monolithic
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Cleaning oxides from Mg metal
Lately I've been working on the NurdRage sodium synthesis and it's a challenging reaction, per my own results and per NurdRage himself. I've had a lot of failures due to poor quality Mg. It seems the majority of amateur suppliers ship their turnings and ribbon
in low quality zip loc style bags. My best result to date is from Mg ribbon which I clipped into small pieces with scissors. The ribbon had visible
oxides so I assume it reacted so well because of the fresh ends on every piece I clipped.
I'm currently running a reaction on tarnished Mg turnings, which I cleaned up using dilute HCl. This sounds simple but is harder than it seems. If the
acid concentration is too high (>3%) it burns through the Mg. If the acid concentration is too low (<1%) or if the concentration is correct but
too little dilute acid is used per unit mass of Mg, it for some reason won't brighten the surface of the Mg. I've found that the following works
great. This is a vigorous reaction with turnings and it evolves considerable amount of hydrogen gas as it burns through the oxide
coating and reaches the pure Mg. Do not do this near ignition sources, and it's probably not a good idea to do this reaction on Mg powder or Mg dust
as it will likely be an uncontrollably violent reaction.
To a 500 ml beaker add 255 g water and 16 g impure Mg turnings. Measure 15 g conc. HCl and then rapidly, over the course of 5 seconds, pour the HCl
into the beaker containing the water and Mg turnings. Swirl for 5-10 seconds until the Mg turnings turn bright and shiny, and then immediately pour
into a Buchner funnel set up for vacuum filtration. Vacuum filter the turnings, wash with 2 x 25-50 ml water followed by 2 x 25-50 ml methanol. If you
want to preserve the shininess then dry under vacuum and store under a blanket of inert gas. Otherwise let air dry in front of a fan for 5-10 minutes.
Total recovery from 16 g impure turnings will be about 14.5 g, so a 90% recovery.
For sodium synthesis I fan dry. The metallic shine fades rapidly but it's a much thinner oxide layer: 5-10 minutes of atmospheric exposure vs months
or years of atmospheric exposure in a poorly sealed plastic bag. I typically use the turnings immediately after drying, so once dried I will grind
them in a mortar and pestle for about a minute and then immediately dump into my reaction solvent. 
[Edited on 9-19-2020 by monolithic]
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SaccharinSlayer157
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How thin is the ribbon you are using? Whenever I wanted clean magnesium I always just hit it with a bit of sandpaper because the oxide is usually
loosely bound and acid was too much trouble to get right. This might not work on too thin of ribbon though.
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monolithic
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Quote: Originally posted by SaccharinSlayer157  | | How thin is the ribbon you are using? Whenever I wanted clean magnesium I always just hit it with a bit of sandpaper because the oxide is usually
loosely bound and acid was too much trouble to get right. This might not work on too thin of ribbon though. |
About 0.25 mm, according to my calipers. Yes, it might eat through all of the Mg if you're using exceptionally thin ribbon.
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