Preparing a New Standard Gas to Find the Diffusion Coefficients

We decided to prepare a new standard gas since the old standard gas was giving us variable, inaccurate results.  We decided to target a standard gas with CF3SF5, SF6, and N2O concentrations of 119 ppmv, 476 ppmv, and 4% respectively. We used the same serial dilution method to prepare the gas.

We found the concentrations of CF3SF5 and SF6 in the standard gas to be 1817 and 816 ppbv respectively.  We were not sure why the actually concentrations were so much higher than the targeted concentrations.  The concentration of CF3SF5 was particularly alarming.  Nevertheless, we decided that the gas could still potentially provide accurate data.

We used the same experimental method to diffuse the gas across a water membrane. and to measure the resulting diffused gas concentration.   We found the diffusion coefficients in pure over a temperature range of 3.0 oC to 30.0 oC  and we found the diffusion coefficients in seawater over a temperature range of 15 oC to 30.0oC.  We found the following results with this gas standard:

SF5 new standard SF6 new standard


The results for SF6 matched previous work quite well, validating the results for CF3SF5. More trials should be conducted to ensure that the diffusion coefficients yield a low standard deviation.

The diffusion coefficients for SF6 were 6-19% lower in sea water than in pure water.  Likewise, the diffusion coefficients for CF3SF5 were 5-7% lower in sea water.  It is not clear whether diffusivity decreases due to the sea water or the variability in the experimental method.  In previous studies, the diffusion coefficient for SF6 did not change between pure water and sea water; however it would make sense physically for diffusivity to decrease in salt water. \

It is recommended to better determine the solubility of CF3SF5 in both pure and salt water to determine a more accurate diffusion coefficient.