Color Changes in Indicator Solutions. An Intriguing and Elucidative General Chemistry Experiment

• César R. Silva ^[1] ; Renato B. Pereira ^[1] ; Edvaldo Sabadini ^[1]
  1. [1] UNICAMP, Brazil
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 78, Nº 7, 2001, pág. 939
• Idioma: inglés
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• Resumen

  • The simple dilution of an aqueous solution of the indicator (In) bromocresol green is proposed as an intriguing experiment for students of chemistry. As the solution is diluted with water, its color changes strikingly from reddish to pale blue. These changes are related to changes in the absorption bands in the visible electronic spectra. The pKIn can be determined through the Henderson-Hasselbalch equation by plotting the pH values as a function of log([In-]/[HIn]). The concentration, in mol L-1, of the HIn species can be determined by measuring the absorbance at 440 nm and using the ε value determined from the Beer's law plot. [In-] is calculated from the difference between the initial HIn concentration and the observed [HIn]. Students will be able to conclude that the dissociated fraction of a weak acid increases with dilution.