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Author: Subject: McLeod Gauge
RiP057
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[*] posted on 14-4-2011 at 19:55
McLeod Gauge


I have recently aquired a swiveling McLeod Gauge
much like this one

actually it is exactly like that one, but it is totally wrecked with oxidized mercury.

I was wondering if the board knew any way to clean this beautiful instrument.

I have been taught to use HNO3, but I was wondering if there were any other milder solutions that can be used to clean this piece of glassware, or if it is just the HNO3.

As far as the nitric acid goes would one use conc.? or 10%?
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mr.crow
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[*] posted on 14-4-2011 at 20:30


Very nice! nitric acid will dissolve Hg metal too, while HCl will just dissolve the oxide. Concentrated HNO3 would be pretty vigorous.



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RiP057
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[*] posted on 14-4-2011 at 20:59


so your recommending dilute nitric, and there is a little Hg left in the left most capillary where the readings are done, would dilute take care of the liquid mercury as well??

and yes I got a smashing price on it only fifty bucks!!!
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mr.crow
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[*] posted on 15-4-2011 at 06:21


Actually dilute HNO3 sounds like the best bet. In case any Hg metal is trapped in the oxides or on the surface of the glass. It might be hard to get good contact in the capillary tube. I have never done it before though, haha

To neutralize the Hg contaminated waste react it with NaOH to turn it back into HgO. Not carbonate, fizzing bad!




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RiP057
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[*] posted on 15-4-2011 at 07:00


Im going to give it a try first with the dilute then ramp it up if nothing occurs. As far as the gauge that I have now it is closed on the reading end like the one in the picture, however the one that I had used in the past had a little white stopper on the end of the reading capillary. This made for an easy clean of it, and would definately help get the stuck mercury out of that little corner..... dont know why this one is a complete body, maybe that is the design
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Wizzard
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[*] posted on 15-4-2011 at 07:51


Just be careful- Mercury compounds are toxic :)
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RiP057
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[*] posted on 15-4-2011 at 15:25


uhhh duhh!!!

but in better scientific prose....

only an amateur would dare mess with the highly toxic and deadly mercuric compounds w/o gloves

definately wearing gloves just alone for the nitric acid part, but doubly for the Hg (II) salts even if they are only Hg (I) salts.... Im pretty sure they are Hg (II)

but enough splitting hairs :)

[Edited on 15-4-2011 by RiP057]
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chemoleo
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[*] posted on 15-4-2011 at 17:14


If the Hg is oxidised, it means it's HgO, then convert it to HgCl2 by adding dilute HCl. With more HCl/NaCl, you can form the [HgCl4]2- complex, which is highly water soluble, so you can flush it out (if the HgCl2 per se isn't sufficiently soluble). Then neutralise the leftover, and add Na2S to precipitate lovely red HgS (pigment, safe).

The toxicity of Hg is undeniable, BUT there are FAR worse things out there. I think this should be said!




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RiP057
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[*] posted on 15-4-2011 at 17:37


thats actually a lot of work, I like what I was taught which was conc HNO3, but for safety's sake cause I havent ever seen one this dirty, I will first try dilute HNO3.

as far as neutralizing or anything of the like I am just going to bottle up the Hg(NO3)2/HNO3 solution and drop it off at hazardous materials where they will burn it. after all I am not that afraid of the mercuric salts, I used to use HgCl2 very often for some side project work ;)

The original post was there because I was wondering if anyone has had any experience with cleaning these gauges, and if they could point me in the right direction.

I will wash with water and then acetone and then let air dry, because I have been told that these things arent pyrex material so they can warp and break due to thermal shock...

however chemoleo I do like your approach the only problem is there is some mercury stuck in the left most capillary at the top where hopefully my vacuum pump will pull the mercury up to giving me .01 mm Hg vacuum which is always nice :)
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Fleaker
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[*] posted on 16-4-2011 at 08:55


Concentrated HNO3 doesn't work that well in dissolving clean mercury, the fastest way to dissolve mercury is much like that with silver, about 30% w/v.

Nitric is the way to go with this. After you've got the mercury dissolved, pour it into a plastic graduated cylinder and reduce it with a copper wire. As long as copper wire is present, the mercury won't redissolve.





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[*] posted on 16-4-2011 at 08:57


finally someone with some solid advice, I think I will start off with the 30% which is still pretty conc but dilute enough....

thanks!!!

ever done this before Fleaker?
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Fleaker
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[*] posted on 16-4-2011 at 09:04


Yes I have done this before, I'm not speaking from a guess--I have cleaned McCleod gauges and glass diffusion pumps that way.

RFNA doesn't dissolve mercury for crap (that's all I had one time). Add some DI water and the rate increases. Same is true with silver. You also get better economy with respect to the nitric acid because the water redissolves the nitrogen oxides.




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[*] posted on 16-4-2011 at 09:42


what do you think about the price??? did I get a steal or what????
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