Phenolphthalein is an acid-base indicator that turns pink in the presence of a weak base and is used to test the pH of a solution. The number of anions increases when the pH of the solution is 8.2 or higher, causing the solution to turn pink.
Indicators are compounds that change color when exposed to acidic or basic solutions. Color indicators are commonly used to measure pH and can be used to identify the titration endpoint/equivalence point by adding them to the reaction mixture.
Several types of indicators are employed in various forms of titration, including acid-base, redox, precipitation, and complexometric. Each has its own pKa and pH range and produces a different color at different pH levels.
| Table of Contents: |
What is phenolphthalein?
Phenolphthalein (C20H14O4) is a chemical compound that belongs to the phthalein family. It was discovered in 1871 by the German scientist Adolf von Baer, who made it by fusing phenol and phthalic anhydride in the presence of H2SO4 or ZnCl2 and is closely related to triphenylmethane dyes.
It is a fine crystalline powder that is yellowish-white to bright orange in color. Its pKa is 9.3, which is mildly soluble in water, and when dissolved in alcohol, it is used as an indicator for acid-base titration.
The pH range of the phenolphthalein indicator is from 08.20 to 10.00. The chemical is colorless over a wide range of acidic pH values, although it turns pink at pH 8.2 and bright magenta at pH 10.00 and higher. Along with methyl red, bromothymol blue, and thymol blue, it is also a component of the universal indicator.
How does phenolphthalein indicator work?
Ionization is the chemical reaction that causes the compound's color to change. Furthermore, ionization happens when a molecule loses or gains an electron, resulting in a positive or negative charge on the molecule.
The ionized molecule attracts the opposite charge while repelling the same charge. It also has an affects on molecular shape. Also, the charge determines how it will react to light.
Generally, phenolphthalein solution appears colorless when all colors of light flow through it. However, when it comes into contact with the alkaline condition, it begins to block the blue color of the light spectrum, turning pale pink.
The stronger the alkaline solution, the more the phenolphthalein molecules will change, resulting in a darker color.
Structure of phenolphthalein:
It has 3 hexagonal structures, 1 pentagonal structure, 2 alcoholic groups, and 1 ketone group in its structure. Phenolphthalein also has a structure that includes carbon, hydrogen, and oxygen chains.
Depending on whether it is in alkali (pink) or acid (colorless) solution, the phenolphthalein indicator has two different structures. Both structures absorb light in the ultra-violet (UV) range, which the human eye cannot see. On the other hand, the pink form is also absorbed in the visible light spectrum.
The structure of the pink form of the phenolphthalein indicator is responsible for the absorption of visible light. The electrons in the molecule are more delocalized due to ionization than in the colorless form. In short, delocalization occurs when electrons in a molecule are not associated with a single atom but are spread among several atoms.
The energy gap between molecular orbital shifts as delocalization increases. The energy required to jump an electron to a higher orbital is less. In solution, the human eye detects a pink color. The phenolphthalein indicator changes more as the solution becomes more alkaline, resulting in a deeper pink color.
References:
- Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, Available Here
- ‘Why Does Phenolphthalein Change Color?’ Sciencing, Updated February 16, 2020 By Rosann Kozlowski, Available Here
- Britannica, The Editors of Encyclopaedia. "phenolphthalein". Encyclopedia Britannica, 23 Nov. 2018, Available Here
- Theory-of-acid-base-indicators.pdf, Dr. Pankaj Kumar Rastogi, Session-2018-19, Available Here
People also ask:
Mechanism of phenolphthalein
What is the mechanism involved during the colour change of phenolphthalein indicator?
What is the mechanism behind color change of an indicator?
How does phenolphthalein indicator work in titration?
What is the mechanism of indicator?
You may also like this:
No comments:
Post a Comment