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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 "phph"

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, phenolphthalin, 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.

FAQ (Frequently Asked Questions)

1.Question: As an Indicator, How Does Phenolphthalein Function?

Answer: 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.

2. Question: Explain the Effects of Phenolphthalein on the Colon.

Answer: Phenolphthalein gets dissolved in intestinal and bile juices and thus stimulates intestinal musculature.

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

Answer: 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.