[ctrl]

Hi.

First of all. Nice forum.

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. After that the gas is heated over 500 degrees Celsius to form Hg0. 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??

Another thought is that HCl and HNO3 is reproduced somewhere in the last steps of the cycle. If there in fact is NOCl in the gas, and there is water present (ca 0.5 vol%), could it be that HCl and HNO3 --> aqua regia is formed again? The flowrate for the gas through the scrubber is approx. 1.1 L/minute. The scrubber is also in roomtemperature.

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...  )

[DrCMS]

I thought mercury and gold formed a liquid amalgum?

[Mitch]

Mercury-Gold amalgam sounds very likely.

[ctrl]

Hi all.

Yes, a gold-mercry amalgam is formed. However, the instrument is working if only (for example) HCl is used instead of aqua regia as a calibration gas. The amalgam is present in both cases, so it can't be the amalgam that is doing the destruction. I guess one alternative could be that the scrubber (11 cm long) is too short for the sampling flow of 1.1 L/minute.

Could it be that the soda lime scrubber isn't effective enough? What passes through? Any suggestions?

Thanks for replying!

[mercury]

i'm not sure what instrument u are using but it looks like some moisture is reaching the Gold.This moisture is definitely acidic and destroying the Gold.To avoid this, check what are the drying agents or moisture filtration before the mercury vapour reaches the Gold in your instrument.