Determination of the pKa Value of Phenolphthalein by Means of Absorbance Measurements

Manuel Alonso, Sebastián P. Chapela, María L. Cristaldo, Inés Nievas, Hilda I. Burgos Oliver Gamondi, Carlos A. Stella
DOI: 10.4236/ce.2010.12020                

Abstract

We here report a laboratory protocol for the determination of the pKa value of an acid by means of determinations obtained with a spectrophotometer. Students determine the acidity constant (Ka) and the pKa associated with phenolphthalein from the absorbance values obtained from phenolphthalein solutions at different pHs. The present protocol for the determination of the pKa takes a very short time and is useful when teaching in conditions with limited equipment.

Keywords

Medical Science Teaching

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	P. Atkins and L. Jones, “Principios de Química, los Caminos del Descubrimiento,” Panamericana, Buenos Aires, 2005.
[2]	[2] L. L. Brunton, J. S Lazo and K. L. Parker, “Las Bases Farmacológicas de la Terapéutica,” McGraw Hill, Espa?a, 2006.
[3]	L. Stryer et al., “Bioquímica,” Reverté, Barcelona, Espa?a, 2003.
[4]	A. C. Guyton, “Tratado de Fisiología Médica,” Elsevier, Espa?a, 2006.
[5]	T. Devlin, “Bioquímica,” Reverté, Barcelona, Espa?a, 2003.
[6]	H. Moncaleano, C. Furió, J. Hernández and M. L. Calatayud, “Comprensión del Equilibrio Químico y Dificultades en su Aprendizaje, Ense?anza de las Ciencias,” Número Extra, 2003, pp. 111-118.
[7]	L. Viennot, “Raisonnement a Plusieurs Variables: Tendances de la Penseé Commune,” Aster, Vol. 14, No. 1, 1992, pp. 127-141.
[8]	B. N. Blanchette, and B. R. Singh, “A Single Protein Research Integrated Advanced Biochemistry Laboratory Course; Spectroscopic Determination of Tyrosyl Side Chain pKa,” Biochemical Education, Vol. 28, No. 2, 2000, pp. 107-109.
[9]	P. L. Alvarado, J. C. Menéndez and M. T. Ramos, “Medida del pKa de Varios Fenoles y su Aplicación a la Determinación de Parámetros Electrónicos,” 2006. http://www.iupac.org/publications/cd/medicinal_chemistry/Practica-III-5.pdf
[10]	G. S. Patterson, “A Simplified Method for Finding the pKa of an Acid-Base Indicator by Spectrophotometry,” Journal of Chemcal Education, Vol. 76, No. 3, 1999, pp. 395.
[11]	K. P. Alter, J. L. Molloy and E. D. Niemeyer, “Spectrophotometric Determination of the Dissociation Constant of an Acid-Base Indicator Using a Mathematical Deconvolution Technique,” Journal of Chemistry Education, Vol. 82, No. 11, 2005, pp. 1682-1685.
[12]	“The Merck Index,” 13th Edition, Merck & Co., Inc., New Jersey, 2001.
[13]	S. W. Tobey, “The Acid Dissociation Constant of Methyl Red,” Journal of Chemistry Education, Vol. 35, No. 13, 1958, pp. 514-515.