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Topic: Oxalic dihyrate acid + Sulphuric acid????  (Read 8676 times)

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Offline iyc

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Oxalic dihyrate acid + Sulphuric acid????
« on: October 07, 2005, 09:52:37 AM »
There is a redox titration experiment using KMnO4 as oxidizing agent.

The objective is to prepare an approximately 0.05M solution of oxalic dihyrate acid and use it to determine the concentration of a given KMnO4.

there is a procedure that 25cm3 of a solution of oxalic acid, and add about 25cm3 of dilute H2SO4 and heat the mixture to about 60oC.

I wanna ask what is the purpose of adding dilute sulphuric acid in a solution of oxalic acid? Can anyone tell me??

But I try first.
I think the oxalic acid in solution would decompose  into carbon dioxide and water in heating, so adding dilute sulphuric acid can provide a path of higher activation energy for decomposition or adding sulphuric acid can increase the strength of intermolecular forces between molecules in mixture so that the ethanidoate acid would not decompse easily in heating. Thus, it will not cause a decrease in number of mole of ethanidioate ion in heating and so for the latter redox titration.

« Last Edit: October 07, 2005, 09:56:23 AM by iyc »

Offline Borek

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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #1 on: October 07, 2005, 10:28:51 AM »
Oxalic acid is pretty stable. You have to try harder ;)

Think about permanganate end, not oxalic acid.
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Offline iyc

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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #2 on: October 07, 2005, 10:39:03 AM »
I think I have got a wrong way of thinking, so I try again.

As oxalic acid is a weak acid, so it ionize slightly to give hydrogen ion. Therefore, adding dilute sulphric acid can provide an acidic medium for MnO4- to react with ethanedioate ion.

I think it should be right. But let me ask a further question.
Why do we need to acidify the some oxidizing agent, e.g. MnO4-? Will MnO4- without acidifiying have oxidizing property??
« Last Edit: October 07, 2005, 10:41:29 AM by iyc »

Offline Borek

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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #3 on: October 07, 2005, 10:48:11 AM »
Why do we need to acidify the some oxidizing agent, e.g. MnO4-? Will MnO4- without acidifiying have oxidizing property??

Write half-reaction for permanganate. Will addition of H+ move the equilibrium? In what pH will it be stronger oxidizer? The same hold for other oxidizers, like CrO42-.

In basic solutions permanganate gets reduced to MnO2 only. You will not want to titrate solution full of brown precipitate, as it hides any color change at endpoint.
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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #4 on: October 08, 2005, 12:53:59 PM »
In basic solutions permanganate gets reduced to MnO2 only. You will not want to titrate solution full of brown precipitate, as it hides any color change at endpoint.

No, in a neutral solution you will get MnO2. In a basic solution you will get MnO42-.

Offline Borek

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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #5 on: October 08, 2005, 02:15:09 PM »
OK, my bad. Still, low pH is needed for permanganate, not for oxalic acid.
« Last Edit: October 08, 2005, 02:20:51 PM by Borek »
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Garneck

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Re:Oxalic dihyrate acid + Sulphuric acid????
« Reply #6 on: October 08, 2005, 02:47:40 PM »
OK, my bad. Still, low pH is needed for permanganate, not for oxalic acid.

Yes, it is  :)

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