Phenolphthalein Indicator

Phenolphthalein indicator is a pH indicator solution that is colorless under acidic and extremely alkaline conditions and fuchsia pink under ordinary alkaline conditions. Here is a look at the colors at different pH ranges, the chemistry of the color change, how to make phenolphthalein indicator, and uses of the chemical.

Phenolphthalein pH Range and Colors

The two main colors of phenolphthalein are colorless and fuchsia, which is a pink-purple-red color, depending on the indicator concentration. Phenolphthalein is colorless under acidic, neutral, and slightly basic conditions. It’s pink between pH values of 8.3 and 10.0 and then becomes colorless again at pH values greater than 10. At extremely low pH, like concentrated sulfuric acid, the color turns orange-red. However, the orange-red color is not seen with all strong acids.

There are at least four states of phenolphthalein in aqueous solution. Under extremely acidic conditions, the indicator is in an orange protonated form (HIn⁺). Under strongly acidic, neutral, and slightly alkaline conditions, it presents the colorless lactone form (HIn). The purple-pink color forms under basic conditions, where the doubly-deprotonated or anion form of phenol (In^2-) predominates. As alkalinity increases, the color slowly fades as the indicator converts into the In(OH)^3- form.

How to Make Phenolphthalein Indicator Solution

You can purchase pre-mixed phenolphthalein indicator solution, but it is easy to prepare yourself. Phenolphthalein is somewhat soluble in water (400 mg/L), but it is highly soluble in either ethanol or ether. So, usually you first dissolve the powdered dye in alcohol and then dilute it with water or else dissolve it in 50% alcohol.

Here is the recipe for 1% phenolphthalein indicator solution:

1. Weigh out 1.0 grams of phenolphthalein.
2. Dissolve it in 100 milliliters of 50% ethanol in water. Alternatively, dissolve it in a solution of 50 milliliters absolute ethanol and 50 milliliters water.

Another common formulation is a 0.5% solution:

1. Weigh out 0.5 grams of phenolphthalein.
2. Dissolve it in 50 milliliters of ethanol.
3. Add distilled water to make a final volume of 100 milliliters.

Phenolphthalein Uses

One of the most common uses for phenolphthalein indicator is for finding the end point of an acid-base titration. However, the chemical has other purposes.

• pH indicator for acid-base titration
• pH indicator for concrete carbonation
• Disappearing ink
• Color change toys, such as “Hollywood Hair” Barbie
• Colored bubbles
• Laxative: The use of phenolphthalein as a laxative has been largely discontinued, due potential carcinogenicity.
• Phenolphthalin, the reduced form of phenolphthalein, is used in the Kastle-Meyer test for blood.
• Phenolphthalein is a component of universal indicator.

References

• Dunnick, J. K.; Hailey, J. R. (1996). “Phenolphthalein Exposure Causes Multiple Carcinogenic Effects in Experimental Model Systems.” Cancer Research. 56 (21): 4922–4926.
• Schwarzenbach, Gerold (1957). Complexometric Titrations. Translated by Irving, Harry (1st English ed.). London: Methuen & Co.
• Wittke, Georg (1983). “Reactions of phenolphthalein at various pH values”. Journal of Chemical Education. 60 (3): 239. doi:10.1021/ed060p239
• Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company.