Magnesium electrolysis

[Donci6552]

I electrolysed a piece of magnesium, connected to the positive terminal, hoping to get a thin MgO layer but instead I got this greyish black layer. The electrolyte I used was sodium bicarbonate, could it have been carbon that deposited on my product? Or does magnesium react with the sodium hydroxide formed? Please help!

[Enthalpy]

Hi Donci6552,

 

electrolysis is tricky and it's hard to predict and even to explain afterwards.

 

Even if you had anodized the metal properly, the layer's colour depends on details of the alloy. "Magnesium" is probably a magnesium alloy. For instance pure Al makes a brilliant oxide layer but Al-Zn alloys a grey one.

 

Apparently, Mg can be anodized in aqueous electrolytes. But is a bicarbonate the best choice?

[John Cuthber]

It's perfectly possible for a clean surface of a metal to look nearly black.

A stack of razor blades can be used as a light absorber.

https://books.google.co.uk/books?id=fdhGGY4Xw_QC&pg=PA216&lpg=PA216&dq=razor+blade+light+trap&source=bl&ots=ixfj6aKDHd&sig=nPvE3hPOyQzO8pNKMv60yjiDeSU&hl=en&sa=X&ved=0ahUKEwjy95bt-KzMAhUKM8AKHdecB_MQ6AEIIjAB#v=onepage&q=razor%20blade%20light%20trap&f=false

[Sensei]

I electrolysed a piece of magnesium, connected to the positive terminal, hoping to get a thin MgO layer but instead I got this greyish black layer.

You electrolyzed piece of magnesium? Do you mean solid magnesium? Underwater?

Magnesium reacts (at higher temperatures violently) with water to form Magnesium Hydroxide and Hydrogen gas.

MgO also reacts with water, forming Mg(OH)₂

So you could have Mg(OH)₂ ,which is white, and almost not soluble, layer on the bottom of container with water.

 

If you want to make MgO from Mg(OH)₂ you should boil entire water,

and then continue, increase temperature above 332 C.

It's mentioned on wikipedia

https://en.wikipedia.org/wiki/Magnesium_hydroxide

 

The electrolyte I used was sodium bicarbonate, could it have been carbon that deposited on my product?

 

When I was doing electrolysis with sodium bicarbonate alone and 2L water, using 230 V, electrodes were also covered by black material.

Even if electrodes were 24 carat Gold. Without presence of Magnesium. It's carbon layer.

 

[Donci6552]

Thanks for the help. And for some reason my magnesium doesn't react at all with room temperature water.

[Sensei]

Thanks for the help. And for some reason my magnesium doesn't react at all with room temperature water.

 

Maybe it is not Magnesium.. ? Can you make photo?

 

Within couple seconds it should be covered by Hydrogen bubbles

 

What happens when water is heated to f.e. boiling point.. ?

Do you have enough of it to measure its density?

It should be much lighter (1.738 g/cm³) than Aluminium (2.7 g/cm³)

Edited April 29, 2016 by Sensei

[Enthalpy]

For some reason my magnesium doesn't react at all with room temperature water.

Just like everyone's magnesium. That's normal life, which differs from YouTube.

[foxy john]

Magnesium don't react violently with water at room temperature, you will get slow bubbling over an hour or so.

[Enthalpy]

And if you put a piece of polymer in water, you also get bubbles on it, from the dissolved air.

 

A hint that magnesium doesn't react quickly nor strongly with water is that you can hold it in your hand without getting burns.