Because the concentration of In- in the acidic solution is very low and the concentration of H+ is quite high, the phenolphthalein is colorless.
Phenolphthalein (C20H14O4) is a chemical compound that belongs to the phthalein family generally used as an acid-base indicator. It has a pKa of 9.3 and a pH range of 8.3 to 10.0. In acidic conditions, the phenolphthalein indicator is colorless; in basic medium, it becomes pink.
Why phenolphthalein is colorless in acid solution?
Phenolphthalein is a weak acid that does not dissociate in an acidic environment and remains in an unionized form. Because the unionized form is colorless, phenolphthalein in an acidic solution is also colorless.
For example,
- Titration of 0.1N HCl versus 0.1N NaOH in the presence of phenolphthalein indicator.
- In a conical flask, take 10.00 ml titrand (hydrochloric acid solution) and 2–3 drops of phenolphthalein were added.
- No titrant (sodium hydroxide solution) is added to the solution at this point.
- Therefore, phenolphthalein is colorless, because it is in an acidic environment.
- The titrant (NaOH) in the burette is now used to titrate this mixture.
- It is completely neutralized as soon as we add some quantity (about 8-10 ml) NaOH. (HCl + NaOH =) NaCl + H2H2O.)
- 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.
- Now, if we add a few drops of NaOH after the equivalence point, phenolphthalein comes under basic conditions gives the solution a pink color.
- This is the endpoint of the titration which indicates that the reaction is completed.
You may also like this:
No comments:
Post a Comment