chemistry

Phenolphthalein

Phenolphthalein is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Phenolphthalein rather than just read about it. In short: Phenolphthalein ( feh-NOL(F)-thə-leen) is a chemical compound with the formula C20H14O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations.

Key takeaways

Phenolphthalein belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phenolphthalein to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phenolphthalein from memory before moving on to harder problems.

Reference excerpt

Phenolphthalein ( feh-NOL(F)-thə-leen) is a chemical compound with the formula C20H14O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes. Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols in experiments. It is a weak acid, which can lose H+ ions in solution. The nonionized phenolphthalein molecule is colorless and the double deprotonated phenolphthalein ion is fuchsia. Further addition of hydroxide in higher pH occurs slowly and leads to a colorless form, since the conjugated system is broken. Phenolphthalein in concentrated sulfuric acid is orange-red due to protonation and creation of a stabilised trityl cation.

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) → CO2−3(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

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. In addition, it has been found to inhibit human cellular calcium influx via store-operated calcium entry (SOCE, see Calcium release activated channel § Structure) in vivo. This is effected by its inhibiting thrombin and thapsigargin, two activators of SOCE that increase intracellular free calcium. 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.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Phenolphthalein

Start with the simplest possible case. Write down what Phenolphthalein claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Phenolphthalein before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Phenolphthalein ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Phenolphthalein

In research: Phenolphthalein appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Phenolphthalein in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Phenolphthalein is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1871 introductions, 4-Hydroxyphenyl compounds, IARC Group 2B carcinogens, so understanding it makes those chapters shorter.
In everyday life: Look for Phenolphthalein outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Phenolphthalein in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phenolphthalein means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phenolphthalein out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phenolphthalein in simple terms?

Why does Phenolphthalein matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Phenolphthalein?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Phenolphthalein.