yields less waste compared to other con ventional methods. Thus it benefits both the analytical chemists and the environment. In this communica tion, microtitrimetry is performed by employing the technique of differential electrolytic potentiometry for the location of the end point. Oxidationreduction titration using platinum electrodes is described. For the first time the endpoint for a sample of 1.0 μL of 0.10 M Fe(II) has been located by titration using a solution of Ce(IV). The optimum conditions such as volume of cerium ammonium sulfate added, current applied to polarize the electrodes in case of dc controlled current potentiometric titration (CCPT). The effect of chang ing the percentage bias of the square wave used to polarize the electrodes on the differential peak in case of ac CCPT has been investigated. The precision of this microtitrimetric method is also reported. Keywords: microtitrimetry, controlled current potentiometric titration, redox, Ce(IV), Fe(II) DOI: 10.1134/S1061934813010024 1
the color change of the indicator near the endpoint. Not only change in color is sometimes very delicate and slow, but different people have also different sen sitivity to colors. Therefore, a more sensitive method is required for the micro titrimetric applications. Differential electrolytic potentiotitrimetry, pro posed by P. Dutoit and G. Weise [7], is the process in which the potential difference between a pair of iden tical electrodes immersed in an electrolytic cell and polarized by a dc or an ac current. In order to avoid the possibility of deleterious effects which might be caused by repeated passages of current through the reference electrode, a third auxiliary electrode is used to conduct the current. The corresponding technique of applying a small current through a concentration cell without liquid junction during the course of titration is termed differential electrolytic potentiotitrimetry (DEP) and was introduced in 1922 [8]. The recommended IUPAC nomenclature corresponding to the terms electrolytic potentiotitrimetry and differential electro lytic potentiotitrimetry are (CCPT) and controlled current potentiometric titration with two indicator electrodes, respectively. CCPT has the term electro lytic which was suggested by Bishop [9], implying that electrolysis is proceeding under the influence of a minute heavily stabilized current. This electrolysis current is less than the diffusion current of the electro active species, and the solutions are normally stirred. Basically, CCPT as a detection technique utilizes two identical metallic electrodes that are polarized by a heavily stabilized current, and the potential differ ence between these electrodes is measured. At the
Titration is a common technique excessively used in various laboratories. As a result, this technique con sumes large quantities of chemicals and in return it yields waste that has negative impact on our environ ment. Microtitrimetry has been applied to reduce the consumption of reagents and minimize their hazard ous waste.
A potentiometric micro titration method was applied for the determination of potassium tetraphe nylborate in acetone/water mixtures using a silver nitrate solution [1]. A micro determination was described in which ascorbic acid was titrated with 2,6 dichlorophenolindolphenol [2]. Chloride levels in Marine worm’s coelomic fluids were also determined by this method [3]. A micrometer syringe assembly with a capillary tip was developed for microtitrimetry [4]. An assay procedure for the determination of ascorbic acid in pharmaceuticals and fruits using an indicator to locate the endpoint was described [5]. A simple titrimetric method for the micro determination of salbutamol sulfate with Nbromosuccinimide was reported [6]. Most of the attempts made to apply microtitrimetry were based either on potentiometry or the use of indicators for the location of the endpoints. It is well known that using dilute solutions will result in titration curve that is less steep, and as a result the location of the endpoint becomes difficult. The same difficulty also arises when indicators are used in dilute solutions. Color changes of indicators are usually not instant which will in turn cause a serious misjudging to 1 The article is published in the original.
