Author Replies to Ault

First, we would like to thank Addison Ault for the useful comments about our recent paper in this Journal (1).

The main goal of our paper is the resolution of the paradox that a simple dilution can change the color of an indicator (bromocresol green) solution. Students, trying to solve this paradox, will realize that the color change is due to the shift in the equilibrium HIn_yellow = H⁺ + In⁻_blue; that is, the degree of ionization increases upon dilution. Therefore, this unusual color change behavior can be easily understood by a very common equilibrium concept.

Certainly, this phenomenon will only be clearly observed in water for indicators whose pK_In is near 5. The approach developed by Ault (2), which is useful to estimate pH from the pK values, can be also used to estimate the pH value of the bromocresol green solution upon dilution. Ault's comment that no color changes will be observed in a buffered indicator solution is correct, and we can also expect no color changes for other cases involving a diluted aqueous acid or basic solution as a solvent.

Literature Cited

1. Silva, C. R.; Pereira, R. B.; Sabadini, E. J. Chem. Educ. 2001, 78, 939–940.
2. Ault, Addison. J. Chem. Educ. 1999, 76, 936–938.