Acta Phys. -Chim. Sin. ›› 2012, Vol. 28 ›› Issue (05): 1030-1036.doi: 10.3866/PKU.WHXB201203025


Determination of Acid Dissociation Constant of Methyl Red by Multi-Peaks Gaussian Fitting Method Based on UV-Visible Absorption Spectrum

ZHANG Jian-Hua, LIU Qiong, CHEN Yu-Miao, LIU Zhao-Qing, XU Chang-Wei   

  1. School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P. R. China
  • Received:2011-12-22 Revised:2012-03-01 Published:2012-04-26
  • Contact: XU Chang-Wei
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (20903028), Scientific Research Foundation for Returned Scholars from Ministry of Education of China, and Scientific Research Foundation for Yangcheng Scholar, China (10A041G).

Abstract: UV-visible electronic absorption spectra of methyl red aqueous solutions are characterized by the overlap of a principal peak at λmax ((520±15) nm) with a shoulder peak at λmax ((435±20) nm), which are assigned to acidic species (HMR) and basic species (MR-) of methyl red, respectively. In this study, the spectra and the integrated absorbance of the MR- and HMR peaks (denoted A1 and A2, respectively) were interpreted using a new multi-peaks Gaussian fitting method. From the absorbance ratio A1/A2 and the concentration ratio cMR-/cHMR, the average acid dissociation constant (pKa) was determined as 4.76 at 298.15 K. The goodness is high and the values of R2 (degree of fitting) and χ2 (chi-square test for goodness of fit) were 0.998 and below 10-5, respectively. The effects of aggregation behavior of sodium dodecyl sulfate (SDS) and cetylammonium bromide (CTAB) on pKa were also investigated via this method. The multi-peaks Gaussian fitting method was shown to determine pKa more reliably and simply than traditional spectrophotometric techniques.

Key words: Methyl red, UV-visible absorption spectrum, Overlap peak, Acid dissociation constant, Multi-peaks Gaussian fitting


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