Phenolphthalein

Uses

pH indicator

Phenolphthalein's common use is as an indicator in acid-base titrations. It also serves as a component of universal indicator, together with methyl red, bromothymol blue, and thymol blue.

Phenolphthalein adopts different forms in aqueous solution depending on the pH of the solution. Inconsistency exists in the literature about hydrated forms of the compounds and the color of sulfuric acid. Wittke reported in 1983 that it exists in protonated form (H3In+) under strongly acidic conditions, providing an orange coloration.

The lactone form (H2In) is colorless and dominates between strongly acidic and slightly basic conditions. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction and becomes completely colorless when pH is greater than 13.

The pKa values of phenolphthalein were found to be 9.05, 9.50 and 12 while those of phenolsulfonphthalein are 1.2 and 7.70. The pKa for the color change is 9.50.

Carbonation of concrete

Phenolphthalein's pH sensitivity is exploited in other applications: concrete has naturally high pH due to the calcium hydroxide formed when Portland cement reacts with water. As the concrete reacts with carbon dioxide in the atmosphere, pH decreases to 8.5–9. When a 1% phenolphthalein solution is applied to normal concrete, it turns bright pink. However, if it remains colorless, it shows that the concrete has undergone carbonation. In a similar application, some spackling used to repair holes in drywall contains phenolphthalein. When applied, the basic spackling material retains a pink color; when the spackling has cured by reaction with atmospheric carbon dioxide, the pink color fades.

Education

In a highly basic solution, phenolphthalein's slow change from pink to colorless as it is converted to its Ph(OH)3− form is used in chemistry classes for the study of reaction kinetics.

Entertainment

Phenolphthalein is used in toys, for example as a component of disappearing inks, or disappearing dye on the "Hollywood Hair" Barbie hair. In the ink, it is mixed with sodium hydroxide, which reacts with carbon dioxide in the air. This reaction leads to the pH falling below the color change threshold as hydrogen ions are released by the reaction: :OH−(aq) + CO2(g) → (aq) + H+(aq).

To develop the hair and "magic" graphical patterns, the ink is sprayed with a solution of hydroxide, which leads to the appearance of the hidden graphics by the same mechanism described above for color change in alkaline solution. The pattern will eventually disappear again because of the reaction with carbon dioxide. Thymolphthalein is used for the same purpose and in the same way, when a blue color is desired.

Detection of blood

Main article: Kastle-Meyer test

A reduced form of phenolphthalein, phenolphthalin, which is colorless, is used in a test to identify substances thought to contain blood, commonly known as the Kastle–Meyer test. A dry sample is collected with a swab or filter paper. A few drops of alcohol, then a few drops of phenolphthalin, and finally a few drops of hydrogen peroxide are dripped onto the sample. If the sample contains hemoglobin, it will turn pink immediately upon addition of the peroxide, because of the generation of phenolphthalein. A positive test indicates the sample contains hemoglobin and, therefore, is likely blood. A false positive can result from the presence of substances with catalytic activity similar to hemoglobin. This test is not destructive to the sample; it can be kept and used in further tests. This test has the same reaction with blood from any animal whose blood contains hemoglobin, including almost all vertebrates; further testing would be required to determine whether it originated from a human.

Laxative

Phenolphthalein has been used for over a century as a laxative, but is now being removed from over-the-counter laxatives over concerns of carcinogenicity. Laxative products formerly containing phenolphthalein have often been reformulated with alternative active ingredients: Feen-a-Mint switched to bisacodyl, and Ex-Lax was switched to a senna extract.

Thymolphthalein is a related laxative made from thymol.

Despite concerns regarding its carcinogenicity based on rodent studies, the use of phenolphthalein as a laxative is unlikely to cause ovarian cancer. Some studies suggest a weak association with colon cancer, while others show none at all.

Phenolphthalein is described as a stimulant laxative.

Phenolphthalein has been added to the European Chemicals Agency's candidate list for substance of very high concern (SVHC). It is on the IARC group 2B list for substances "possibly carcinogenic to humans".

The discovery of phenolphthalein's laxative effect was due to an attempt by the Hungarian government to label genuine local white wine with the substance in 1900. Phenolphthalein did not change the taste of the wine and would change color when a base is added, making it a good label in principle. However, it was found that ingestion of the substance led to diarrhea. Max Kiss, a Hungarian-born pharmacist residing in New York, heard about the news and launched Ex-Lax in 1906.

Synthesis

Phenolphthalein can be synthesized by condensation of phthalic anhydride with two equivalents of phenol under acidic conditions. It was discovered in 1871 by Adolf von Baeyer.

References

References

1. "Phenolphthalein".
2. "Universal Indicator".
3. (1 March 1983). "Reactions of phenolphthalein at various pH values". Journal of Chemical Education.
4. (December 1996). "Spectrophotometric Analysis of the Relationship between Dissociation and Coloration, and of the Structural Formulas of Phenolphthalein in Aqueous Solution". Analytical Sciences.
5. (1 September 1998). "The Adsorption Behavior of Phenolphthalein at a Mercury Electrode in Water-Ethanol Solutions". Monatshefte für Chemie - Chemical Monthly.
6. (February 2001). "Molecular structure and vibrational spectra of phenolphthalein and its dianion". Spectrochimica Acta Part A.
7. (20 February 2019). "Is this Solution Pink Enough? A Smartphone Tutor to Resolve the Eternal Question in Phenolphthalein-Based Titration". Journal of Chemical Education.
8. "Ready to use spackle/repair product containing dryness indicator".
9. "Toystore".
10. Spiller, H. A.. (May 2003). "Skin Breakdown and Blisters from Senna-Containing Laxatives in Young Children". The Annals of Pharmacotherapy.
11. Dunnick, J. K.. (1996). "Phenolphthalein Exposure Causes Multiple Carcinogenic Effects in Experimental Model Systems". Cancer Research.
12. Tice, R. R.. (1998). "Measurement of Micronucleated Erythrocytes and DNA Damage during Chronic Ingestion of Phenolphthalein in Transgenic Female Mice Heterozygous for the p53 Gene". Environmental and Molecular Mutagenesis.
13. "Phenolphthalein".
14. Stolberg, Sheryl Gay. (August 30, 1997). "3 Versions of Ex-Lax Are Recalled After F.D.A. Proposes Ban on Ingredient". The New York Times.
15. Cooper, G. S.. (2004). "Ovarian Cancer Risk and Use of Phenolphthalein-Containing Laxatives". Pharmacoepidemiology and Drug Safety.
16. (2000). "IARC monographs on the evaluation of carcinogenic risks to humans". International Agency for Research on Cancer.
17. Dobrydneva, Y.. (2009). "Phenolphthalein as a Prototype Drug for a Group of Structurally Related Calcium Channel Blockers in Human Platelets". Journal of Cardiovascular Pharmacology.
18. "Phenolphthalein - Substance Information - ECHA".
19. "Phenolphthalein - Molecule of the Month - March 2022 (HTML version)".
20. (2018-10-25). "Phenolphthalein".
21. Baeyer, A.. (1871). "Ueber eine neue Klasse von Farbstoffen". Berichte der Deutschen Chemischen Gesellschaft.
22. Baeyer, A.. (1871). "Ueber die Phenolfarbstoffe". Berichte der Deutschen Chemischen Gesellschaft.
23. Baeyer, A.. (1871). "Ueber die Phenolfarbstoffe". Polytechnisches Journal.
24. Max Hubacher, {{US patent. 2192485 (1940 to Ex Lax Inc)