Determining conc of H2O2 by titration

[electric.avenue]

Homework Statement

I am planning a titration to determine the concentration of commercial 20 volume hydrogen peroxide. I will be titrating it against potassium permanganate. I know that acid, H2SO4, must come into it too, but am unsure as to whether this is added to the H2O2 solution, or the potassium permanganate solution.

I have done very little practical work in chemistry, as I attend an evening class, and I need to know:

is diluting the 20 volume hydrogen peroxide on a 1: 9 basis with distilled water an adequate dilution?

what volume of diluted solution should I make up?

what volume of diluted solution for each titration?

what conc of potassium permanganate to be used?

what conc of H2SO4, and what to add it to?

Any help gratefully received.

Thanks in adv.

Homework Equations

The Attempt at a Solution

 

[Borek]

First things first. To understand what is going on and why you need to know reaction equation. Some information (like titrant concentration and some procedure details) will be probably given by TAs, you won't be able to guess them. Optimal amount of titrant is about 80-90% of burette volume. Then it all boils down to simple stoichiometry.

You may check some books on analytical chemistry, this is a standard assay.

Borek

 

[electric.avenue]

Thanks for getting back to me, Borek.

Yes, I do have a reaction equation for this:

2 KMnO4 (aq) + 5 H2O2 (aq) + 3H2SO4(aq) --->

2 MnSO4 (aq) + 5O2 (g) + 8H2O (l) + K2SO4 (aq)

No indicator needed because the solution will turn pink when all the H2O2 has reacted.

Do I add the H2SO4 to the potassium permanganate solution that goes into the burette, or is it added to a solution of the hydrogen peroxide?

I am also very uncertain about what concentrations to use, as different websites recommend different ones, and I have done v little labwork due the the fact that the class I attend is an evening class, and we have very little time.

The hydrogen peroxide that we have to determine the concentration of is 20-volume, so would a potassium permanganate solution of 0.05 M be reasonable for this?

And to what extent would I need to dilute the commercial hydrogen peroxide? Would 10 ml of commercial 20-volume hydrogen peroxide to 90 ml of distilled water be reasonable?

 

[chemisttree]

When I plan titrations like this, I assume that I will use about half of the graduated buret volume but you could do as Borek suggests and plan to add between 80% to 90% of the buret volume if you wish. You should be using a 50 mL buret so plan on adding about 25 mL of titrant.

You will add an easily measurable amount of peroxide to the erlenmeyer flask and dilute it with some water (100 mL sounds fine). How much H2O2 solution (analyte) you add depends on your glassware. Ideally you would add the peroxide using a Class A volumetric pipette. One or five ml should be appropriate. These are calibrated 'to deliver' (TD) so don't push out the last bit of peroxide that is trapped in the narrow capillary at the tip. (Technique points) Just touch the pipette tip to the inside of the erlenmeyer and let gravity do ALL the work. If you are using graduated cylinders to add the peroxide, use about half of the volume of the graduated cylinder. I would use a 10 mL cylinder filled to 3 to 7 mL (you decide how much). This glassware is calibrated 'to contain' (TC) and so the analyte needs to be quantitatively transferred to the erlenmeyer flask. This involves at least two rinses with water, added to the erlenmeyer each time, to complete the transfer. Add the acid to the flask and begin the titration. Add the acid as a dilute solution (0.1M) to prevent decomposing the peroxide. You will need to determine how much 0.1 M H2SO4 to add from the equation you have. Add about 20% excess acid to this reaction.

In a 20% H2O2 solution, how much (moles) H2O2 do you have? For the "approximate" calculation you can assume a w/v solution. Examine your reaction and determine how many moles of permanganate need to be added to neutralize this. This is the amount of permanganate that should be present in about half of the volume of the graduated buret or 80% to 90% of the buret as Borek suggests. What is this target concentration of permanganate?
Make up APPROXIMATELY this strength of permanganate, ie. use an even number. For example, don't try to make up 0.0483 molar when approximately 0.05 molar will do. Express the concentration to at least 3 significant figures. If you are aiming for 0.05 M, you can express this as 0.0512 M, for example. If you make up one slightly stronger, you will add less during the titration. If it is slightly weaker, you will add more during the titration, no big deal.

Don't forget to do at least three replicates with your analyte and one blank. Blank = water + acid only.
Analyte = 3 - 7 mL H2O2 + water + acid.

 

[Borek]

Note: 20 volume H2O2 is not necesarilly 20%. Most likely it is 6%. 20 volume refers to the fact that 1 volume of the solution - when decomposed - produces 20 volumes of oxygen.

Borek

 

[electric.avenue]

Thanks v much. I ended up getting an A for Chemistry.

 

[Spring lady]

Hey. I have a problem with this titration. Specifically, I need to determine the concentration of hydrogen peroxide during his catalyzed decomposition. But I do not understand how can I determine the concentration of hydrogen peroxide after 15 minutes of the beginning of reaction when hydrogen peroxide is continuously decomposed? Please help me. The answer to this question I need as soon as possible. Thanks in advance. :)