Error by a factor of 10 in peroxide/permanganate titration

[mrbelliveau]

Hi all,

Trying to get this titration working correctly and for the life of me, I can't figure out where my error is. This is what I have:

Burette contains 0.01M KMnO4 (I'm positive that my error does not come from preparing this - 0.4g in 250ml of water, with a molar mass of 158g/mol. Additionally, when I standardize it with Mohr's salt, everything works out fine)

The anylate is prepared by doing the following (I suspect the error must be somewhere in here):
- add 1 ml of industrial (30% or 9.8M) H2O2 to 99ml of water, making 0.3% (or 0.098M, essentially 0.1M) H2O2
- then add 5ml of this dilute H2O2 to the Erlenmeyer flask to be titrated, along with ~20ml of 3M sulfuric acid (more than enough to provide the acidic medium).

In the balanced redox reaction, it shows 5 peroxides reacting per 2 moles of permanganate ...so using a bit of math, it should take about 20 ml's of 0.01M permanganate ion to reach the endpoint. I use this equation to account for the balanced stoichiometric coefficients:

(volume of H2O2) * (concentration of H2O2) * (2) = (volume of permanganate) * (concentration of permanganate) * (5)

I created this experiment a year ago and in my chicken-scratchy notes, I claimed to have used an average of ~21ml of KMnO4, but this year I feel like I'm following the same steps, and my endpoint is being reached after about 2.0 - 2.3ml, off by a factor of about 10 from the theoretical endpoint (and from previous years). Not sure where else I am going wrong. I'm sure it's something very simple! Anyone out there familiar with this titration (or titrations in general) who may be able to shed some light on the location of my error?

Thanks in advance for any help you can provide!

Postscript: I know there is a phenomenon that exists where a pseudo end-point is reached in this type of redox titration early on in the titration, but I've added a substantial amount of MnO4 after the end-point I'm reaching and it has shown no sign of fading.

 

[Borek]

Expected 20 mL looks more or less correct (I got 19.6, so exactly the same order of magnitude).

Perhaps you used 3% peroxide this year? Or the old one decomposed, it is not that stable after all.

 

[mrbelliveau]

Thanks for the suggestion. The bottle was purchased and opened for the first time a little over a year ago and has been in a sealed, clear, plastic bottle in a dark cabinet in a warm climate. I also thought about it decomposing a bit, but find it hard to believe that it dropped from 30% all the way to 3%. Other people had access to it besides myself, so I guess I can't positively attest to its purity.

I might just try purchasing a bottle of 3% commercial peroxide and trying the same thing (but with a 1:10 dilution).

Thanks again!