Indicators for titration and its end-point

Question

In titration, if a strong acid mixes with a strong alkaline, we will use phenolphthalein as an indicator.

Does the end point change from red to colourless, or form red to pale pink?
If the answer is former, does it mean that the solution is neutralized, it means the pH value is about 7 ?

To replier Michael :

As you said, if alkaline is put in the conical flask with phenolphthalein, the end point would be red to pale pink, but not colourless.

Before I asked this question, I see eye to eye with you, however, after I did the past paper, the model answer said that the end point of phenolphthalein turns from red to colourless. Is it wrong at all?

Moreover,if the end point of it is from red to colorless, the pH value would be slightly below 8.3, so the pH is nearer to pH 7, isn't it likely to be neutralized? Although the molarity is not clear, it would be neutralized more.That isn't what titration means?

Answer 1

The pH range of colour change is 8.3-10.0, which is included in the pH change (about 4-10) near the equivalence point of the titration between a strong acid and a strong alkali. In the pH range 8.3-10.0, the colour of the solution appears pale pink (not colourless !), and this colour shows the end point of titration.

If phenolphthalein is used as the indicator, the strong alkali (NOT the acid !) is usually put in the burette. The origin colour of the solutionin the conical flask (i.e. the acid) is colourless, and thus the colour change at the end point is from colourless to pale pink. It is easy to determine the end point by human's eyes.

If the acid (as the first replier recommended, but it is more difficult to perform the titration) is put in the burette, the origin colour of the solution in the conical flask (i.e. the strong alkali) is red (or pink), and the colour change at the end point is from red (or pink) to pale pink. It is very difficult to recognize such colour change. Therefore, the titration is usually done a little bit over, such that 1 or ½ drop of the acid is in excess to make the colour change from red (or pink) to colourless, and this colour change is recognized much more easily. The pH at this point is below 8.3, and the exact pH value depends on how much the acid is in excess.

Answer 2

in the titration,
strong acid-strong alkali----------methyl orange, phenolphthalein
strong acid-weak alkali-----------methyl oragne
weak acid-strong alkali----------phenolphthalein
weak acid-weak alkali -----------no indicator is suitable
from the above we can see that for each titration type, there is a particular indictor suitable for testing the end point. in the case of strong acid and strong alkali, if we use phenolphthalein, the solution in burette has to be the acid so that the end point must change from red to colourless. if the solution in burette is alkali and the phenolphthalein is added in the acid, we can't see the colour change. also the pale pink is indeed very pale, so we usually don't consider it as a change.
the process is neutralization, however the pH is not exactly 7, maybe slightly higher. it is shown in the phenolphthalein which changes to colourless at 8.3, meaning the resulted solution would be around 8.3. This is because the salt formed from the reaction would be slightly alkaline. this applies to all strong acid and strong alkali. the special pH of salts formed from different strength of acid and alkali determines which indicator to use.