Lab 12.4: Effect of Catalyst on Reaction Rate: Do we need to add H2SO4 to Control Hydrogen Peroxide Solution (Beaker A) before Titration ?

[Meena Iyer]

As per the instructions there is no need to add Sulphuric Acid to the 'pure' Hydrogen Peroxide Solution (Beaker A) before titration. However when we titrate with potassium permanganate, with just a few mm of potassium permanganate the solution became dark brown and some more drops dark purplish. But book says about 35 mm of titrant will result in the desired purple color. So we thought maybe a slight error in the instructions. Repeated after adding sulphuric acid just like the rest of the beakers. This time it was colorless for more than 20 mm of the titrant but after adding some more ml of titrant the solution became 'medium' brown with some residue at the bottom. What is this residue? And do we need to add H2SO4 before titration?

I am not sure what is happening. I strongly suspect potassium permanganate which was purchased from the medical store. It came in a small satchet in small purple/blue crystals. When we used this potassium permanganate solution on filter paper (activity in lab 10.2) the color changed from purple to brown - no green color.

Please help. After donating my blood for this exp. don't want to give up.

Meena

[Robert Bruce Thompson]

The instructions are correct as written.

The brown precipitate is manganese dioxide (Mn+4). It's unlikely that
your KMn04 is the problem. More likely, the hydrogen peroxide is of
lower concentration than expected. Drugstore H2O2 is nominally 3%, but
that concentration decreases rapidly if the bottle is opened, and even
an unopened bottle may contain much less than 3% if it's more than a few
months old.

[Meena Iyer]

Our hydrogen peroxide was 6% which we diluted to 3%. After your response I purchased another bottle again 6% but from a different manufacturer. This time I didn't dilute and used as is. With just a few drops of potassium permanganate brown precipitate started forming and adding more drops made the solution darker and darker and more precipitate. Never we got anything close to pink.

I checked online and in many cases they are adding sulphuric acid to H2O2. I tried titrating after adding sulphuric acid. This time each time I add a drop of potassium permanganate slight pinkish hue which disappeared after some shaking and this continued for quite a few drops. Then at some time it started turning brown and then it started behaving like the earlier case without H2SO4.

Meena

[Meena Iyer]

I tried again with a supposedly lab-grade quality of potassium permanganate for the rest of the beakers. For Beaker C (30 secs) the vol of KMnO4 was between 13-14 ml and that for Beaker F (240 sec) it was 5 ml. For Beaker A (with H2SO4) the vol was 28.8 ml. Note that Beaker C - F have been sitting for few days while Beaker A was freshly made. For Beaker D (60 sec) it was less than 3 ml and for Beaker E (120 sec) it was 1.8 ml. Little wierd results for D and E.

Now my question is: Are these results acceptable? Have we screwed up big time?

And also though you are insisting that there is no need to add sulphuric acid, but without making the solution slighly acidic for Beaker A the exp failed?

And another related question: in an redox-titration for determining the concentration of potassium permanganate, the potassium permanganate solution was put in the buret. I thought that the analyte is placed in the flask/beaker but here it is put in the buret. Is this an exception or it does not really matter which is placed where?

Sorry for bothering you with my stupid questions.

Meena