Make Manganese Dioxide Electrodes - Revisited


Uploaded by NurdRage on 15.04.2012

Transcript:
Warning: Nitric acid is corrosive and nitrogen dioxide is toxic.
This experiment should be performed outside or in a fume hood.
Greetings fellow nerds.
When doing preparative electrolysis a big issue is the severe corrosion or passivation of most anodes.
Manganese dioxide electrodes make good cheap anodes
and two years ago I experimented with them according to some posts I found in sciencemadness.
While I could reproduce cobalt oxide with manganese dioxide on titanium electrodes,
I couldn’t reproduce straight manganese dioxide on titanium electrodes
as they kept passivating with less than an hour of use.
I suspected it was because I had impure manganese nitrate
but my trick of removing iron by using manganese hydroxide didn’t work very well on manganese nitrate.
The source of the problem with the original method was that we used crude manganese dioxide from batteries.
But in another video I showed how to make highly pure manganese dioxide
from potassium peroxymonosulfate and purified manganese sulfate.
Today we’re going to revisit manganese dioxide electrodes
but this time ensuring we make pure manganese nitrate at the start.
Take 1 gram of highly pure manganese dioxide that we made in a previous video.
Now add in 1.45g of oxalic acid dihydrate.
Then add in 5 mL of water.
The oxalic acid will quickly react with the manganese dioxide
to form manganese carbonate along with water and carbon dioxide.
When the reaction slows down, carefully add in 1.7mL of nitric acid a few drops at a time.
The nitric acid will neutralize the manganese carbonate
and produce manganese nitrate and carbon dioxide.
After all the nitric acid is added, wait until the bubbling stops.
Now pipette the liquid to another container, away from the excess manganese dioxide.
This solution of manganese nitrate can be used directly but I recommend evaporating it
until you produce a saturated solution for best results.
You can tell when it starts to precipitate pink or white powder or crystals of manganese nitrate.
Now to make the manganese dioxide electrodes.
Just like the previous video,
get a titanium strip and after sanding it etch it with hot hydrochloric acid until it the surface starts bubbling hydrogen.
Over here I’m covering my hot plate with aluminum foil to keep it clean.
Now with the titanium in the acid when it bubbles evenly it’s ready to go.
Wash off the excess acid with distilled water.
Apply a layer of manganese nitrate solution to the titanium
and then place the titanium on the hot plate set to least 300 celsius.
Press down with a rod or spatula and as the electrode heats up the manganese nitrate will decompose
into manganese dioxide and release nitrogen dioxide.
When the electrode stops belching out brown nitrogen dioxide,
immerse the electrode in water and wipe away any loosely bound manganese dioxide.
Then apply another layer of manganese nitrate and repeat the process as many times as necessary.
Eventually you’ll have a manganese dioxide on titanium electrode.
This electrode can be tested by electrolyzing a weak sulfuric acid or a sodium chloride solution.
Unlike the impure electrode I made last time this one passivates far slower and is quite usable as an anode.
I’ve seen it last for weeks when used with low current density.
So there you have it,
I failed the first time but through this we found purity is very important to making manganese dioxide electrodes.
Special thanks to the sciencemadness discussion board.