Determination of pK, Using the Half-Volume Method: A Laboratory Experiment With the advent of relatively inexpensive pH meters, the determination of the titration curve of a weak acid or base has hecome feasihle in first year laboratory courses and is included in some laboratory manuals designed for first year students. This exoeriment. in which the eauivalence noint or the eauivalent weieht of an unknown acid is found. mav be extended to the determination of pK, withour further exprrimenrnl uork.Thr vnlue of pK, u n be rend directly from rhp titration rune, using rhe "half-volume" mcthnd' and,sincc A p l I ~ ~ V ~ ~isl ucmnll m r in this region, i r man be determined inirly accuratrly. The method depcndaon the fact that,nt a volumeof added lw+eequolro half that required torearhthrrqu~rnlenctpoint, [A-] = [HAJ and pK, = pH, making the usual assumptions. In our laboratory, this experiment is part of a sequence in which students prepare and standardize 0.1 N NeOH and 0.1 N HCI and then obtain the titration curve for a weak acid. Theacid may be supplied asasolution of a known acid, such as acetic acid, for which the student determines the concentration and pK. from the titration curve. A more interesting experiment is for the student to identify an unknown weak acid, through determination of equivalent weight and pK, values. Students can be given approximate equivalent weights, or asked to design the experiment themselves. Time must he allowed for students to work out themethad; it is suggested to them that quick roughexperiments to determine the amount of acid to weigh out and to find the inflection point of the curve will save time in the long run. Generally, one rough titration curve and two carefully determined curves are obtained during the 4-hr laboratory period; each curve is plotted as it is determined. Students consult a handbook and submit an identification of their compound and are told what the compound actually is. Before the next laboratory period, they prepare a detailed report, with discussion of theory and procedure, assumptions made and sources of error. Despite the approximations made, student results with the half-volume method agree well with literature values of pK.. If pure, well-dried compounds are used as unknowns, accuracy appears to he limited by the graphical method used to determine pK. rather than by approximations made in the theory. For example, in a group of determinations of pK, for acetic acid made by nine students, values ranged from 4.65 to 4.90, with an average value of 4.74. The handbook value is 4.75.2 Seven of the nine values were within 2% of this value. Results were similar with other weak monoprotic acids, with values generally agreeing with literature values within 5%. We have found that including the determination of pK, in the experiment enhances students' interest in and understanding of acid-base equilibria.
-
Waser, Jiirg, "Quantitative Chemistry. A Laboratory Teat," W. A. Benjamin, Inc., New York, 1964, p. 183. Weast, Robert C., (Editor) "CRC Handbook of Chemistry and Physics," 53rd ed., Chemical Rubber Co., Cleveland, Ohio, 1972, p. D-120. New College of the University of South Florida Sarasota, 33580
S. J a n e Stephens Micheal J. Joncich
Volume 54. Number 11, November 1977 1 711
