A Simple, Effective Demonstration of Titration Curves and Indicator Selection The concepts of acid-base titration curves and selection of the appropriate indicator for an acid-base titration can he easily and effectively demonstrated by use of a potentiometric titration. After having calculated the p H versus volume of titrant curve for a titration of 25 ml of 0.10 M HC1 (diluted to 50 ml) with 0.10 M NaOH a t representative points1 (for example: after addition of 0, 5, 10, 20, 24, 25, 26, 30, and 40 ml of 0.10 M NaOH), and after,discussing the factors influencing the choice of an indicator for such a titration, the titration can he performed as a demonstration using hoth a p H meter and the chosen indicator (such as phenolphthalein). Students then observe first hand that the p H changes very slowly during the initial stages of the titration, very rapidly at the equivalence point, and again slowly after the equivalence point, just as the calculation predicts. In addition, they simultaneously observe the correlation of the raoid change t dis~laved - in .DH at the eauivalence ~ o i n as . . hv the DH meter with the color change of the indicator. An even more effective demonstration is based on the titration of 25 ml of 0.10 M HC2Hs02 (diluted to 50 ml) with 0.10 M NaOH. Again the p H versus volume of titrant curve is calculated at various representative points,' followed again by some discussion of indicator selection. The potentiometric titration is then performed twice, once with the appropriate indicator (such as phenolphthalein, p H range 8.3-10) and once with an unsuitable indicator (such as methyl red, p H range 4.2-6.3). Students may then see verification of the shape of the calculated titration curve and again correlate the change in p H as displayed hy the p H meter with the change in color of the indicator. When phenolphthalein is used, the color change is very sharp and correlates well with the rapid change in p H at the equivalence point. However, when methyl red is used, the color change is drawn out over 15-20 ml of added titrant and the change is premature. Experimental aspects include the following: Prior to use, the p H meter is standardized with a buffer solution of known pH. Pipet 25.0 ml of the 0.10 M acid into a 150-ml beaker and dilute with 25 ml water to orovide sufficient volume. Use a maanetic stirrinn bar to keep the solution stirred. Emerse the electrohes, add the indicator, and titrate with the 0.10 M NaOH. he demonstration has been successfully performed in general chemistry and quantitative analysis courses, hoth in laboratory sections of about 30 students and also in lecture sections with as many as 100 students. Student response has been most gratifying. -
1 For example, see Mortimer, C. E., "Chemistry-A Company, New York, 1971, pp. 604-610.
Conceptual Approach," VanNostrand Reinhold Lawrence C. Nathan
University of S a n t a Clara Santa Clara, California 95053
262
/ Journal of ~ h & i c a l Education
