Phenolphthalein (C20H14O4) is an acid-base indicator that belongs to the phthalein family of chemical compounds. It also is used as a component of the universal indicator, along with methyl red, bromothymol blue, and thymol blue.
Compounds that change color when exposed to acidic/basic solutions are known as indicators. Color indicators are widely used to detect pH and can be added to the reaction mixture to determine the titration endpoint/equivalence point.
In chemistry different types of titration, including acid-base, redox, precipitation, complexometric, and several types of indicators are used. Each one has its pKa, pH range, and produces a particular color at different pH values. The two theories that explain the acid-base indicator are Ostwald's theory and the quinonoid theory.
What is phenolphthalein?
Phenolphthalein (C20H14O4) is an indicator that belongs to the phthalein family of chemical compounds. It's a yellowish-white to light orange thin crystalline powder. It has a pKa of 9.3, is slightly soluble in water, and is prepared as an indicator for acid-base titration experiments when dissolved in alcohol. It appears colorless in the liquid form up to pH 8.5, after which it changes from pink to dark red.
Phenolphthalein which is closely related to the triphenylmethane dyes was discovered in 1871 by German scientist Adolf von Baeyer, who made it by fusing phenol and phthalic anhydride in the presence of sulfuric acid (H2SO4) or zinc chloride (ZnCl2).
Structure of phenolphthalein:
The chemical compound phenolphthalein has the formula C20H14O4 and is commonly abbreviated as "HIn" or "phph" in shorthand notation.In- is the ionic form of phenolphthalein, while HIn is the molecular form. In the basic solution, the molecule loses a hydrogen ion and becomes a negative ion. When titrating weak acids with strong bases, phenolphthalein is the ideal indicator.
The ionic form of phenolphthalein is In-, while the molecular form is HIn. In the basic solution, the molecule loses a hydrogen ion and becomes a negative ion.
Properties of phenolphthalein:
Molecular Weight/ Molar Mass: 318.32 g/mol
Formula: C20H14O4
Density: 1.277 g/cm3
Appears: White powder
Melting Point: 258–263°C
FAQ (Frequently Asked Questions):
Phenolphthalein is which type of indicator?
Phenolphthalein (C20H14O4) is a phthalein-family chemical molecule that is commonly used as an acid-base indicator.
What is the phenolphthalein indicator made of?
Under acidic conditions, phthalic anhydride and two equivalents of phenol can be condensed to prepare phenolphthalein.
Is phenolphthalein a natural indicator?
Phenolphthalein is a synthetic indicator, not a natural indicator, which is colorless in an acidic medium but pink in a basic medium.
How does phenolphthalein work as an indicator?
Phenolphthalein changes color when the molecule undergoes a rearrangement in the structure, transforming it from a colorless molecule to a colorful ion.
Why do we use phenolphthalein as an indicator in acid-base titration?
Phenolphthalein is chosen because it changes color in the pH range of 8.3 to 10, it is inexpensive and commonly available. The solution turns pink from colorless when the concentration of OH- ions increases somewhat, indicating the endpoint. In basic solution, it will appear pink; in acidic solution, appears colorless.
What is the pH range of phenolphthalein to produce permanent pink?
The pH range for the phenolphthalein indicator is 08.20 to 10.00. The chemical is colorless over a wide range of acidic pH values but begins to turn pink at pH 8.2 and continues to a brilliant magenta at pH 10.00 and higher.
What color is phenolphthalein indicator in a neutral solution?
Phenolphthalein is colorless in a neutral solution, as it begins to changes from a colorless to a pink solution at pH = 8.3, whereas the pH of a neutral solution is 7.00.
What color is phenolphthalein indicator in an acidic solution?
Phenolphthalein is colorless in acidic conditions, as does not react with acid. It is a weak acid that reacts only with strong bases and changes color.
What color is phenolphthalein indicator in a basic solution?
In basic solutions, phenolphthalein turns pink due to the formation of ions, which makes it very easy to visualize the stoichiometric end-point.
How do you make a 1.0 percent w/v solution of phenolphthalein indicator from phenolphthalein?
Take 01.00 g of phenolphthalein (C20H14O4) and dissolve in 100 ml of ethanol (95 percent).in a volumetric flask, and properly mix it. Once it has completely dissolved, transfer it to an indicator bottle.
Which indicator is best for weak acid strong base titration?
Phenolphthalein (pH range 8.3 - 10.0) or thymol blue (pH range 8.0 - 9.6) would be good indicators for titration of the weak acid CH3COOH and the strong base NaOH,
Why is methyl orange used as an indicator?
Methyl orange, with a pKa of 3.8, is an often-used pH indicator in titration because of its clear and distinct color variance in different pH ranges of 3.1 - 4.4.
References:
- Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, The Free Encyclopedia. Available Here
- ‘Why Does Phenolphthalein Change Color?’ Sciencing, Available Here
- Britannica, The Editors of Encyclopaedia. "phenolphthalein". Available Here
- Theory-of-acid-base-indicators.pdf, Dr. Pankaj Kumar Rastogi, Session-2018-19, Available Here
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