Sciencemadness Discussion Board - Why has my solution of potassium permanganate and copper sulfate turned orange?

Why has my solution of potassium permanganate and copper sulfate turned orange?

itsallgoodjames - 15-12-2020 at 17:09

A few days ago I made a solution of potassium permanganate and copper sulfate in isopropanol. The aim was to oxidize the isopropanol to acetone. I had read somewhere that the reaction was catalyzed by sulfate ions. I used copper sulfate. The reaction worked, though with further reading, I don't think the CuSO4 was necessary. What's weird is over the last few days of sitting, it's turned orange. I have absolutely no clue why this has happened. Any ideas? As you can probably tell, I'm not very knowledgeable in organic chem.

Thanks in advance for the help

Edit- my only thought is that maybe it's some weird double salt, but that doesn't really explain why it's orange...

[Edited on 16-12-2020 by itsallgoodjames]

DraconicAcid - 15-12-2020 at 21:20

That's....weird. you also have a pale-coloured precipitate at the bottom?

itsallgoodjames - 15-12-2020 at 21:54

I as a matter of fact do have a pale blue precipitate at the bottom. I just assumed it was undissolved copper sulfate, but it's too light to be that.

DraconicAcid - 15-12-2020 at 22:05

Actually, if it's anhydrous, or even the tri/monohydrate, it will be quite pale.

woelen - 16-12-2020 at 00:30

I think that yopur permanganate is reduced to MnO2 and that the latter is very finely dispersed in the liquid. This brown/orange color is quite common with permanganates, reduced to MnO2. You can also observe this in aqueous solution (e.g. add a dilute solution of KMnO4 to a solution of sugar in dilute NaOH). You get all kinds of intermediate colors (green and bluish tints), but finally you end up with an orange/brown and somewhat turbid solution of hydrous MnO2.

The pale blue precipitate is either partially dehydrated CuSO4.xH2O (x < 5), or maybe some impure copper(I) compound. Copper(I) is colorless/white in many of its compounds, albeit not in all of them (it can form brick red anhydrous Cu2O or yellow hydrous CuOH/Cu2O).

DraconicAcid - 16-12-2020 at 00:32

Quote: Originally posted by woelen

The pale blue precipitate is either partially dehydrated CuSO4.xH2O (x < 5), or maybe some impure copper(I) compound. Copper(I) is colorless/white in many of its compounds, albeit not in all of them (it can form brick red anhydrous Cu2O or yellow hydrous CuOH/Cu2O).

Copper(I) halides would be white, but it's tough to imagine that an alcohol would reduce a copper(II) salt to copper(I), especially when there's manganese salts around to reduce.

woelen - 16-12-2020 at 00:37

I can imagine that reduction. Once the manganese is used up and all is reduced to MnO2, then there still is a lot of alcohol around. I think that in this experiment, the initial amount of permanganate was low, given the weakness of the orange color and the absence of a brown precipitate of MnO2. Under these conditions I do not think that manganese will be reduced all the way to Mn(2+). Maybe a little Mn(3+), but definitely not further. The Mn(3+), if it forms, will also be brown/orange in solid Mn2O3 and solid Mn3O4.

DraconicAcid - 16-12-2020 at 00:39

I'm pretty sure that it's easier to reduce Mn(III) to Mn(II) than to reduce Cu(II) to Cu(I).

woelen - 16-12-2020 at 01:11

This only is true in acidic solutions. E.g. a solution of sugars at high pH is capable of reducing copper(II) to copper(I), in the form of the brick-red Cu2O or the yellow/orange hydrous CuOH/Cu2O.nH2O. Such a solution, however, does not reduce manganese all the way down to its +2 oxidation state. It goes to +4, maybe to +3, but not further. So, in this experiment with the isopropanol I can also imagine that a similar thing occurs.

itsallgoodjames - 16-12-2020 at 06:04

Quote: Originally posted by woelen

I think that yopur permanganate is reduced to MnO2 and that the latter is very finely dispersed in the liquid. This brown/orange color is quite common with permanganates, reduced to MnO2. You can also observe this in aqueous solution (e.g. add a dilute solution of KMnO4 to a solution of sugar in dilute NaOH). You get all kinds of intermediate colors (green and bluish tints), but finally you end up with an orange/brown and somewhat turbid solution of hydrous MnO2.

The pale blue precipitate is either partially dehydrated CuSO4.xH2O (x < 5), or maybe some impure copper(I) compound. Copper(I) is colorless/white in many of its compounds, albeit not in all of them (it can form brick red anhydrous Cu2O or yellow hydrous CuOH/Cu2O).

Thanks! I'll have to see if the MnO2 settles out on the bottom over the course of the next few days. I wasn't there while the reaction was happening, as I had to leave right after it began and wasn't present for multiple days. It very well could have done that color thing, but I'm not sure, as I wasn't there when the reaction was happening.

RustyShackleford - 16-12-2020 at 08:12

suspension of MnO2

Bedlasky - 16-12-2020 at 08:13

Quote: Originally posted by woelen

This only is true in acidic solutions. E.g. a solution of sugars at high pH is capable of reducing copper(II) to copper(I), in the form of the brick-red Cu2O or the yellow/orange hydrous CuOH/Cu2O.nH2O. Such a solution, however, does not reduce manganese all the way down to its +2 oxidation state. It goes to +4, maybe to +3, but not further. So, in this experiment with the isopropanol I can also imagine that a similar thing occurs.

This isn't necessarily true. If you add excess of glucose or fructose in to alkaline permanganate/manganate/hypomanganate/MnO2/MnO(OH) solution/suspension, there is a slow formation of colourless Mn(II) complex with glucose/fructose. This complex is stable if excess of sugar is present. You can see formation of brown ring at the surface, which disappear when you swirl the test tube.

https://www.sciencemadness.org/whisper/viewthread.php?tid=32...

http://www.sciencemadness.org/smwiki/index.php/Hypomanganate