Today’s electrode lab went fairly well. This is the lab I always panic about, because the students have to move concentrated salt water (up to 1M NaCl) around the lab, and salt water and electronics equipment must not mix! I only had to chide one student once for not using the secondary containment tub, and nothing was spilled.
I had the students do more measurements this year than last, having 4 different concentrations (1M, 0.1M, 0.02M, and 0.005M NaCl) and an unknown (tap water). Last year they only had 3 known concentrations, but had to do both stainless steel and Ag/AgCl electrodes in the same lab. Most of the students finished the lab. One group still has one set of measurements to do on Thursday, and I stayed an hour late with the singleton student—this is a lab that goes much faster if one student records data while the other reads the meters, so I served as meter-reader for him after everyone else had left.
I spent some time at the beginning of class to teach each pair of students how to use a vernier caliper to that they could measure the dimensions of their electrodes. I also had them measure the thickness of the electrodes using my micrometer. I’ll have them do both again (without my instructions) for the silver wire electrodes on Thursday. Ah—I need to bring in some salt so that I can make a strong salt solution for electroplating on the chloride! It does not need to be pure, so I won’t use up the 1M NaCl that we have for them to measure their electrodes with.
I do have to get the students to start lab more efficiently. Once everyone had their setup built, they took measurements fairly quickly, but they came to lab with no schematic of their test circuit and no table set up for recording their measurements. The first hour of lab was wasted by almost everyone doing the pre-lab work that they should have done over the weekend and asked about in class on Monday. I did insist on seeing their schematics before I would let them have any of the salt water. Several had to redo their circuits a few times, because they made no sense (only one wire of the function generator drawn, or a circuit that did not include their current measuring resistor).
Next year, I may have to add more explicit instructions in the pre-lab to make a schematic of their test setup and a table for recording results. Tomorrow I’ll talk to them about preparing for lab so that they can start work immediately, though I don’t know if it will do any good. For next year, I should also add some discussion to the prelab about adjusting the resistor size after making a measurement. Almost all the groups chose to use a 1Ω resistor, which means that the voltage drop across the resistor was generally quite small (1mV to 180mv). It would be better to use a slightly larger resistor (10Ω or 100Ω) to get larger readings. I’ll also have to tell them to set the amplitude on the waveform generator to 10v p-p, so that the signals are large enough, as the default setting when the generator is turned on is only 100mV peak-to-peak. I think everyone got the amplitude up to at least 1v, which may be good enough.
Tomorrow I’ll spend some time helping them write gnuplot scripts to model their impedance data. I’m assuming it will look a lot like the data I collected last year, which means that the conventional model of polarizing electrode will not fit all that well.
I hope that we also have time for some complex impedance and voltage divider problems, so that they have a little more practice before Friday’s quiz (which I still haven’t written).