Jesse Hefterl and Richard w. Zuehlke University of Bridgeport Bridgeport. Connecticut 06602
The s~ectroohotometricdetermination of the pK of an acid-has; indicator has long held (1)a time-honored place in the general and analytical chemistry lahoratory. As virtually all laboratory procedures point out, however, the time allotment for such an experiment is usually three hours, based as it frequently is on the use of manual, single-beam spectrophotometers. Having been convinced of the intellectual value of this exoeriment. but also auestionine" the value of so much mechanical repetition (and its attendant cost in lahoratory supervision), we decided to produce a computer-simulated experiment in its stead. The eeneral orocedure in this exoeriment involved deter" mining the absorption spectra (usually in the visible region) for the acidic and basic forms of the indicator at appropriately
Computer Simulation of Acid-Base Behavior extreme pH's, followed by selection (by inspection) of an intermediate DH in the transition interval and subsequent determination of the associated spectrum. By use of the computed molar ahsorptivities of the two forms a t two different wavelengths (usually the points of maximum absorption for the two forms), the student can produce two simultaneous equations in concentration of the acid and hase forms, which, when used with the known value of intermediate pH, permits evaluation of the pK value. The heart of the computer program involves reproduction of the acid and hase form spectra "
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' Project begun by J. H. as a student in Analytical Chemistry. 'Spectra were obtained by J. H. in the laboratory using both manual and recording instruments.
Figure 1. Computer-simulated memyl red spectrum at pH 1.50.
Figure 2. Computer-simulated methyl red spectrum at pH 12.50.
Volume 54, Number 1, January 1977 / 63
nomial expression (2) and a Fortran program originally designed by Dr. M. W. Dixon (3)and altered by us to fit the current problem. The absorption spectra are fitted well using 4-6 terms typically, and these coefficients are then read as DATA into the Basic simulation program. The simulation Dronram is o~eratedin the interactive mode. and the m i m a n but'put is a n absorption spectrum for a specified indicator concentration and pH (see Figs. 1-3). Although the spectra can easily be used in this form, we have given students the option of interrogating the computer for an exact printout of the absorbance at any specified wavelength (see insert under each spectrum). The unique feature of this program is its ability to store and reproduce a variety of visible absorption spectra. It can therefore be adapted to simulate other spedrophotometric experiments, including simultaneous quantitative determination of twoor more absorbing species, demonstration of the isosbestic point, the influence of various parameters (e.g., temoerature. ionic strenpth. concentration) on chemical equilibria studied hy abs~~rptinmetry, and the free-electron thwr\, rn(dc.linr of the sDectra of coniueated dves. While work on some of these and ocher systems~sstillbeing done in this Laboratory, the present program is in useable form, and a listing of same may be obtained from R. W. Zuehlke. Acknowledgment
The interest of the Exxon Education Foundation in and their partial support of this work is gratefully acknowledged. Literature Cited im"lated
red spectNmat pH 4,
cator pK is 4.99. a t any selected concentration, as well as their concentrationweighted sum a t any chosen pH. Since the 'pectra2 are broad and oscillatory, we chose to fit them with a Chebyshov poly-
64 / Journal of Chemical Education
'1,
Melimn,rliRnn &and Kiser, Rohen W.,"Pmhlomsand
ExpeKmentainlnsfrumenfal Anrlyris." Charles E. Merrill Bmkr. Inc. Cslurnhus, Ohio. 1963. Exprrirnent 10. H r ~ mWilliam . R.and Camnbel1.J. A..J.CHEM.EOUC.45.674 (196sl,sndasro~ cialed releronres. Fliedmsn, Lester end Barrow, Gordon. "sophomore Chemistry I.nl,
