Question

How do you choose an appropiate indicator for a titration?

Why it is important to use such a small quantity of indicator in a titration?

What is the difference between the end point and equivalence point?

Can the End point be used quantitatively? why or why not?

If we rinse the buret with 33% NAOH solution instead of the 0.1M solution of NAOH and Water , How does this affect the result of our first trial? Assuming this is the only trial, how would this impact the calculated concentration of NaOH solution?

Answer 1

1. How do you choose an appropriate indicator for a titration?

To determine the equivalence point (the actual point where the reaction has just been completed and refers to the stoichiometric quantities of reactants) of a reaction an appropriate indicator should be used. The equivalence point should lie between the pH range of an indicator so that the equivalence point can be detected by the change of the color of the indicator.

For example to determine the equivalence point of the titration of CH₃COOH and NaOH, phenolphthalein indicator is used. Phenolphthalein changes color between 8.3 and 10 and pH at the equivalence point of the reaction lies in that range. If we use methyl orange indicator it will change color before the equivalence point has been reached as pH range of methyl orange indicator is 3.1 - 4.4.

2. Why it is important to use such a small quantity of indicator in a titration?

Indicators are themselves either acids or bases. The protonated form(acid form/conjugate base) and the unprotonated form(base form/conjugate acid) have different colors. The color of the indicator solution hence depends on the pH of the solution. Little amount of indicator will not affect the equivalence point of the titration. If too much indicator is added, the effect of the indicator upon the pH of the solution can no longer be neglected.

3. What is the difference between the end point and equivalence point?

The equivalence point is the actual point where the reaction has just been completed and refers to the stoichiometric quantities of reactants. The end point is the position at which the indicator changes color. The end point generally is not the exact point where the reactants have completely reacted but simply shows that a particular pH change has occurred.

For example, consider a titration between HCl and NaOH, pH at the equivalence point is 7. If we use the phenolphthalein indicator (8.3 - 10) , more NaOH is required to see the color change of the solution although it has reached the equivalence point (a pH of 7). So we would have to add a fraction more NaOH after neutralization to get the change. This does not really matter as pH change is very rapid at the end point.

4. Can the End point be used quantitatively?

End point can be used quantitatively as pH at the equivalence point and at the end point does not differ very much when small amount of indicator is used.

5. If we rinse the burette with 33% NAOH solution instead of the 0.1M solution of NAOH and Water , How does this affect the result of our first trial? Assuming this is the only trial, how would this impact the calculated concentration of NaOH solution?

33% NaOH is too more concentrated with respect to a 0.1 M NaOH solution. When rinsed with this concentrated solution the inner wall of the burette is coated with a thin layer of NaOH and it will affect the of titration. The NaOH adhered to the inner wall of the burette will increase the pH of the titrant.

If the titrant used is an unknown solution of NaOH, the calculated concentration of the NaOH will be higher than the original concentration of the solution. Volume of NaOH needed to reach to the equivalence point would be lower(because of the addition of 33% NaOH solution to the unknown NaOH solution) than it would have been needed to reach to the equivalence point when its pH was not increased .