In this lab experiment, students use sol-gel chemistry to prepare silica gel monoliths from tetraethylorthosilicate (TEOS). Carrying out the hydrolysis and condensation under acid-catalyzed vs. base-catalyzed conditions leads to different morphologies at the nanoscale for the resulting gels. These differences are probed by incorporating the pH sensitive dyes Congo Red or Bromothymol Blue into the sol before gelation. Molecular diffusion of acids and bases such as HCl and NH3 into the gel nanoarchitectures causes vivid color changes in the dye-modified silica gels. Leaching of the larger dye molecules from the gels into solution can also indicate the relative pore sizes of the gels prepared under different conditions. This has been a very popular lab with students for two years in a row in a second year inorganic chemistry laboratory. The lab could easily be adapted for the general chemistry level.
At the end of this experiment, students will be able to:
- Observe how hydrolysis and condensation reactions of metal alkoxides (thinking of Si here as a metal) can lead to porous metal oxide gels and effectively describe the experimental procedure used to obtain these gels.
- Describe the differences between gels obtained from acid-catalyzed vs. base-catalyzed reactions and explain how one might experimentally probe these differences.
- Illustrate how to make functional materials out of porous gels by incorporating pH-sensitive dyes into the gels and evaluate the effectiveness of these materials as pH sensors in either the vapor phase or liquid phase.
0.5 M NH4F(aq)
pH buffer test solutions (pH 3 and pH 8 work well)
Glassware and Lab Equipment:
Two micropipettes (85µL and 360 µL)
Magnetic stir bar/stir plate
Polypropylene centrifuge vials (10 per student)
Dessicators for vapor phase pH sensing tests
A brief document is attached describing some implementation tips and observations of typical student results in the lab.
Students write a formal lab report after completing all of the experimental work. A document is attached that describes the expectations for the format and content of the report. This was the second formal lab report that the students wrote in the semester, so they already had some feedback on their writing. Some of the comments in the lab report expectations document resulted from my evaluation of their previous lab report and did help to improve for example, the abstract and experimental sections.
A grading rubric is also attached for evaluating the lab reports. Students were asked to print out and attach a copy of the rubric to the front of their lab report. The intention was that they would read through the rubric and consider any final revisions in light of my expectations.
Using the grading rubric provided above, out of 20 possible points and 21 students, 6 students scored 18-20 points, 6 students scored 15-17 points, and 9 students scored 12-14 points.
If students that scored poorly on one or more sections as described in the grading rubric took the opportunity to submit a revised version of that section, I typically rewarded 80% of the possible points for that section. Sadly, in my experience students rarely take the time to resubmit.