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Topic: Why does Phenolphthalein turn pink and colorless?  (Read 16131 times)

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Offline summerrains

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Why does Phenolphthalein turn pink and colorless?
« on: June 23, 2010, 08:29:25 AM »
Okay so I observed an experiment that went like this. When a colorless solution was poured from one beaker to another, it turned pink, then with continued pouring, the solution became colorless again.

Basically the experiment goes like this:

The first plastic cup is charged with 250 mL of water, 1 mL of 1% w/v phenolphthalein solution, and
10 drops of 2 M sulfuric acid solution. The plastic second cup is charged with 15 drops of 2 M potassium hydroxide solution. Slowly pour the contents of the first cup into the second cup. At first, the pouring will result in a red color. With continued pouring, the acid will neutralize the base, and the red color will disappear.

I know that phenolphthalein turns pink in basic solutions and is colorless in acidic solutions.
That's why it was colorless in sulfuric acid at first.

But what I don't understand is how come the phenolphthalein turned pink when the contents of the first cup was poured into the second cup.

I thought that this reaction would occur:
2 KOH + H2SO4 → K2SO4 + H2O

So why would the phenolphthalein turn pink if K2SO4 was produced, and why did it turn colorless again in the end?

Thank you for any *delete me*

Offline Borek

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Re: Why does Phenolphthalein turn pink and colorless?
« Reply #1 on: June 23, 2010, 08:45:26 AM »
Think is terms of limiting reagents. When you add small amount of acid, what is left in the solution?
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Offline summerrains

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Re: Why does Phenolphthalein turn pink and colorless?
« Reply #2 on: June 23, 2010, 08:55:45 AM »
Okay, I get it.

In the first cup you have an acid solution, plus phenolphthalein. No colour.

In the second cup you have a basic KOH solution. When you first start to pour the acid solution into the basic solution, there is not enough acid yet to neutralise all the KOH. The phenolphthalein is now present in a basic solution so it turns pink

When you continue to pour the acid solution into the KOH basic solution, a time will be reached when sufficient acid has been added to neutralise all the base. The solution then becomes acidic, and the phenolphthalein once again becomes colourless.

Right?

But how do I phrase the above in a scientific way, for a report, instead of using layman terms?

Thank you!!

Offline Borek

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Re: Why does Phenolphthalein turn pink and colorless?
« Reply #3 on: June 23, 2010, 09:45:50 AM »
You've got it right.

Explain it in terms of limiting reagent and excess, should do.
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Offline summerrains

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Re: Why does Phenolphthalein turn pink and colorless?
« Reply #4 on: June 23, 2010, 11:32:03 PM »
How do I explain the part where when you first start to pour the acid solution into the basic solution, there is not enough acid yet to neutralise all the KOH? (in scientific terms)

Offline Borek

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Re: Why does Phenolphthalein turn pink and colorless?
« Reply #5 on: June 24, 2010, 02:37:59 AM »
I hate to repeat myself, but let's give it a try. Scientific language is not about being pompous, but about being precise and using correct terms, an keeping it as simple as possible. LIMITING REAGENT and EXCESS REAGENT are simple enough, correct enough and precise enough.

You won't get other answer.
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