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Data acquisition
PC
Injector holder
Power supply Electrode Magnetic stirrer ac, dc polarization source Fig. 1. Microtitrimetry by CCPT using square wave and the LabVIEW software.
endpoint, this potential difference produces a sharp symmetrical peak. CCPT technique does not require a reference electrode and thus the difficulties of the salt bridge are eliminated. Moreover, polarization enhances the response of the electrodes. This tech nique has been applied to various types of titrimetric reactions using different types of electrodes, such as antimony electrodes for acid—base reactions [10– 13], silversilver halide electrodes for precipitation reactions [14–16], platinum electrodes for oxidation reduction reactions [17–19] and gold amalgam elec trodes for complexation reactions [20–22]. CCPT has been applied also to various types of titrimetric reac tions in nonaqueous media using different types of electrodes [23–27]. EXPERIMENTAL A small cell that can accommodate the two elec trodes and the tip of a micropipette or a microinjector was designed as shown in Fig. 1. Wires of platinum were cleaned properly using aqua regia, then rinsed with deionized water and used as an indicating system. Solutions of 0.10 M Ce(IV), Fe(II), and 5% sulphuric acid were prepared using distilled deionized water. Micropipettes that deliver volumes at 1, 10 and 100 µL levels were used to transfer the sample into the cell. A microliter injector was designed to deliver precise vol umes of the titrant at microliter levels. The microliter Injector: the L12100 linear actua tor is used as the core element for the injector. It has Precession and accuracy of the method (n = 5) Method Fe(II) amount (μmol) Recovery, % RSD, % ACDEP DCDEP
1.00 1.00
101 104
4.14 7.90
been designed to push or pull loads along its full stroke length. The L12 linear actuator has an embedded internal position controller that will send position commands which the actuator will follow. The control signal is a voltage source that spans from 0 to 5 V in steps. The resolution is very high: 0.118 V is the equivalent to deliver or suck a volume of 1.0 µL. The injector is controlled by a LabVIEW pro gram that allows delivering a precise volume of 1.0 µL. The program developed is easy to use and very interac tive in a way that the user has a complete control on the injector. A polarization source that provides both the dc and the markspace biased square wave was also designed and applied to polarize the electrodes. The ac or dc polarizing currents used to polarize the electrodes can be monitored directly using the Labview software. The markspace biased square wave applied for the polar ization and the output voltage are shown in Fig. 1. RESULTS AND DISCUSSION For DCCCPT the effect of changing the dc cur rent on the resulting differential curve was studied. A volume of 10 µL of 0.1 M Fe(II) was titrated with 0.1 M Ce(IV) solution at different current densities. The resulting differential curves are shown in Fig. 2. It is obvious that the differential curves obtained at higher current densities such as 69.0 and 34.5 µA/cm2 are broad and not suitable for the location of the end point. However, the end point can be easily located from the curve obtained at a current density of 17.3 µA/cm2. This current density was considered as optimum and used to investigate the precision of this method. The titration of 10 µL of 0.1 M Fe(II) with 0.1 M Ce(IV) was repeated five times and the results obtained are shown in table where a standard deviation of 0.011 was obtained. The effect of the sample volume on the resulting titration curve was also investigated.
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MICROTITRIMETRY BY CONTROLLED CURRENT POTENTIOMETRIC TITRATION Potential difference ΔE (mV)
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Fig. 2. Titration of 10 µL 0.1 M Fe(II) with 0.1 M Ce(IV) at current densities of 17.3, 34.5 and 69.0 μA/cm2.
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Fig. 3. Titration of samples of 2.0 and 5.0 µL of 0.1 M Fe(II) with 0.1 M Ce(IV) at a current density of 17.3 µA/cm2.
Potential difference ΔE (mV) 900 Potential difference ΔE (mV) 800
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Fig. 4. Titration of 10 μL 0.1 M Fe(II) with 0.1 M Ce(IV) at different markspace percentage biases, %: 4, 20, 67, and 85.
Figure 3 shows the titration curves obtained by titrat ing samples of 2 and 5 µL of Fe(II). It is clear that the endpoints can be easily located even for a sample vol ume of 2 µL. The effect of changing the percentage bias of the markspace bias periodic wave was studied while titrating a volume of 10.0 µL of 0.1 M Fe(II) with 0.1 M Ce(IV) at different percentage biases. Figure 4 shows the resulting differential curves based on the potential changes per volume of Ce(IV) added. Several trials were carried out at the percentage bias of 20 in order to investigate the reproducibility and accuracy of this method in locating the endpoint, as shown in table. A standard deviation of 0.004 was obtained which means high precision. Furthermore, the effect of the volume of the sample has also been investigated. The resulting differential curves which JOURNAL OF ANALYTICAL CHEMISTRY
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Fig. 5. Titration of 1.0, 2.0 and 5.0 µL of 0.1 M Fe(II) with 0.1 M Ce(IV) at a markspace percentage bias of 20%.
