A Simplified Undergraduate Laboratory Experiment to Evaluate the Effect of the Ionic Strength on the Equilibrium Concentration Quotient of the Bromcresol Green Dye

HERNÁN B. RODRÍGUEZ; MARTÍN MIRENDA

JOURNAL OF CHEMICAL EDUCATION

AMER CHEMICAL SOC

Lugar: Washington; Año: 2012 vol. 89 p. 1201 - 1204

0021-9584

A modified laboratory experiment for undergraduate students is presented to evaluate the effects of the ionic strength, I, on the equilibrium concentration quotient, Kc, of the acid−base indicator bromcresol green (BCG). The two-step deprotonation of the acidic form of the dye (sultone form), as it is dissolved in water, yields suitable hydronium concentration in the solution to carry out the experiment successfully and avoids the use of any buffer solution to fix the hydronium concentration of the media. The 95% confidence interval for the slope parameter (−2.08 ¡À 0.13), obtained from the linear fit of pKc versus ¡ÌI/ (1 + 2.30¡¤¡ÌI) plot, contains the theoretical value of −2.04 predicted by the Debye−Hückel extended law. From the intercept, a value for the equilibrium dissociation constant of BCG, pK = (4.91 ¡À 0.02), is obtained, showing a reasonable agreement with literature data. The simplification makes it possible for students to complete the experiment in the course of an ordinary 3-h laboratory session.fied laboratory experiment for undergraduate students is presented to evaluate the effects of the ionic strength, I, on the equilibrium concentration quotient, Kc, of the acid−base indicator bromcresol green (BCG). The two-step deprotonation of the acidic form of the dye (sultone form), as it is dissolved in water, yields suitable hydronium concentration in the solution to carry out the experiment successfully and avoids the use of any buffer solution to fix the hydronium concentration of the media. The 95% confidence interval for the slope parameter (−2.08 ¡À 0.13), obtained from the linear fit of pKc versus ¡ÌI/ (1 + 2.30¡¤¡ÌI) plot, contains the theoretical value of −2.04 predicted by the Debye−Hückel extended law. From the intercept, a value for the equilibrium dissociation constant of BCG, pK = (4.91 ¡À 0.02), is obtained, showing a reasonable agreement with literature data. The simplification makes it possible for students to complete the experiment in the course of an ordinary 3-h laboratory session.