This experiment is designed to measure the adsorption characteristics of nitrogen molecules on activated charcoal particles maintained at liquid nitrogen temperature. The basic outline and theory of this experiment can be found in Experiment 26 of Garland and Shoemaker.
This experiment also incorporates a specific web-based module designed to help students come to grips with the underlying physical nature of adsorption. The web-site can found at http://www.jhu.edu/~chem/fairbr/teach/BET/isot.html.
The essence of this experiment involves a series of volume expansions, ultimately to determine the adsorption properties of nitrogen on activated charcoal. A pictorial representation of the experimental set-up can also be found at http://www.jhu.edu/~chem/fairbr/teach/BET/betexptl.pdf
The pre-lab is constructed as a series of questions – answers to these questions should constitute a working knowledge of adsorption and enable you to carry out the experiment.
Basis for Pre-Lab:
- What is the pressure-to-volume relationship for an ideal gas expansion between two vessels with volumes V1, V2 and initial pressures P1 and P2? After expansion the pressures can be written P1` and P2`.
- What is the fundamental chemical concept underlying the pressure-to-volume relationship; conservation of mass, energy, number of moles or volume? Show how this leads to the relationship used in (a)
- Answer the following question: If gas at 500 Torr is expanded into another container, initially under high vacuum (i.e. P2 = O Torr) having four times the volume of the initial container what will the final pressure of the overall system be? Show your working clearly and carefully.
- How would the volume expansion relationship be modified in the case of adsorption? Explain either in words or using an equation how volume expansions be used to determine adsorption characteristics?
- Why do we out gas the carbon prior to adsorption experiments?
- What the liquid nitrogen is used for (2 reasons)?
- The Langmuir adsorption isotherm (see web page) is a simplified version of the BET isotherm that gives the same qualitative form of the variation in adsorption as function of external pressure at low gas pressures. Using the Langmuir isotherm explain in words how the number of moles adsorbed on the activated charcoal should vary as a function of the external pressure. (e.g. number of moles of gas adsorbed is constant as a function of the applied pressure)
- Also from the animated gifs provided on the webpage explain at a molecular level how adsorption is effected by external parameters such as temperature and pressure?
- Give some examples of practical everyday situations where adsorption is important.
- Identify likely sources of error in the experiment?
(A) Preparation of "activated" charcoal:
- Carefully remove the sidearm flask from the glass line
- Measure approximately 2 grams of activated carbon and transfer into the sidearm flask without contaminating the glass wool filter inside the sidearm flask
- Reattach the sidearm flask to the glass line, and begin pumping through the sidearm valve to reestablish the vacuum
- Attach the heating tape to the flask so that as much of the carbon is covered without overlaying the heating tape on itself. Power the heating tape at 100/140 on the Variac (Heat for 30 minutes)
- Remove heating tape from flask and allow to cool for 15 minutes
- Once cool, slowly open the valve connecting the sidearm flask to the glass line making sure that the vacuum in the glass line does not disturb the carbon in the flask
Close the valve and repeat the helium expansion from above, making sure that when you evacuate the side arm flask the you do it through the sidearm or else you may lose some of your now activated carbon
(B) Helium Expansion into glassware to determine the volume of the line and the volume of the sidearm vessel containing the charcoal:
- check to make sure that all relevant stop cocks are open for initial pumping
- start the Alcatel mech. pump and pump the glass line and both gas line to the regulators, (both the Helium and Nitrogen) – MAKE SURE THAT THE NEEDLE VALVES ON BOTH NITROGEN AND HELIUM TANKS ARE CLOSED.
- fill the 1025 mL round bottom flask with a known pressure of He (200-800 Torr in »
100 Torr increment). Throughout all experiments pressure should be read off the Baratron Digital Pressure Gauge.
- Pump the line and side arm flask back to base pressure (< 5 Torr)
- (a) Expand the gas from the round bottom flask into the line, recording the pressure drop.
- Then expand the round bottom and line pressure in the sidearm flask containing the characoal, again recording the pressure drop.
- Fill the round bottom to another known pressure, evacuate the line and sidearm flask and repeat 5(a) and (b) until a total of 7 data points have been acquired.
Once all helium expansions are complete, evacuate the helium line and open the nitrogen line.
(C) Determine the adsorption characteristics of nitrogen in contact with the activated charcoal:
- Immerse the activated carbon in liquid nitrogen.
- Fill the round bottom flask and the line with a known pressure of nitrogen (starting low, »
10-20 Torr and going to higher pressures »
700-800 Torr), open the sidearm flask and watch pressure drop stabilize. It will typically take 5-10 minutes for the pressure to stabilize as the system comes to equilibrium.
- Close the stopcock between the sidearm flask and the main glass line
- Refill the glass line and round bottom to next higher pressure as in 2) and carry our successive expansions into the sidearm.
Make sure to watch the liquid nitrogen level does not fall below the level of charcoal in the sidearm vessel as the experiment continues.
Once done bring line back to atmosphere before removing the liquid nitrogen for the charcoal, ensuring that the gas has a place to escape.