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Phenolphthalein is a weak organic acid, being colorless in an acidic solution and pink ( because of the color of its conjugate base) in a base solution. The Experimental Procedure suggests the addition of 2 drops of phenolphthalein for the standardization of the sodium hydroxide solution. Explain why the analysis will be less accurate with the addition of a larger amount, e.g., 20 drops, of phenolphthalein.

2006-12-01 14:03:41 · 6 answers · asked by ZG786 1 in Science & Mathematics Chemistry

6 answers

First, depending upon the size of the drops, 20 drops could change the volume of the sample enough to change the final reading (this is due to the dilution of the sample).

In addition, if the concentration of the Phenolphthalein is sufficiently high enough, it will react with some of the Sodium hydroxide lowering the concentration which is available to react with the titration acid solution.

2006-12-01 14:18:11 · answer #1 · answered by Richard 7 · 30 1

Phenolphthalein in larger amounts actually ends up neutralizing the acid/base in solution, thus biasing the results by providing an endpoint sooner than expected. 2 drops would be insignificant bias, but 20 drops (~ 1 ml) could move the endpoint significantly.

Many titrations used in laboratories today use an electronic means of determining the endpoint, so as not to bias the endpoint with this "interference" from the indicator.

2006-12-01 22:20:45 · answer #2 · answered by sep_n 3 · 1 0

ask yourself what's going on in the solution when you add phenolphthalein
why is it reacting, what's going on during the reaction?
indicators used in acid-bases trition change color when neutralized

so dropping phenolphthalein in the base is neutralizing the base up to the point where the solution gets neutral (in fact not really but let's say the reaction stops when the tested solution gets neutral)
when the tested solution gets neutral the more acid you drop the more acid the test solution will be so the pinkish solution will make the opposite reaction and will be colorless back

so if you drop too much phenolphthalein in the base you will have a very fast pink reaction that will disapear, so unless you are really paying attention to the color you might not even see it

ok now that you see what's the problem you'll have to prove it =)

2006-12-01 22:28:54 · answer #3 · answered by Anonymous · 0 1

The change of an indicator from one form to another involves use of or liberation of H+ Ions. The amount of H+ Ions concentration is negligible in comparison to the amount of acid or base involved in the titration reaction.But the magnitude of the indicator error increases with the concentration of the indicator.hence only a minimum required quantity should be added to indicate the end-point. The experimental endpoint and the equivalence point do not coincide because of the above reason.

But indicator errors are determinate errors.It can be corrected by performing a blank titration and subtracting it from the total volume consumed in the real titration.

2006-12-02 01:16:11 · answer #4 · answered by pal 1 · 0 0

I guess because since phenolphthalein is an acid, its addition would make the receiving solution more acidic than it was before, so that it would test as more acidic than one would want it to.

2006-12-01 22:18:41 · answer #5 · answered by yahoohoo 6 · 0 0

It gets dissolved?

2006-12-01 22:12:20 · answer #6 · answered by Rachel T. 2 · 0 0

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