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Topic: Complexiometric titration  (Read 5391 times)

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Offline Rutherford

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Complexiometric titration
« on: March 04, 2013, 10:35:48 AM »
Reactions of complex formation are frequently used in titrimetric methods of determination of various inorganic ions. For example, fluoride forms a stable complex with aluminum(III):
6F-+Al3+ = AlF63–
In water the complex gives a neutral solution. This process can be used for the direct titration of fluoride and indirect determinations of other species.

In the first experiment, a sample solution containing fluoride was neutralized with methyl red, solid NaCl was added to saturation, and the solution was heated to 70–80°C. The titration was performed with 0.15 M AlCl3 until yellow color of the indicator turned pink.

1.What process occurred at the endpoint?
2.Why heating increased the endpoint sharpness?
3.What is the purpose of adding sodium chloride?


I think that the process is:
6F-+Al3+ = AlF63–
I don't know 2 and 3. Maybe heating increases the reaction rate. Any help would be appreciated.

Offline Borek

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Re: Complexiometric titration
« Reply #1 on: March 04, 2013, 11:09:39 AM »
I think that the process is:
6F-+Al3+ = AlF63–

That's not what the question asks - you are expected to explain the color change.
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Offline Rutherford

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Re: Complexiometric titration
« Reply #2 on: March 04, 2013, 12:12:32 PM »
The pink color would mean acidic environment. The complex is neutral so what could cause the color change?

Offline Hunter2

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Re: Complexiometric titration
« Reply #3 on: March 05, 2013, 04:24:24 AM »
Think what happens if all Fluoride is consumed to the aluminum chloride.

Offline Rutherford

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Re: Complexiometric titration
« Reply #4 on: March 05, 2013, 08:27:07 AM »
AlCl3+3F- :rarrow: AlCl3F33-?

Offline Borek

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Re: Complexiometric titration
« Reply #5 on: March 05, 2013, 10:49:47 AM »
The pink color would mean acidic environment. The complex is neutral so what could cause the color change?

These are both very important points. Actually they contain the key to the answer.
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Offline Rutherford

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Re: Complexiometric titration
« Reply #6 on: March 05, 2013, 12:08:08 PM »
Maybe:
AlCl3+H2O :rarrow: Al(OH)3+H++Cl-
Acidic H+ ions are produced here, but Al makes a precipitate so it can't react with the chlorine. I am really out of ideas  ???.

Offline Hunter2

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Re: Complexiometric titration
« Reply #7 on: March 05, 2013, 12:15:31 PM »

Offline Rutherford

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Re: Complexiometric titration
« Reply #8 on: March 05, 2013, 12:59:56 PM »
So a gas is produced.

Then I have [Al(H2O)6]3+ in the solution. Then it hydrolyses to produce the precipitate (Al hydroxide). How would it react with fluoride?

Offline Borek

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Re: Complexiometric titration
« Reply #9 on: March 05, 2013, 01:33:43 PM »
Check here: http://www.rod.beavon.clara.net/AlCl3_and_water.htm

Nonsense - that would work in the case of ANHYDROUS AlCl3. You have SOLUTION of AlCl3.

List all substances present in the neutral solution.
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Offline Rutherford

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Re: Complexiometric titration
« Reply #10 on: March 05, 2013, 01:34:23 PM »
Then what's actually happening here  ?

Offline Hunter2

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Re: Complexiometric titration
« Reply #11 on: March 05, 2013, 02:27:54 PM »
Check here: http://www.rod.beavon.clara.net/AlCl3_and_water.htm

Nonsense - that would work in the case of ANHYDROUS AlCl3. You have SOLUTION of AlCl3.

List all substances present in the neutral solution.

And this is written in the second part of the article. Please read all and dont call nonsense. Aluminiumsalts in water are acidic. Did you ever shave your bear wet. If you get cut you can stopp the bleeding with aluminiumsulfate, because of acidic reaction with the blood.

To solve the miracle Aluminium get complexed by flouride until it is consumed, the free aluminium gives acidic reaction, what can be seen by change of colour of the indicator.

Offline Rutherford

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Re: Complexiometric titration
« Reply #12 on: March 05, 2013, 02:44:48 PM »
Thanks.
Isn't that what I wrote here:

I think that the process is:
6F-+Al3+ = AlF63–
I don't know 2 and 3. Maybe heating increases the reaction rate. Any help would be appreciated.
Then the rest of Al3+ hydrolyses to produce the hydroxyde as you said. What about question 2 and 3?

Offline Borek

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Re: Complexiometric titration
« Reply #13 on: March 05, 2013, 04:36:21 PM »
And this is written in the second part of the article. Please read all and dont call nonsense. Aluminiumsalts in water are acidic.

Sorry, my comment was addressed at Raderford, who blindly claimed that the gas is produced.

Quote
To solve the miracle Aluminium get complexed by flouride until it is consumed, the free aluminium gives acidic reaction, what can be seen by change of colour of the indicator.

There is another process that can be responsible for the pH change. Solution we start with after neutralization contains some HF, and Al3+ grabs all F- it can. That shifts the HF dissociation equilibrium. Judging which of these processes is dominating is not easy without exact calculations.
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Offline Rutherford

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Re: Complexiometric titration
« Reply #14 on: March 06, 2013, 06:25:37 AM »
Never mind then, I skip this.

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