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 AJAC  Vol.5 No.17 , December 2014
Spectrophotometric Determination of the pKa, Isosbestic Point and Equation of Absorbance vs. pH for a Universal pH Indicator
Abstract: The pKa and the isosbestic point of the universal pH indicator Carlo Ebra 1-11 (catalog number 45712) were determined using UV-Vis spectrophotometry. Aqueous buffer solutions with pHs ranging from 3.83 to 10.85 were mixed. Four methods—two graphical and two mathematical— were used to estimate the acid dissociation constant (pKa) and isosbestic point using absorbance measurements. The equation for the dependence of the absorbance on pH at λ = 600 nm was obtained using calibration curves. The resulting average pKa of the four methods was 8.277 with a standard deviation of 0.1728. The results obtained using the mathematical methods were very similar, with a deviation of 0.0014; the average pKa determined using these methods was 8.263 ± 0.001. The literature contains no previous reports of the pKa of this indicator. The isosbestic point occurs at a wavelength of 494 nm, with an absorbance of 0.46.
Cite this paper: Salgado, L. and Vargas-Hernández, C. (2014) Spectrophotometric Determination of the pKa, Isosbestic Point and Equation of Absorbance vs. pH for a Universal pH Indicator. American Journal of Analytical Chemistry, 5, 1290-1301. doi: 10.4236/ajac.2014.517135.
References

[1]   Zscherp, C., Schlesinger, R., Tittor, J., Oesterhelt, D. and Heberle, J. (1999) In Situ Determination of Transient pka Changes of Internal Aminoacids of Bacteriorhodopsin by Using Time-Resolved Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy. Biophysics: Proceedings of the National Academy of Sciences, 96, 5498-5503.

[2]   Baran, Y., Baran, S. and Kemal, N. (1997) Spectofotometric Determination of the pKa Values of Some Aminoacid Complexes of Pentacyanoferrate (II) and Pentacyanoruthenate (II). Journal of Chemistry, 21, 105-110.

[3]   Meloun, M., Bordovská, S. and Vrana, A. (2007) The Thermodynamic Dissociation Constants of the Anticancer Drugs Camptothecine, 7-Ethyl-10-hydroxycamptothecine, 10-Hydroxycamptothecine and 7-Ethylcamptothecine by the Least-Squares Nonlinear Regression of Multiwavelength Spectrophotometric pH-Titration Data. Analytica Chimica Acta, 584, 419-432.
http://dx.doi.org/10.1016/j.aca.2006.11.049

[4]   Meloun, M., Bordovská, S. and Galla, L. (2007) The Thermodynamic Dissociation Constants of Four Non-Steroidalanti-Inflammatory Drugs by the Least-Squares Nonlinear Regression of Multiwavelength Spectrophotometric pH-Titration Data. Journal of Pharmaceutical and Biomedical Analysis, 45, 552-564.
http://dx.doi.org/10.1016/j.jpba.2007.07.029

[5]   Mitchell, R.C., Salter, C.J. and Tam, K.Y. (1999) Multiwavelength Spectrophotometric Determination of Acid Dissociation Constants Part III: Resolution of Multi-Protic Ionization Systems. Journal of Pharmaceutical and Biomedical Analysis, 20, 289-295.
http://dx.doi.org/10.1016/S0731-7085(99)00041-2

[6]   Tam, K.Y., Hadley, M. and Patterson. W. (1999) Multi-wavelength Spectrophotometric Determination of Acid Dissociation Constants Part IV: Water-Insoluble Pyridine Derivatives. Talanta, 49, 539-546.
http://dx.doi.org/10.1016/S0039-9140(99)00010-7

[7]   Amador-Hernández, J., Rojas-Hernández, A. Madaí, E., De La Garza-Rodríguez, M., Velázquez-Manzanares, M. and Medina-Vallejo, L.F. (2014) New Chemometric Strategies in the Spectrophotometric Determination of pKa. European Journal of Chemistry, 5, 1-5.
http://dx.doi.org/10.5155/eurjchem.5.1.1-5.901

[8]   Ivanov, V.M., Adamova, E.M. and Figurovskaya, V.N. (2010) Acid-Base, Spectrophotometric, and Colorimetric Properties of 1, 2-Dihydroxyantraquinonone3-sulfoacid (Alizarin Red S). Journal of Analytical Chemistry, 65, 473-481.
http://dx.doi.org/10.1134/S1061934810050072

[9]   Kong, X., Zhou, T., Liu, Z. and Hider, R.C. (2007) pH Indicator Titration: A Novel Fast pKa Determination Method. Journal of Pharmaceutical Sciences, 96, 2777-2783.
http://dx.doi.org/10.1002/jps.20959

[10]   Meloun, M., Bordovská, S., Syrovy, T. and Vrána, A. (2006) Tutorial on a Chemical Model Building by Least-Squares Non-Linear Regression of multiwavelength Spectrophotometric pH-Titration Data. Analytica Chimica Acta, 580, 107- 121.
http://dx.doi.org/10.1016/j.aca.2006.07.043

[11]   Meloun, M., Bordovská, S. and Syrovy, T. (2007) A Novel Computational Strategy for the pKa Estimation of Drugs by Non-Linear Regression of Multiwavelength Spectrophotometric pH-Titration Data Exhibiting Small Spectral Changes. Journal of Physical Organic Chemistry, 20, 690-701.
http://dx.doi.org/10.1002/poc.1235

[12]   Meloun, M., Ferencíková, Z. and Vrána, A. (2011) Determination of the Thermodynamic Dissociation Constant of Capecitabine Using Spectrophotometric and Potentiometric Titration Data. Journal of Chemical Thermodynamics, 43, 930-937.
http://dx.doi.org/10.1016/j.jct.2011.01.012

[13]   Panigrahi, S., Suna, P. and Misra, P.K. (2012) Effect of Organized Assemblies, Part VIII: Spectrophotometetric Study on the Effect of Micellar Media on the pK of Some Substituted N-Benzylideneanilines. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 415, 349-357.
http://dx.doi.org/10.1016/j.colsurfa.2012.08.051

[14]   Zapata, L., Kalembkiewicz, J. and Sitarz-Palczak, E. (2009) Studies on Equilibrium of Anthranilic Acid in Aqueous Solutions and in Two-Phase Systems: Aromatic Solvent-Water. Biophysical Chemistry, 140, 91-98.
http://dx.doi.org/10.1016/j.bpc.2008.11.012

[15]   Smith, S.A. and Pretorius, W.A. (1994) Spectrophotometric Determination of pKa Values for Fluorescein Using Activity Coefficient Corrections. Water SA, 28, 395-402.

[16]   Skoog, D. and Leary, J. (1994) Análisis Instrumental. McGraw-Hill, Madrid.

[17]   Harris, D.C. (1999) Quantitative Chemical Analysis. W.H. Freeman, New York.

[18]   Vogel, A. (1969) Química Analítica Cuantitativa: Teoría y Práctica. Editorial Kapelusz, Buenos Aires.

 
 
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