How to prepare methane solution in water?

[JI2002]

We are trying to run method RSK 175: Analysis of dissolved methane in ground water using a GC headspace equilibration technique. Our problem is to figure out how to prepare a methane standard in water. We spiked 5 ul pure methane into 40 ml water(without headspace), we still see methane bubble in the water solution after cooling it to 4C. Any ideas? Thanks.

[Bruce Hamilton]

I presume an email to the method originators might elicit the best method.

In the absence of instructions, my normal technique would be to first create a saturated solution ( under pressure if necessary - needs lots of mixing ), dilute 1:1, measure that concentration by another method, and dilute to 100 ppb ( or whatever you are trying to achieve ).

Note that quantitative gaseous hydrocarbon solubility into water is quite difficult without loss, even when using hermetic devices.

Please keep having fun,

Bruce Hamilton

[AICMM]

I asked a customer or mine who did this analysis and they said they used methane in nitrogen for the gas standard. Then, they inject a known volume directly into the water with no headspace in the container.

Best regards.

[Ron]

Dissolution of gases into liquid tends to be a slow process. Inject the methane into the water with no headspace, then keep turning and agitating the container until the bubble disappears. We prepare TOGAS standards this way, and it can take a while for the bubble to disappear.

[JI2002]

Thank you all for the replies. After injecting methane into water in a 40 ml vial and cooling the container to 4C, I tried to roll it on the bench, the bubble did get smaller. The process is time consuming. So I tried to sonicate it in cool water, the bubble disappeared in a matter of minutes. At same time, I sent an email to the people who developed the method, hopefully they will get back to me soon. Thanks again for the input.

[Bruce Hamilton]

Interesting. I've used ultrasonication ( with vacuum ) to remove HCs from water for GC analysis, but wouldn't have considered using it to dissolve gases to make trace standards. I'm glad it worked for you.

Methane is quite soluble in water ( 33 ul/mL at 20C and 1 atm partial pressure ), but requires good agitation, and the US clearly provides it.
Saves the shaking of 100ml SS pressure bombs that I used.

Please keep having fun,

Bruce Hamilton