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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: upsidedown on April 24, 2009, 04:09:56 PM

Title: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 24, 2009, 04:09:56 PM
I writting a plan on how to determine the concentraion of hydrogen peroxide, H2O2, by a redox titration with potassium mangante (VII), K2MnO4.
The solution of hydrogen peroxide is know to be approximately 100-volume.
I know that I should start by diluting the H2O2, but im not sure what to do after to find the exact concentraion of H2O2.

The equation ifor this is:
5H2O2 + 2KMnO4 + 3H2SO4  ----> 2MnSO4 + K2SO4 + 5O2+ 8H2O         

Thanks for any help.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: plankk on April 24, 2009, 04:32:13 PM
Then you should acidize your solution of H2O2. The next step is doing the titration by using the solution of KMnO4 of known concentration. After all you have to calculate how much H2O2 you have by using the reaction's equation.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 24, 2009, 05:48:59 PM
Final calculations won't be different from any other titration.

http://www.titrations.info/titration-calculation
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 25, 2009, 08:21:40 AM
I know I have to used acidified KmnO4,
do a acidify it just by adding H2SO4, and will this effect the concentration of KmnO4?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: UG on April 25, 2009, 08:54:28 AM
Should be no problem.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: plankk on April 25, 2009, 09:04:53 AM
If you add the solution of H2SO4 to the solution of KMnO4, the concentration of KMnO4 of course changes (the volume grows). If you do in this way your calculation will be harder and titration could be done with bigger measuring error. The better idea is to acidify the solution of hydrogen peroxide. Then it isn't important how diluted is your solution. From calculation you will find a n (the number of moles H2O2). And to find concentration of your solution (which you dilute in the first step), you have to know how volume you take for titration.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 25, 2009, 09:11:36 AM
How much H2SO4 should I add?

Does it decrease the concentraion the same as just adding distilled water?

Thanks for the help.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 25, 2009, 03:25:19 PM
You should take aliquot of hydrogen peroxide solution, add sulfuric acid, titrate with pemanganate. Note, that amount of hydrogen peroxide after the aliquot was taken doesn't change - concentration does, but number of moles is constant. Thus changes in concentration can be safely ignored.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 05:38:20 AM
Quote from: Borek on April 25, 2009, 03:25:19 PM
You should take aliquot of hydrogen peroxide solution, add sulfuric acid, titrate with pemanganate. Note, that amount of hydrogen peroxide after the aliquot was taken doesn't change - concentration does, but number of moles is constant. Thus changes in concentration can be safely ignored.

Im not sure what you mean, if water is added the concentraion changes, but if H2SO4 is added it does not?

Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 26, 2009, 05:44:10 AM
Concentration changes in both cases, but it doesn't matter, as the amount of the substance titrated is the same.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 06:03:02 AM
So if 25cm3 of 0.02 mol dm -3 Potassium manganate was needed titrate against H2O2
You work you work out the concentraion of H2O2 the same no matter how diluted it is?

Also, how much sulfuric acid would be enough to acidify H2O2?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 26, 2009, 07:00:21 AM
You don't calculate concentration of H2O2 in the titrated solution. Instead, you calculate number of moles of H2O2 titrated, and as you know volume of aliquot of hydrogen peroxide taken, you can calculate concentration in the ORIGINAL sample. So yes, dilution of your aliquot doesn't matter, as it doesn't change number of moles of H2O2.

Quote from: upsidedown on April 26, 2009, 06:03:02 AM
Also, how much sulfuric acid would be enough to acidify H2O2?

Recipe from my book: put solution containing 40-70 mg H2O2 into 500 mL flask. Dilute to 200 mL, add 20 mL 30% H2SO4, titrate with 0.1n permanganate.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 07:21:19 AM
If I calculate moles of Potassium Manganate by : volume (cm3) × concentration
                                                                                    1000

Then work out the moles of H2O2 by the 5:2 ratio.

Then calculate concentraion by: =                  moles                   
                                                         (volume (cm3) / 1000)

Would this give me the concentraion of H2O2 in my orginal solution?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 26, 2009, 08:04:32 AM
Looks OK. That's assuming you used correct volume in the second part of your calculations.

Note:

Quote from: upsidedown on April 26, 2009, 07:21:19 AM
If I calculate moles of Potassium Manganate

PERmanganate.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 08:17:00 AM
Would I need to calculate the moles of permanganate ?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 26, 2009, 08:52:55 AM
Sorry, I have not noticed you were referring all the time to manganate. That's strange, I have never heard about manganate being used in potentiometric titration, are you sure that's what you are asked/plan to do?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 09:13:24 AM
Thanks for helping me Borek.

Yes, the plans says:
"One of your experiments should involve a redox titration with potassium manganate (VII)."

The plan also says that I have to used the end-point colour change to explain the redox chemistry in the titration reaction. I know that no indicator is required beacuse it should turn pink/purple from clear, but im unsure how to  explain the redox chemistry in the titration reaction. 
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on April 26, 2009, 09:35:30 AM
Well, manganate (VII) is just a permanganate.

No need for indicator:

http://www.titrations.info/potentiometric-titration-end-point-detection

Not sure what

Quote from: upsidedown on April 26, 2009, 09:13:24 AM
explain the redox chemistry in the titration reaction

means, other than what is already obvious from the reaction equation.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: upsidedown on April 26, 2009, 09:53:16 AM
In my second experiment I will be doing a gas collection, which seems easy enough, just add a metal oxide, messure O2 released, then work out the conc of H2O2.
The only problem is that this will give me different concentration from the first experiment, because all measurements are predictions. Do you know if this is acceptable in plans?
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: victoria_kay on May 09, 2009, 11:19:31 AM
I am also doing this plan... and I'm glad to hear I'm on the right lines because nobody at my college seems to have a clue with what they're doing.

I know that I need to dilute the hydrogen peroxide... but I do not know how. In other titrations I have used a 250cm³ volumetric flask to put 25cm³ of the solution being titrated in and then filled the rest up with distilled water or acid. Is it the same with this titration? Should I use 25cm³ of hydrogen peroxide? And how can I calculate the concentration of potassium permanganate to use?

Any help would be fantastic!
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on May 10, 2009, 04:24:50 PM
Permanganate commonly used in labs is 0.1M.

Not kowing - approximately - concentration of hydrogen peroxide, it is difficult to calculate dilutions. However, you should take into account optimum volume of titrant used - that is, your samples should contain enough hydrogen peroxide so that when titrating you will be using about 8o-90% of burette volue. That should allow backward calculation of dilution.
Title: Re: Finding concentration of H2O2 by redox titration.
Post by: victoria_kay on May 11, 2009, 01:30:25 PM
Thanks that was helpful :)

I have another question aswell... I am asked to describe another experiment to determine the concentration of the hydrogen peroxide. I know to do a gas collection of O2 from the catalytic decomposition of hydrogen peroxide. However I am asked to do a calculation before to determine how much hydrogen peroxide I should use in the decomposition. Obviously I have to take the capacity of gas syringe into account but I'm not sure whether to use the fact the hydrogen peroxide is "100-volume" to calculate this or use the equation 2H2O2 -> 2H2O + O2 will it make any difference?

Thanks again

Title: Re: Finding concentration of H2O2 by redox titration.
Post by: Borek on May 11, 2009, 02:00:37 PM
Using 100-volume should give an instant answer.