The formation of ions through a chemical reaction causes the solution to turn pink. In acid-base titrations, phenolphthalein is commonly employed as an indicator. In the presence of an acid, it becomes colorless, while in the presence of a base, it becomes pink.
The chemical compound phenolphthalein (C20H14O4) belongs to the phthalein family. It was discovered in 1871 by Adolf von Baer, a German scientist, who created it by fusing phenol and phthalic anhydride in the presence of H2SO4 or ZnCl2. It is related to triphenylmethane dyes.
The pH range of the phenolphthalein indicator is from 08.20 to 10.00. It is colorless over an acidic pH range, although it turns pink at pH 8.2 and bright magenta at pH 10.00 and higher. It is a component of the universal indicator, along with methyl red, bromothymol blue, and thymol blue.
Why does the phenolphthalein turn pink?
Phenolphthalein (HIn) is a slightly acidic compound that dissociates into H+ and In- ions in an aqueous solution. The concentration of In-ions in the solution gives the solution its pink color.
The solution is colorless because the concentration of In- in the solution is very low under acidic conditions and the concentration of H+ is quite high.
Likewise, under basic conditions, the concentration of H+ ions is extremely low while the In-concentration is high, resulting in a pink-colored solution.
For example, using phenolphthalein as an indicator, titration of 0.1N sodium hydroxide against 0.1N hydrochloric acid.
In a conical flask, 10 ml HCL and 2-3 drops of phenolphthalein were added. At this point, no titrant (NaOH) is added to the mixture; hence the phenolphthalein is colorless because it is in an acidic environment.
The 0.1N NaOH solution is now being used to titrate this solution. It is completely neutralized as soon as we add about 10 ml of the titrant. At this stage, the phenolphthalein indicator is in a neutral state and gives the solution a very pale pink color. This is an equivalence point of the reaction.
Following the equivalence point, we add a few ml of NaOH, and the addition of phenolphthalein occurs under basic conditions, giving the solution a pink color.
Why is phenolphthalein pink in the basic solution?
The ionization of phenolphthalein increases dramatically in an alkaline medium. The OH- ions from the alkali continuously remove H+ ions released from HPh. As a result, the concentration of Ph ions in the solution increases, making the solution pink. Therefore phenolphthalein becomes pink in the presence of basic solution.
Why does the pink color of phenolphthalein fade away?
Right after the endpoint of the titration, the basic solution slowly absorbs CO2 from the air and becomes more acidic, causing the color to fade from pink to colorless. When titrating strong or weak acids with a strong base, phenolphthalein is one of the most commonly used indicators.
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