Diffusing the Gas and Measuring its Concentration

We diffused the standard gas through agar gel rather than water.  Agar gel provides support for the water barrier without strongly influencing diffusion through it.  The effects of diffusing gas through agar rather than water are well documented.  We decided to use agar gel with a mass percent of 0.8%.

We wanted to diffuse the gas through the agar gel and then measure the concentration of the diffused gas.  We used the following experimental method to do so:

Diffusion 2

The standard gas flowed from its gas cylinder into a flowmeter to keep the flow of gas controlled at 0.003 standard liters per minute.  The flow of gas then passed into a diffusion cell housing the water barrier.  The diffusion cell consisted of a lower chamber below the water barrier and an upper chamber above the water barrier. The undiffused gas passed through the lower chamber and into a needle valve where it was vented into the atmosphere. The diffused gas passed from the lower chamber, through the water barrier, and into the upper chamber of the diffusion cell.  Ultra high purity nitrogen with a controlled flow then swept the diffused gas from the upper chamber into the valco valve equipped with a 1.012 ml sample loop.  Pure nitrogen purged the sample loop into the GC and ECD.

We measured the peak areas of the diffused gas every 5 minute throughout the diffusion.  The diffusion process took 4-6 hours to reach steady state.   Here’s a graph that shows the peak areas of CF3SF5  throughout a diffusion experiment.  You can see that the peak areas sharply increase initially, but then start to level off as the process reaches steady state.

Diffusion graph

Once the process reached steady state, we used the calibration curves to convert the peak areas to concentrations.  The peak areas were below the range that we calibrated with the calibration curves; therefore we linearly extrapolated the curves to find the concentrations.  This created potential for error in the experiment because we did not verify that the curves were linear over this range.

We used the ratio of the concentration of the diffused gas and the concentration of the undiffused gas to determine the diffusion coefficient of CF3SF5 in water.