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Last updated date: 17th Apr 2024
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An Introduction

Phenolphthalein is a slightly acidic compound, hence considered a weak acid. Phenolphthalein has the chemical formula of C20H14O4and is a large organic molecule. In crystalline form, phenolphthalein appears to be white to yellow in colour. In short, it can be written as "HIn" or "php".


Phenolphthalein Solution

It is readily soluble in alcohol and mildly soluble in water. So, it is dissolved in alcohol to be used in experiments. Phenolphthalein acts as an indicator in acid-base titrations.


Phenolphthalein Structure

There are three hexagonal structures and one pentagonal structure, two alcoholic groups, and one ketone group in the structure of Phenolphthalein. Also, the carbon, Hydrogen and Oxygen chains form the Phenolphthalein structure.

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Properties of Phenolphthalein



Molecular Weight/ Molar Mass

318.32 g/mol


1.277 g/cm³


White powder

Melting Point

258–263 °C


Synthesis of Phenolphthalein

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

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Phenolphthalein Uses

1. In acid-base titrations, the popular use of phenolphthalein is as an indicator (phenolphthalein titration).

  • To determine the concentration, titration is an experiment where a volume of a solution of known concentration is applied to a volume of another solution. Most titrations are acid-base neutralization reactions.

  • Non-ionized forms of phenolphthalein are colourless. The protonated form of phenolphthalein in acidic solution is orange in colour. The deprotonated form of phenolphthalein in the basic solution is pink in colour.

  • Phenolphthalein, although its ion is pink, is a weak acid and is colourless in solution. The equilibrium would shift if hydrogen ions (H+, as found in an acid) were applied to the pink solution, and the solution would be colourless. The phenolphthalein will be converted into its ion by adding hydroxide ions (OH-, as found in bases) and the solution will turn pink.

  • As a result of pH modifications, Phenolphthalein adopts at least four distinct stages in an aqueous solution. It occurs in the protonated form (HIn+) under highly acidic conditions, producing an orange colouration. The lactone type (HIn) is colourless in both highly acidic and slightly simple conditions. The familiar pink colour is given by the doubly deprotonated (In2-) phenolate form (the anion form of phenol). Phenolphthalein is converted to its In(OH)3 form in highly simple solutions, and its pink colour undergoes a very slow fading reaction and becomes completely colourless above 13.0 pH.

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Note: It also serves, along with methyl red, bromothymol blue, and thymol blue, as a part of the universal indicator.


2. Cement Carbonation: Cement naturally has a high pH as it forms calcium hydroxide when it reacts with water. In the atmosphere, concrete reacts with carbon dioxide and its pH is reduced to 8.5-9 pH. If phenolphthalein is applied to the cement undergoing carbonation, it remains colourless. Phenolphthalein turns pink when it's applied to normal cement.

3. Phenolphthalein has been used as a laxative before.

4. Medical Uses:

Kastle–Meyer Test

In a test to classify substances believed to contain blood, widely known as the Kastle-Meyer test, a reduced form of phenolphthalein, phenolphthalein, which is colourless, is used. Through a swab or filter paper, a dry sample is obtained. A few drops of alcohol are dripped onto the sample, then a few drops of phenolphthalein, and finally a few drops of hydrogen peroxide. If the sample contains haemoglobin, and phenolphthalein is applied, it will turn pink immediately upon the peroxide addition.


A positive test means that the sample contains haemoglobin and is thus likely to contain blood. The presence of substances with catalytic activity similar to haemoglobin will result in a false positive. This test is not harmful to the specimen; it can be preserved and used in further experiments. Every species whose blood contains haemoglobin, including almost all vertebrates, has the same reaction to blood in this test; further research will be appropriate to determine if it came from a human being.


Harmful Effects of Phenolphthalein

Phenolphthalein is believed to be carcinogenic in nature. Facing concerns about its carcinogenicity, it is doubtful that the use of phenolphthalein as a laxative would induce ovarian cancer via SOCE (Calcium release-activated channel and  Structure), it has been found to inhibit human cellular calcium influx. This is accomplished by inhibiting thrombin and thapsigargin, two SOCE activators that increase free calcium intracellularly.

