Sciencemadness Discussion Board

Storing Potassium Manganate(VII) Solutions

NEMO-Chemistry - 27-8-2016 at 03:52

Sorry i couldnt get the subscript to work in the title, so i used the IUPAC name.

Anyway I have several sub molar solutions of KMnO4, stored in clear glass reagent bottles with ground stoppers, kept in a cupboard.

Most are around 4-5 weeks old and have started to go brown! The bottles are stained inside and there is a light brown precipitate? on the bottom of the bottles.

My question is about storage. How do you normally store fairly dilute solutions (<0.1M) of KMnO4 safely and without it degrading?

I assume ideally a dark bottle but i do keep mine in the dark.

Is there anything i should watch for on storage?

I did read a MSDS on it but the storage info was for the crystals, my crystals are kept in a proper container with some other Oxidizer's, well away from other chemicals etc.

Sorry its a crap question, i did try to find information but i got conflicts. I prefer to keep some Solutions as i am experimenting with it at the moment.

NEMO-Chemistry - 27-8-2016 at 03:58

Its ok i had a brainwave, wrap the bottle in tinfoil to exclude light. but havnt found anything to suggest it degrades into anything nasty, mainly Mn2 (ref. random forum post elsewhere).

MSDS that i have checked just say keep away from the obvious like Reducers and strong acids, nothing about decomposition, so i am going to assume it wont become a hazard over time.

Sorry for the waste of time question.

blogfast25 - 27-8-2016 at 04:39

Light is certainly the enemy here but so is too high pH. Adding 2 - 10 % H2SO4 increases the shelf-life no end. Another source also mentioned adding small amounts of MnSO4.

Using good quality starting materials (absent of oxidisables!) should really go w/o mentioning.

A study I found years ago showed that properly formulated KMnO4 0.1 N (Normality = 0.1) solutions could be stored for years without appreciable loss of titre.

[Edited on 27-8-2016 by blogfast25]

NEMO-Chemistry - 27-8-2016 at 05:11

Quote: Originally posted by blogfast25  
Light is certainly the enemy here but is too high pH. Adding 2 - 10 % H2SO4 increases the shelf-life no end. Another source also mentioned adding small amounts of MnSO4.

Using good quality starting materials (absent of oxidisables!) should really go w/o mentioning.

A study I found years ago showed that properly formulated KMnO4 0.1 N (Normality = 0.1) solutions could be stored for years without appreciable loss of titre.


Thanks thats really interesting, H2SO4 would be ideal for some of the solutions.

At the moment the solutions are not of any known concentration, for the type of experiment i was conducting the concentration wasnt critical.

I will however be making some for titration's, so i will take more care with this.

I like the old style glass reagent bottles, but they are not always easy to get with the correct name frosted on the Glass. I havnt as yet come across any cheap amber ones, I do get the odd couple of old reagent bottles from school though.

I should spend some of my upcoming exam money on containers and reagent bottles, i find it hard to spend on containers when i still have a long list of reagents to get.

Thanks for the info, I find KMnO4 fascinating in its chemistry. I have hardly scratched the surface yet but its a really interesting chemical, and pretty safe to use as long as stupidity avoided.

I am interested in the oxidation states and colour changes etc, and havnt bothered with the Glycerine etc for now.

Texium - 27-8-2016 at 06:18

Storing the solutions in amber bottles and keeping them in a dark cabinet will also help a lot. Better yet, only prepare a solution fresh when you need it. I try to avoid storing permanganate solutions, and just make up a new solution whenever I need one.

Also keep in mind that the photodecomposition reaction will also affect the solid albeit at a much slower rate, so make sure to keep your solid potassium permanganate in an amber or opaque container too. Mine is in an amber glass jar at the back of my cabinet behind all of the other oxidizers.

blogfast25 - 27-8-2016 at 06:20

http://nvlpubs.nist.gov/nistpubs/bulletin/08/nbsbulletinv8n4...

Quite old but relevant on permanganate solutions, standardisation of, and titrations with.

NEMO-Chemistry - 27-8-2016 at 07:13

Thanks for the advice and the PDF link. There is alot of information in the Pdf, i will give it a good read tonight.

My crystals are in a standard white square HDPE reagent container, but i do keep most of my chemicals in cupboards with the doors closed so its dark inside.

AJKOER - 28-8-2016 at 06:09

Re-iterate the comment on purity, especially with respect to possible transition metal presence in low grade H2SO4 upon adding to KMnO4.

Test by adding a very small amount of Fe(ll)/Fe(lll) to a sample of the H2SO4/KMnO4 mix. The reaction is given by (see http://www.chemteam.info/Redox/WS-redox-titration-problems.h... ):

5Fe(2+) + 8H+ + MnO4¯ ---> 5 Fe(3+) + Mn(2+) + 4 H2O

With light, I would expect an even faster reaction as the Fe(3+) is converted back to Fe(2+).

[Edited on 28-8-2016 by AJKOER]