A Program to Determinepk's from Spectrophotometric Data A Fortran IV computer program has been written t o calculate the pK.'s of very weak monoprotic acids that have a significantly different absorbance between their acid and anion forms a t a specific wavelength. This has been applied to the determination of the pK,'s of substituted nitrophenols with simple spectrophatometen and calorimeters such as the Spectronic 20. The anionic form of these substituted phenols are yellow, while the protonated forms are transparent a t 540 nm. Typically. a student titrates a weak acid with 0.1 M NaOH and continuously measures the p H , volume of NaOH added, and absorbance of solution. The program corrects the measured absorbance for dilution, generates a table of corrected absorbance versus p H , a table of the first derivative versus p H , and a table af the second derivative versus p H . The student can plot the data and evaluate the p K , from the maximum slope of the corrected absorbance versus pH plot, or the maxima of the first derivative plot, or the zero intercept of the second derivative plot. Alternately, the program will plot the data for the student. A listing of the program and sample output are available from the authors.
Midwestern University Wichita Falls. Texas 76308
448
1 Journal of Chemical Education
Robert J. Palma, Sr. J o h n W. Meux
