iota - 14-11-2006 at 01:09
Hi.
First of all. Nice forum. I have checked in now and then when I need some info regarding chemistry. Seems like nice people here, and I could really
need some feedback on my questions. 
I use a instrument for calibration and messuring elementary mercury, Hg0, in gases. There are several other compounds in there aswell. The normal
stuff you get when running a coalplant etc.
Elemental mercury is transformed from approx. 1*10^(-6) M HgCl2 (which is the most common Hg-formation in the gas) with approx. 1*10^(-3) M HCl/HNO3
(aqua regia) and H2O. Before the elemental mercury is analysed the inlet gas (smoke from the plant with HgCl2) is heated to maximum 180 degrees
Celsius and diluted 100 times. The carriergas for transporting the elemental mercury is N2.
Now we have the elemental mercury in the N2 carrier-gas. This gas will go through a scrubber with solid soda lime to remove stuff like SO2, HCl and
other compounds. After that the remaining gas is led through a gold-trap and mercury will stick on it. However... The gold-trap is destroyed within a
2 minute range. Not visual to the naked eye, but with micro-photo. I know that aqua regia will destroy gold, but since it's heated before the analysis
step, it shold have fallen appart? Or?
Normaly the HNO3 should be separated into NO2, H2O and O2 at 180 oC...
One thought is that nitrosyl chloride (NOCl) is created and it somehow passes through the soda lime-scrubber and then destroyes the gold. Is this a
reasonable thought? The concentration of the HCl/HNO3 is quite low... What could in that case "scrubb-off" the NOCl??
It has to be a mixture of HCl and HNO3 in the calibration. The messurements work perfectly if only HCl is used. The problem is that certain rules by
the EPA (Enviromental Protection Agency (?)) states that calibration should be made with HCl/HNO3...
I am quite lost on this one, so if someone out there could help me I would really appriciate it!
(Sorry if there's a lot of misspelling... 
)
YT2095 - 14-11-2006 at 01:24
it`s probably forming an amalgam.
Mercury is used in gold refining for this reason.
edit: it`s just another possibility, but have you checked for Cyanides as well?
[Edited on 14-11-2006 by YT2095]
iota - 14-11-2006 at 01:43
Hi.
Thanks for replying!
First. Yes, the gold and Hg forms an amalgam. When the gold is heated the mercury will leave the gold and head to the analysis. There is a cooldown
timer on the gold aswell before the next analysis.
I haven't checked for cyanides yet. But is it really possible that a cyanide compound could destroy the gold? Maybe I've missunderstood you.
YT2095 - 14-11-2006 at 02:05
yes cyanide will dissolve gold, take Sodium Cyanide for instance:
4 Au + 8(NaCN) +O2 + 2 H2O = 4 NaAu(CN)2 + 4 NaOH
edit: also NaAu(CN)2 is water soluble.
[Edited on 14-11-2006 by YT2095]
iota - 14-11-2006 at 02:09
Hi.
Thanks for replying.
I checked in a bunch of articles and yes, the gold can react like you discribed... sorry for my last post. 
Is there any way that can verify the occurrence of cyanides in the gas? Or on the gold?
Thanks in advance.
woelen - 14-11-2006 at 07:02
Cyanides can be detected with great sensitivity with a slightly acidified solution of a mix of an iron (II) salt and an iron (III) salt. Bubble your
gas mix through a solution of FeCl2 and FeCl3 (or FeSO4 and Fe2(SO4)3) for some time. When there is cyanide in the gas mix, the solution will turn
blue. This test is very sensitive. It is based on formation of prussian blue.
Nitrosyl chloride is a very stable gas. It does not decompose with temperatures lower than appr. 600 C. So, heating it to 180 C certainly does not
destroy it. NOCl is hydrolysed by NaOH, but it might be that the bubbling through the solution is too fast.
unionised - 14-11-2006 at 12:59
My best guess is that there's a lot of mercury there and so it's destroying the gold's structure.
Can you try a smaller sample?
I doubt it's cyanide because one of the normal ways to destroy cyanide is to burn it.
HCl would be removed by a soda lime trap. NOx wouldn't attack gold and I don't think you would get much NOCl.
iota - 17-11-2006 at 02:33
Hi again all.
The flowrate for the gas passing the scrubber is slightly over 1 L/min. The scrubber tube is approx. 11 cm long.
It doesn't seem like much NOCl is generated, but maybe a chunk of aqua regia is formed before the scrubber. But wouldn't HCl/HNO3 be stopped by the
scrubber??
This is starting to really anoy me... Thanks again for helping me with comments and experience!