are depicted in Fig. 5 can be used to locate the end points even for a sample of a volume of 1.0 µL. *** For the first time the endpoint for a sample of 1.0 µL of 0.10 M Fe(II) has been located by titration with Ce(IV) using the technique of CCPT. The opti mum conditions which give differential titration curves that can be used to locate the endpoints have been selected. The optimum dc current range used for the polarization of the electrodes was found to be 1.0– 10 µA. For markspace bias method, higher biases give better curves, and they are more sensitive compared to dc current method, where an endpoint for a sample of 1.0 µL of Fe(II) could be easily located. No. 1
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ACKNOWLEDGMENTS The financial support of the Deanship of Scientific Research (FT 100020) at King Fahd University of Petroleum and Minerals and the support of the Chem istry Department, KFUPM are acknowledged. REFERENCES 1. Karrman, K.J., Einar, B., and Gedda, P., Microchim. Acta, 1959, vol. 47, p.775. 2. Vallant, H., Microchim. Acta, 1969, vol. 2, p. 436. 3. Walsby, J.R., Anal. Chem., 1973, vol. 45, p. 2445. 4. Gruen, L.C. and Human, J.P.E., Microchim. Acta, 1975, vol. 2, p. 43. 5. AminudDin, M., Asma, R.N., Siddiq, M., Ghauri, S., Jabbar, A., and Khan, M., J. Pure Appl. Sci., 1982, vol. 1, p. 11. 6. Geeta Nadadhur and Baggi Tulsidas, R., Microchim. Acta, 1990, vol. 1, p. 95. 7. Dutoit, P. and Weise, G., J. Chem. Phys., 1911, vol. 578, p. 608. 8. Willard, H.H. and Fenwich, F., J. Am. Chem. Soc., 1922, vol. 44, p. 2504. 9. Bishop, E., Analyst, 1958, vol. 83, p. 212. 10. Bishop, E. and Short, G.D., Analyst, 1962, vol. 87, p. 467. 11. Short, G.D. and Bishop, E., Analyst, 1962, vol. 87, p. 724.
12. Short, G.D. and Bishop, E., Analyst, 1962, vol. 87, p. 415. 13. Bishop, E. and Short G.D., Analyst, 1964, vol. 89, p. 587. 14. Bishop, E.R. and Dhaneshwer B., Analyst, 1962, vol. 87, p. 207. 15. Bishop, E.R. and Dhaneshwer, B., Analyst, 1962, vol. 87, p. 845. 16. Bishop, E. and Dhaneshwer, R.B., Anal. Chem., 1964, vol. 36, p. 726. 17. Bishop, E., Microchim. Acta, 1956, vol. 40, p. 619. 18. Bishop, E., Analyst, 1958, vol. 83, p. 212. 19. Bishop, E., Analyst, 1960, vol. 83, p. 422. 20. Malmstadt, H. and Fett, V.E.R., Anal. Chem., 1955, vol. 27, p. 1757. 21. Monk, R. and Steed, G.K.C., Anal. Chim. Acta, 1962, vol. 26, p. 305. 22. Abulkibash, A.M., Sultan, S.M., AlOlyan, A.M., and Alghannam, Sh.M., Talanta, 2003, vol. 61, p. 239. 23. Abdennabi, A.M.S. and Bishop, E., Analyst, 1982, vol. 107, p. 1032. 24. Bishop, E. and Abdennabi, A.M.S., Analyst, 1983, vol. 108, p. 1349. 25. Abdennabi, A.M.S. and Bishop, E., Analyst, 1983, vol. 108, p. 71. 26. Abdennabi, A.M.S. and Rashed, M., AJSE, 1986, vol. 12, p. 82. 27. Abdennabi, A.M.S. and Bishop, E., Analyst, 1983, vol. 108, p. 1227.
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