Expert: Dana Krempels, Ph.D. - 10/1/2009

Question
An experiment was conducted to observe diffusion in selectively permeable membranes and I was asked to use phenolphthalein (pH indicator)in 10 ml of water in a dialysis tube to be tied tightly on both ends and placed in a solution of NaOH (sodium hydroxide solution). I would like to know if the purpose of the phenolphthalein (pH indicator) was to "show" evidence for the passage through the semipermeable membrane? I just want to be pointed in the right direction for understanding the purpose of the phenolphthalein.

Answer
Dear Vern,

Because phenolpthalein turns pink in a basic solution, it will tell you whether the NaOh has crossed the selectively permeable membrane where the indicator molecule has been placed.  If the solution remains colorless, then NaOH hasn't crossed the membrane and reacted with the phenolpthalein, as it remains colorless in neutral or acidic solutions.

As you should know, NaOH is a strong base.  So if your phenolpthalein solution comes in contact with the NaOH, it should turn pink.

Hope that helps.

Dana