M. Natarajan and A. Ramasubramanian Madura College Madurai-625011, South India
II
The Carbon Electrode in Potentiometric Titrations
The use of the carbon electrode in place of the conventional platinum electrode and quinhydrone electrode offers an attractive alternative in certain types of potentiometric work1 and the nresent article is a reo& of our studv of the effectiveness bf carbon electrode in h o x and acid-dase titrimetry. We have found that consistent results of hieh accuracv (error less than 0.5%) are obtained as a matter ofcourse. Whiie it is not our intention to claim that carbon can replace platinum or quinhydrone in every type of potentiometric titration, we belleve that for m a w routine titrations I't is not indiswnsible. Not the least important of the advantages of the carbon electrode is its very low (zero!) cost, in contrast to the high price of Pt. For this reason, and for its other advantages, the carbon electrode deserves to be popularized among teachers and students also. The carbon electrode (center rod taken from a dead dry cell) is kept immersed in concentrated hydrochloric acid for about 15 min to dissolve out the impurities, washed well with water and dried. A set of redox titrations and acid-base titrations (see table) were performed potentiometrically using the pretreated carbon electrode, the other half-cell being standard calomel electrode, and the results were accurate to within 0.5% of the theoretical value. In extending the work, we have found that carbon electrode and a tungsten (or silver)2,3 electrode both immersed in the same test solution would serve excellently well in a series of titrations, thus obviating the necessity for a calomel electrode and a salt bridge (see table). The results are reproducible. A survev of the literature of the cornoarable Pt-W electrode syst~msuggests that in the redm type of titrations the curbon electrnde mirbt be responding co the redox ~otential-ofthe ~ y s t e mT. ~~ ~ - c - w elecirode &ern did not give accurate results for the titrations of potassium iodide with any of the oxidizing agents. When the C-Ag electrode system was used, however, titrations were successful. Probably, carbon electnxle must hr responding tothe redox potential of thesystem and the silver plectrode to the iodide omxntrations, which does not change abruptly at the equivalence point. In addition, the C-Ag bielectrode system proved to be quite successful in precipitation titrations involving the addition of silver nitrate solution to the halide solution. Here, the carbon is acting as a reference electrode and the silver as the halide indicator electrode. In the case of titrations using potassium iodide, the carbon 'Miller, F. J.,Anal. Chem., 35,929, (1963). %Willard,H. H., and Fenwick, F., J. Arner. Chem. Soe, 44,2504, 2516 (1922). Ueno, Koyo, and Tachikawa, Tetshuhei, Kagaku, Hunsseki, I , 757 (1958). "Kthoff, I. M., and Nightingale, E. R., Anal. Chem. Acta, 19,593 (1958).
electrode showed a slight tendency to disintegrate in iodine solution probably due to the binding material of the electrode being attacked. This, however, did not pose any problem during titration or affect the accuracy of the results. The work is being extended to other types of titrations.
Some simple titration curves using ceric ammonium sulfate.
Summary of the
Nature of Tifration I
Tifrations Carried Our II
Redox
Ceric ammonium sulfate: Ferrous ammonium sulfate ~ o t a r r i u mferrocyanide Potassium iodide Potassium perman-
POtalliUm ferrocyanide ~ o t a l l i u mIodide ~otassiumdichromate: Ferrous ammonium sulfate Pota6lium ferrocyanide Sodium hydroxide: Acid-BaSB H y d r o ~ h l o racid i~ Acetic acid Ammonium hydroxide: H y d r o ~ h l ~ racid ic PrecipiSilver nitrate: tation sodium chloride potassium ibdidi
Results
Bielectrode ~lectroder Pair used Used Conditions Ili-A 111-8 IV
Camon with standard calomel electrode (Fig1
C-W CW C-A9
All 0.1 M 101ution6 with2M ruliuric acid.
C-W C-A9 C-W C-W C-W C-W C-W C-A9 C-Ag
111-A:Potentiometric titrationr wing carbon electrode with calomel electrode using saturated potarrium nitrate-rait bridge. 111-B;Bielectrode pair urea in the same test solution without salt bridge.
Volume 53, Number 10, October 1976 / 663