FAQs on Phenolphthalein

1. As an Indicator, How Does Phenolphthalein Function?

Phenolphthalein is a colourless and weak acid that is commonly used to signify the endpoint of the titration as an indicator in titration experiments. As this compound dissociates to form pink anions when dissolved in water, the endpoint is indicated by the formation of pink colour. This is how Phenolphthalein works as an indicator. To learn more about Phenolphthalein and other chemical indicators, you can check out Vedantu’s official website where they’ll find several questions and answers for their practice. Students can also find previous year paper’s questions and their solutions which help them to understand what type of questions are asked in the exam. This also helps to prepare for the exam by making practice more similar with time limits and exam-like pressure.

2. Explain the Effects of Phenolphthalein on the Colon.

Phenolphthalein gets dissolved in intestinal and bile juices and thus stimulates intestinal musculature. This was the effect of Phenolphthalein on the Colon. To study more about Phenolphthalein, the student can check out the free content by Vedantu on Vedantu’s Youtube Channel. Students can also find solved questions and NCERT Solutions at Vedantu’s official website. On the website, they can find tons of free content of all chapters of subjects that will assist them in understanding concepts better and getting good grades in their exam. Make sure that one uses the FREE content that will help them and change their lives drastically.

3. What is the Colour of Phenolphthalein in the Water?

Phenolphthalein is an indicator, based on whether it reaches an acid or a base, a chemical that changes colour. If it meets something basic, such as ammonia, it turns purple; if it meets an acid like vinegar or a neutral material like water, it remains colourless. This was one of the properties of Phenolphthalein. To study more properties of Phenolphthalein and other indicators, one can check out Vedantu’s Youtube Channel and Vedantu’s official website. Here they will find a ton of FREE content waiting for them to open and explore. Students can benefit a lot from the important list of questions that Vedantu’s best teachers have prepared for the students. Students should use these resources to the fullest to score good marks and understand the concepts better. A little practice and concentration can drastically increase a student's confidence which helps to boost their marks and eventually ranks in competitive exams like JEE, etc.

4. How to Study Chemical Indicators for an Exam?

Chemical indicators are substances that tell us about the acidic or basic nature of a substance. Chemical indicators can neither be naturally occurring nor prepared artificially. There are numerous chemical indicators but all of them can not be used at all times. They are used upon the availability and feasibility. Litmus Test is one of the most popular tests that tell one about the chemical behavior of any element. Like litmus tests, there are several tests that can be tested in the exam. One must make notes of the chapter while the teacher is teaching the chapter so that they do not miss the important information that had been told in class. Students can also refer to Vedantu's official Youtube Channel Where they can learn more about indicators through video lectures from Vedantu's best teachers. To bring good marks in indicators, the student needs a ton of practice to understand the topic from the base. These are a few of the steps that the students can follow to get good marks in chemical indicators. There are a lot of FREE resources available on Vedantu's official website and the student should definitely check that out.

5. What are Indicators and What are Some Qualities Required to Qualify as a Chemical Indicator?

Chemical Indicators are those substances whose solutions change colour due to changes in their pH. These are also called acid-base indicators. Chemical Indicators are generally some weak acids or bases, but their conjugate base or acid forms have different colours due to differences in absorption spectra. These Chemical Indicators are mostly organic and weak acids or bases with much-complicated structures.

Chemical indicators are supposed to be both sensitive and also easily detectable. They do not need to show a visible change, however. The type of indicator depends on how it is being used. For example- a sample detected by spectroscopy might use some indicator that would not be visible to a normal human naked eye, while testing for calcium in an aquarium must produce an obvious colour change. One more important quality of these indicators is that they don't change the conditions of the sample. For example, methyl yellow adds a little yellow colour to the alkaline solution, but if acid is added to the solution, the colour remains yellow until the pH is neutral. At this point, the colour changes from yellow to red. At low levels, methyl yellow does not itself change the acidic properties of the given sample.

The typical concentration of methyl yellow is in the parts per million range. In this small amount, there is a visible colour change, but not enough to change the sample itself. with an enormous amount of methyl yellow added to a specimen shows some different results. Not only, any colour change might be invisible, but the addition of so much methyl yellow would completely change the chemical composition of the sample. In some other cases, small samples of a large volume of water are separated from the larger volume so that they can be tested with indicators that produce significant chemical changes.