What are the structural changes happen in acid base titration using Phenolphthalein as it develops color?
Indicators used in acid/base titrations , such as phenolphthalein, are weak bases or acids. Phenolphthalein is a weak acid. It has a complicated molecular structure, so I will write it as HP. If you add phenolphthalein to an acid solution, there is an abundance of H+ ions: HCl ↔ H+ + Cl- The phenolphthalein being a weak acid also wants to ionise and produce: HP ↔ H+ + P- But because of the high concentration of H+ ions from the main acis, the ionisation of the phenolphthalein is inhibited and it exists as the undissociated weak acid. The undissociated acid molecule is colourless. That is why, in an acid solution, phenolphthalein is colurless, Now a base is added to the acid solution. A neytralisation reaction takes place: HCl + NaOH → NaCl + H2O The concentration of H+ ions decreases. When the “end point” is recached, and all the H+ ions from the acid have been reacted, the phenolphthalein is able to ionise and form , as above: HP ↔ H+ + P- The P- ion is a bright pink colour, so as soo
