In the Laboratory
Potentiometric Acid–Base Titrations with Activated Graphite Electrodes P. Riyazuddin* and D. Devika Department of Analytical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India It is now well established that the dry cell graphite (DCG) rod is a completely acceptable substitute for platinum and glassy carbon electrodes in a variety of electroanalytical techniques (1–8). This communication describes the use of a DCG electrode pretreated (activated) with permanganate as a hydrogen ion sensor. For some titrations, a combination of DCG electrodes was used, where one was activated and the other nonactivated. Experimental Method
Electrodes and Equipment The DCG electrode sticks and multipurpose electrode assembly were fabricated as described earlier in this Journal (8). A single electrode stick or one of the electrodes of the assembly was activated by keeping it in acidified 1 M potassium permanganate solution (0.5 M with respect to sulfuric acid) for 10 min; it was then rinsed with water. Potential measurements were made with a digital pH/mV meter (precision ± 0.01 pH unit) at ambient temperature (33–35 °C). The solution was stirred continuously with a minimagnetic-stirrer. Procedure Forty milliliters of HCl (0.0147 M) was titrated with NaOH (0.084M) taken in a 10-mL semimicro buret (0.05mL divisions). In the vicinity of the endpoint, the titrant was added dropwise and the potential was read 1–2 min after each addition, against a calomel electrode. The endpoint was located graphically.
Figure 1. Titration of 40.0 mL HCl (0.0147 M) with NaOH (0.084 M) using DCG electrode shortly after activation (curve A); titration repeated after 1 day (B), 5 days (C), and 8 days (D). Curve E— titration with nonactivated DCG electrode.
Results and Discussion Figure 1 shows a case when the DCG had been activated only shortly before the first titration. The titrations were repeated after 1, 5, and 8 days, the electrode being stored dry between titrations. As can be seen, the pH sensitivity of the electrode gradually decreased. After about 12 days its sensitivity approached that of a nonactivated DCG electrode. Since the response of activated DCG electrodes varies with time, these electrodes may not be useful as sensors in direct pH-metry. On the other hand, for endpoint detection in acid–base titrations, any electrode having a hydrogen ion function is appropriate, and a theoretical electrode response to a change in pH (60 mV/pH unit at 30 °C) is not necessary. Though the magnitude of potential jump (pH sensitivity of the electrode) slowly decreased with passage of time, an activated DCG may still be used advantageously as an indicator electrode for the titration of strong, weak, monobasic, and polybasic acids (9). In addition, the electrode response to a change in hydrogen ion concentration during titration was fast (the potential became stable within 1–3 s); only near the endpoint was a wait of 1–2 min required. The response of activated graphite towards hydrogen ion is attributed to the formation of quinone-hydroquinone system on the surface of the electrode (10). *Corresponding author.
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Figure 2. Titration of 40.0 mL HCl (0.0147 M) with NaOH (0.084 M) using combination DCG electrodes (curve A) and with glass– calomel electrodes (curve B).
Journal of Chemical Education • Vol. 74 No. 10 October 1997
In the Laboratory DCG Combination Electrodes
Conclusion
The endpoint in acid–base titrations may also be detected by using a combination of an activated DCG (as indicator electrode ) and a nonactivated DCG (as reference electrode). During acid–base titration, the potential of combination electrodes was measured, and the endpoint appeared as a peak on the graph of potential against volume of NaOH added. The general shape of several graphs obtained resembled a second derivative titration curve, as shown in Figure 2. For example, the small dip in potential always appeared just before the peak, and the peak corresponded with the endpoint. Several titrations were also performed with different sets of combination electrodes and also with different brands of dry cells. In almost all cases, the endpoint (the peak) was slightly shifted from the true endpoint (7.0 mL, the value obtained with glass vs. calomel electrode) by one drop of titrant (0.05 mL), leading to a systematic error of 0.7%. However, a more accurate estimate of the endpoint was obtained when we read/measured the volume of titrant corresponding to the midpoint of the line joining the dip and the peak. A possible explanation for the behavior of combination electrodes may be kinetic differences in the establishment of equilibrium potential, for the surfaces of the electrodes are not identical (11). Connecting the combination electrodes in opposite order to the pH/mV meter also resulted in a curve similar to that in Figure 2, but with diminished peak height.
Activated DCG electrodes seem to be suitable as potentiometric indicator electrodes. The electrodes in the form of simple probes behave similarly to metal/metal oxide electrodes in acid–base titrations. These electrodes are easily prepared (a valuable laboratory experience for students), inexpensive, strong (unlike fragile glass electrodes), light, and nontoxic, and hence most suitable for teaching laboratories. Students can ordinarily activate the electrodes and use them in the same laboratory session. Furthermore, DCG combination electrodes help in precisely locating the endpoint without plotting time-consuming second derivative graphs. Literature Cited 1. Natarajan, N.; Ramasubramanian, A. J. Chem. Educ. 1976, 53, 663. 2. Sane, K.; Sane, K. V.; Srivastava, P. K.; Bhattharjee, S. Indian J. Chem. Educ. 1981, 8, 1. 3. Palanivel, A.; Riyazuddin, P. J. Chem. Educ. 1984, 61, 920. 4. Vema Reddy, B.; Janardhan, A. S. Chem. Educ. (India) 1985, 2, 48. 5. Riyazuddin, P.; Hussainy, S. M. Y. Chem. Educ. (UK) 1986, 23, 146. 6. Lakshmi, G. J. Electrochem. Soc. India 1988, 39, 377. 7. Upadhyay, P. K. Proc. Indian Acad. Sci. (Chem Sci). 1989, 101, 251. 8. Riyazuddin, P. J. Chem. Educ. 1991, 68, 342. 9. Devika, D. M.Ph. Dissertation, University of Madras, 1994. 10. Selig, W. J. Chem. Educ. 1984, 61, 80. 11. Kekedy, L; Makkay, F. Talanta 1959, 16, 1212.
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