Question

Phenolphthalein was the indicator you used in your titration.

a. What made phenolphthalein a good choice of indicator? (Meaning, how do you choose an appropriate indicator for a titration.)

b.You used only a couple drops of a 1% solution. Why is it important to use such a small quantity of indicator in a titration?

c.The point where the color changed is known as the end point. What is the difference between the end point and equivalence point?

d.Can the end point be used quantitatively? Why or why not?

Answer 1

a)

the acidic solution of phenolphtalein is colourless while the basic solution is pink in colour. The indicator range is 8.3-10.0, which is a good range while doing the titration of a weak acid with a strong base, i.e titrations where we expect the mixture to be basic. It large detection range and apparent colour change makes it a good indicator.

b)

The indicators are themselves acid or bases, the only difference is that their protonated and de protonated forms have different colours. So if large quantities are added, the indicators will have an effect on the pH of the solution, especially in the case of weak acid.

c)

The end point is the one where the indicator changes the colour, while the equivalnece point is the actual point where the reaction is completed. end point and equivalence point need not be equal because end point depends on the pH, for example phenolphtalein will give colour change in the range 8.3-10 but equivalnece point is a single point.

d)

Yes it can be used, because end point and equivalence points are approximately same, and in the titration we only have a way of knowing end point(colour change).