Sciencemadness Discussion Board

Removing magnesium tarnish

Esplosivo - 22-1-2006 at 07:43

I've managed to buy approx 400g of Mg powder at a very cheap price (approx. $15). The only problem is that it is somewhat tarnished. I know that Mg reacts quickly with oxygen in humid environments to form a surface coating, in many ways similar to Aluminium. The colour of the 'tarnish' is grey/black. I now this colour is normal but since MgO2 is white, what might this grey layer be?

Now a couple of questions. Will this layer affect the reactivity of my Mg powder so much? For example, can it be used for prep. of Grignard's reagent? If no, is there any possible way to remove this surface layer? I was thinking about a quick wash with a dilute acid, say nitric or hydrochloric, the Mg being in a filter paper in a funnel. The acid quickly flows and reacts with the Mg oxide layer making up the tarnish. After this wash acetone is used to dry the Mg, and this Mg is then sealed. Does this seem effective to you? Thanks for the help.

[Edited on 22-1-2006 by Esplosivo]

solo - 22-1-2006 at 08:01

Magnesium [7439-95-41 M 24.3, m 651°, b llOOo, d 1.739. slowly oxidises in moist
air and tarnishes. If dark in colour do not use. Shiny solid should be degreased by washing with dry Et20, dry
and keep in a N2 atmosphere. It can be activated by adding a crystal of I2 in the Et20 before drying and storing.

...................."Purification of Chemicals in the Lab", pg.347

F i f t h E d i t i o n
Wilfred L. E Armarego Christina Li Lin Chai

Esplosivo - 22-1-2006 at 08:06

So removing the tarnish should be as easy as adding some I2 dissolved in ether? I read that entry myself before posting here but ended up posting anyway because I wasn't sure what it really was referring to and if the 'activation' part meant to remove the oxide coating. Thank you solo.

[Edited on 22-1-2006 by Esplosivo]

Magpie - 22-1-2006 at 09:37

I have a similar quest: to obtain some Mg that is suitable for Grignard work, at the lowest price. I have found a source but it seems too expensive. I have even thought of buying plate Mg and making my own turnings with a drill. But plate Mg I've found is rather impure with up to 4% Zn or such. In looking at the specifications of what the chemical suppliers sell it seems that it is important that the Mg be 1) turnings, and 2) of high purity.

I have often heard of this technique of adding a crystal of iodine to initiate a recalcitrant Grignard reaction. Does anyone know how the iodine works to do this?

garage chemist - 22-1-2006 at 11:03

Magpie, buy only chemically pure magnesium.
I have some plates of 98% magnesium (technical alloy) and shavings from those plates do NOT work for grignards.

However, granulated magnesium (grain size about 0,5mm) from a chemical supplier works nicely after addition of a small iodine crystal (to the dry filings, followed by short heating) and refluxing the ether/alkyl halide solution for a few minutes over it.
When the grignard starts, the initially waterclear liquid turns turbid and greyish and the Mg gets thrown around in the liquid.
Ethyl bromide forms a grignard very easily, others, like iodobenzene, require longer refluxing before the reaction initiates.

Tarnish is best removed from magnesium by grinding the filings with a mortar and pestle until every particle has at least one bright spot. This is recommended before any kind of grignard and even with fresh filings!

[Edited on 22-1-2006 by garage chemist]

Esplosivo - 22-1-2006 at 11:34

Thank you for the hint garage chemist. I bought the Mg powder from a chemical supplier, but it was somewhat old, and since I am a frequent customer he offered me a discount. It wasn't really my intention to buy so much Mg, but I reasoned out that a similar offer would not present itself so easily again so I took it. Thank you for the help everyone.

Swany - 22-1-2006 at 18:41

It should be noted that the coating of oxide of the Mg, while it has a similar appreance to Al; is totally different, chemically. The MgO is not strongly bonded to the metal, and thus acts less like a protective layer. On the other hand, the Al2O3 oxide forms a rather strong bond lattice with the Al metal. To treat Mg to make it less reactive to water for pyrotechnic compositions, it is treated with K2Cr2O7. This forms a chromium oxide/Mg complex that acts like the Al2O3 layer, I think. Mumbles posted a large explination for this at APC, and if interested I could provide a link.

I also have some 99.99% pure Mg, in ingot form. If interested contact me, I am sure I could somhow make some shavings for anyone who really wants some.

evil_lurker - 22-1-2006 at 20:39

You might want to try this outfit:

Purity is 99.95%, magnesium is sold in ingot or shaving form, rices seem to be reasonable $7.00 per pound for 1-3 pounds and $3.85 for shipping, and they deal with small quantities.

A full ingot is trapezoid in shape and roughly 2.5" high X 3.25-3.75" wide X 23"-24" long

I had planned on purchasing a few pounds and drilling them under a dry volatile non-flammable solvent using a drill press set to the lowest speed and a large silver & deming drill bit then washing them with dry ether.

I'm not for sure which is best size/shape for the turnings, but I am going for the long thin "curly q's" to start out with. If that don't work, I'm switching over to a reamer, upping the speed, and going for some chips.

The_Davster - 22-1-2006 at 21:31

There is a picture of a 3lb block(I could not resist the price break) in Bromic's book under magnesium if you are wanting to see what Mg from looks like. The bottle of shavings beside it came from cutting the piece shown out of the block with a hacksaw. The shavings have lost reactivity overtime, they used to burn with a bright white light and giving off a cloud of Mg3N2, but now they emit an orange glow and do not really combust, there is no flame usually and you end up with a pile of the oxide. So for any reaction, fresh shavings are definatly an advantage. I would like to get a full ingot sometime, not that I'll ever get through the 3 pounds I have now, but for use as a decoration!.

Explosivo, how fine is your powder anyway? Would it permit a quick dilute acid wash and then dry ether wash, or is it so fine it would dissolve completly quickly.

Does anyone know if the "If dark in colour do not use" from solo's post is a "do not use or bad things will happen" or a "do not use or you will get no reaction"?

I was also under the impression that the grignard synth could be violent, would a powdered Mg be too reactive here?
1,2-dibromoethane can also be used instead of I2 to get the reaction going.

[Edited on 23-1-2006 by rogue chemist]

Esplosivo - 22-1-2006 at 22:07

The Mg powder is quite fine, <1mm actually. It would require a v. dilute and weak acid to remove only the coating without dissolving the rest of the metal with it. I was thinking of acetic acid.

Regarding the quote from 'Purification of lab chemicals' by Solo, I too was and am in doubt about it's meaning. It doesn't seem clear enough.

[Edited on 23-1-2006 by Esplosivo]

woelen - 22-1-2006 at 23:33

Just based on logic, I would expect "no reaction will occur" or "reaction will occur slowly and with lot of impurities". The reason I expect this is that MgO is not very reactive and besides that, it is basic and as such may interfere with the grignard reactions also (forming water, if any HX is formed in the organic reaction). Grignard reactions are quite critical. So, I would give it a try with a small volume and would not worry too much about a very violent reaction occurring. Of course, always first try with small test tube quantities.