Phenolphthalein: Definition, Structure and Uses

Phenolphthalein is a considered as a white or yellowish-white crystalline compound C₂₀H₁₄O₄ which is used in analysis as an indicator as its solution is brilliant red in alkalies and is loses its colour by acids and in medicine as a laxative.

We all know Acids and Bases i.e., acidic or alkaline. But, the function of a pH indicator is far more advanced. 

One of the pH testers is called Phenolphthalein, which is usually colorless but, in contact with any acid or base, it changes color from pink (acidic) or purple (base).

It is also used as an indicator in acid–base titrations, and turns colorless in acid based solutions and pink in basic solutions. It basically belongs to the class of dyes known as phthalein dyes.

Read More: Degree of Unsaturation Formula

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Phenolphthalein

Phenolphthalein is a chemical compound. The formula is C₂₀H₁₄O₄ as It contains 20 carbon atoms, 14 hydrogen atom, and 4 oxygen atoms, and written as "HIn" or "phph" as notation. Phenolphthalein does not dissolve well in water, alcohol is used to prepare this solution, and when you add a drop of acid in the solution may sometimes see a white cloud generated in it. Moreover, it turns pink when it comes into contact with a substance of a pH of level 8.2 and purple at higher pH. The change in color is the result of ionization; a process that alters the charge and shape and color of Phenolphthalein molecule. 

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It is a weak acid, which loses H⁺ ions in solution. The nonionized phenolphthalein molecule is colorless, the protonated phenolphthalein molecule is orange in color, and the deprotonated phenolphthalein is fuchsia, when the base is added to the solution containing phenolphthalein, the equilibrium between the nonionized and the deprotonated states shifts in favor of deprotonated state as the H⁺ ions are removed from solution.

Read More: Specific Heat of Water

This table represents the molecular weight and other features of Phenolphthalein:

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Structure of Phenolphthalein

It has 3 hexagons, one pentagon, 2 alcoholic groups, and one ketone group. Besides, the structure forms the carbon, hydrogen, and oxygen chains.

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The phenolphthalein molecule contains a total of 41 bond(s) There are 27 non-H bond(s), 19 multiple bond(s), 2 rotatable bond(s), 1 double bond(s), 18 aromatic bond(s), 1 five-membered ring(s), 3 six-membered ring(s), 1 nine-membered ring(s), 1 ester(s) (aromatic) and 2 aromatic hydroxyl(s).

This image represents the structure of Phenolphthalein.

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Uses of Phenolphthalein (C₂₀H₁₄O₄)

• Used in acid base titrations as an indicator

Titration is an experiment, where the volume of one solution concentration is applied to the volume of another solution. Most titrations are acid-based.

Read More: Number of Moles Formula

• It is used in Kastle-Meyer test

The Kastle–Meyer test is a blood test, firstly described in 1903, in which the phenolphthalein is used to detect the presence of hemoglobin in human body; it will turn pink immediately by the addition of the peroxide, because of the phenolphthalein.

Read More: What is Enthalpy

•  Medical uses

Laxative products used to contain phenolphthalein but have now been reformulated and used as alternate active ingredients such as; Feen-a-Mint switched to bisacodyl and Ex-Lax switched to a senna extract. Phenolphthalein has also been added to the European Chemicals Agency's list for SVHC substances.

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Change of Color of Phenolphthalein

Phenolphthalein is weak and colorless in solution and its ion is pink. Ionization is the process that is responsible for the changing of the color of the Phenolphthalein. Ionization happens when one molecule loses or gains an electron, this process gives the molecule a negative or positive electric charge.

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The ionized molecule attracts other molecules and repels the same charge, and it also affects the shape of the molecule.

Normally all the colors of the light passes through a phenolphthalein appears colorless but when it is exposed to the alkaline it blocks the clue color light and turns pink, and stronger the alkaline solution, the more Phenolphthalein molecules will change and the darker color will appear.

This image represents the change in color of Phenolphthalein.

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Working of Phenolphthalein Indicator 

• The pH scale has a marking i.e., from 0 to 14 for detecting the acids and bases. 0 to 6.9 are the acids, the less the number the more it is acidic. While the pH 7 is neutral, the pH of water.
• From 7.1 to 14 are the bases. The higher the number of the more basic nature they have. In common practices a chemist uses litmus paper to test the pH of a compound. The litmus paper turns red with acid and turns blue with the base.

Read More: Solvent Examples

• Phenolphthalein is naturally colorless and works differently than litmus paper. It also turns pink in color in an alkaline solution or base. The compound remains colorless in acids but starts turning pink on the pH 8.2 and continues to turn bright in strong bases.

This image represents the phenolphthalein indicator.

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Uses of Phenolphthalein for Entertainment

Phenolphthalein is used in toys, for example; disappearing inks or disappearing dyes on Barbie hair. The ink was mixed with sodium hydroxide, which reacts with carbon dioxide. This reaction leads to the falling level of pH below, and the color changes as hydrogen ions are released by the reaction.

OH⁻(aq) + CO₂(g) → CO₂^-

3(aq) + H⁺(aq)

Thymolphthalein is also used for the same purpose, when a blue color is required.

Read More:

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Things To Remember 

• Phenolphthalein is a bit soluble in water and usually dissolves in alcohols for the experiment purpose. It is a weak acid, which loses H⁺ ions in solution. 
• The formula is C₂₀H₁₄O₄ as It contains 20 carbon atoms, 14 hydrogen atom, and 4 oxygen atoms, and written as "HIn" or "phph" as notation.
• It has 3 hexagons, one pentagon, 2 alcoholic groups, and one ketone group.
• Phenolphthalein is naturally colorless and works differently than litmus paper. It also turns pink in color in an alkaline solution or base. 

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