One thing I must say: TAing is helping my walk with God. As I was prepping the lab for the two experiments I'm in charge of, I was terrified. That probably sounds ridiculous, but I kept on running into problems. Then, a TA meeting would come and I wouldn't have done what I needed to have done.
Take experiment 8 for example! From the getgo, I wondered what water to use. Once again, that probably sounds ridiculous, EXCEPT that I was making up 20+ L of solution (I actually need to make more than that!), and if I needed Millipore water I would have to truck it in from a different building across the street! That question was answered: use DI water. I won't belabor the point and go into the clumpy KCl. Okay, I will! I opened up the first jar of KCl and discovered a uniform clump inside. From that point on, I became a percussionist. I would take a jar and beat it on something. I tried beating it on my leg, but obviously, I had better things to do than bruise myself with KCl. So I struck (ha ha) on a new idea -- which is why at 10:00 one night I could have been found beating quarter notes and eighth notes into the floor with a bottle of KCl in each hand. I was so afraid that a night watchman was going to walk into the lab and find a demented TA. Because I would have been that demented TA.
So the stock solutions were made. That was a plus, because the students had to use them! So I finished the stock solutions 10 minutes before the students came into lab last week. Last week was just an activity: a "meet and greet the instrumentation day." Basically, after shaking the UV'vis' hand and learning the Mass Spec's nickname, you went out the same door you came in -- if you were a student, that is. (Will they ever know the pain and suffering we TA's endure for their sake?).
This week was a different story: I needed to have the experimental solutions made up for two completely different experiments, run through 2 completely different experiments, and have data in hand for the two experiments in case someone's experiment failed. I actually had fun running the experiments -- it was just that I was going crazy. (Please note: I never, ever, in a million years, ever exaggerate!). Exp. 8 used a Ca-selective electrode, and Exp. 9 used a tiny liquid chromatography column, TLC, and then HPLC.
So... I was happy that the activity went well. One other TA, Ann, a veteran, had mentioned that the pH meters never seemed to settle on a certain value, but I wasn't too worried about that. After all, back when I was doing prep with a pH meter, the thing never could make up its mind what the pH was. I usually resorted to pH paper! (Which is like going from the Renaissance to the Ice Age, supposedly!).
So... I TA'd on Friday, then went to an incredible Grad-IV meeting. A doctor spoke on career choices (see my previous blog). That was EXCELLENT. Then we had ALGE -- an "After Large Group Event." That was the Cops and Robbers Game. I didn't have much time to work on lab prep after ALGE, because I talked to some of the Grad-IV people until about 1:30 in the morning. So by that time it was Saturday. I went home and slept until about 8:30. There was a chemistry picnic at 11:30 that day, and I thought about going in to work on the lab prep then, but I figured that by the time I hit my stride, pull out the parafilm and pack the place up. Basically I was lazy and skilled at rationalizing.
The picnic and the games (relay races, volleyball, watching football, and then tug-o-war) were incredible. Actually, I think it was the PEOPLE that made it so fun! Anyway, I dropped off my friend Jen at about 6:30, and then went to the store. I had been without a can opener and milk for over a week and I felt like I was deprived. Once again, I contemplated going to the lab, but seeing as how the sun sets around 7:30ish and it took me 2 hours to get "necessities," I skipped the "working in lab" idea.
This is why exactly at 2:00 on Sunday I was feeling slightly panicky. Tomorrow was the TA meeting when I would have to present the instrument and tell its life history. There was no more shaking of the hands and tipping the hat. But I had not run either experiment, and I had no clue how long it would take me to do so!
The long and short of it is, Exp. 9 went as smooth as silica (which was used in the experiment, by the way), and Exp. 8 was a headache with a toothache thrown in. That's why I said that Exp. 8 was improving my walk with God -- every step, it seemed to me, came with fresh difficulties. But God helped me at each one of those steps. Here's a list of mistakes I made:
1) No filling solution in the reference electrodes.
Believe it or not, for an entire day of the activity, I let the kids use dried-out reference electrodes. I figured that since it was sitting in buffer, it had all the hydration it needed! I just thought the electrodes looked grungy, because they had crystalline deposits coating the inside sleeve. After Ann told me about the electrodes jumping high and low, she mentioned that I might want to fill the electrodes. So I did. I looked online and found the electode’s manufacuring site. I printed the ref. electrode’s manual, which said the ref. electrode’s typical filling solution is Cat. No. 900001. But just getting the electrodes open almost proved too much for me. After heaving and howing on the caps for a good while, I gave up that and seized some pliers. When the run-of-the mill model failed, I went for the needle-nose variety (you must understand that I was acting out of desperation! I HAD to get the electrodes open – otherwise, life wouldn’t be worth living). Finally, the cap popped off. Seeing the force I had to apply to get the electrode open initially, I couldn’t believe the description of basically a gentle tug that the manual described. All I can figure is that when the electrode dried out, some of the solid accumulated near the O-ring on the electrode. I dismantled the electrodes, rinsed them out, and filled them with fresh solution #900001.
2) Wrong CaCl2.
I consulted with my dad on this one. Basically, the dihydrate form of calcium chloride is pretty unstable, and it likes to grab any and all water it can access. That’s fine as long as you’re not trying to make a concentration standard with it! I’ve made 3 different CaCl2 stock solutions so far: one with material from an old dihydrate CaCl2 bottle, one from material in an anhydrous CaCl2 bottle (which is made up of pellets designed for use with a dessicator), and one from a bottle of fresh CaCl2 dihydrate. I was going to make up a new solution of anhydrous CaCl2 that was reagent grade, but the Stores were closed, and my card wouldn’t scan me in. While my professor recommended the anhydrous form, I’m going to wait until I get a free moment to buy the anhydrous and compare the solution I make from that to the solution made with the new dihydrate form.
3) Wrong filling solution.
It was Monday night when I found this out. I had presented at the TA meeting earlier that day by describing Exp. 9 in full, and then blurting out, “But I haven’t run through Exp. 8 yet.” Then I added, “But I don’t anticipate any problems.” I can’t tell you how many times that phrase echoed through my head since then. Sometimes I wanted to laugh, when I thought, “I sure didn’t anticipate THIS problem!”
Back to Monday night. I had prepared the dilutions to measure. Firstly, I prepared two solutions to calibrate the electode. One solution was made up of 1mL of 0.1 M CaCl2 in 100 mL of water, and the other solution was made up of 11mL of 0.1 M CaCl2 in 100 mL of water. The instruction manual said that ideally there would be a difference of 25 to 30 mV between the electrode’s reading of these two solutions.
I think that the biggest difference I saw was an 8mV difference. I was comparing the two reference electrodes, and I found that reproducibility was nil. While the first time through the reading was –180 and –184 (approximately), the next time the reading might by 1020. Then the drift started. The meter would show a reading that would steadily decrease by 100 units in a few minutes. I was beginning to despair. I was writing an email in my head: “Dear Dr. ____, I know this is the eleventh hour. That is why I am writing. I have not been able to get Exp. 8 to work, and I need help. I realize that labs are supposed to start tomorrow, and I just wonder if there is another ISE that we can use instead of this one. Thanks, Hannah.”
Fortunately, it was never sent. Two different times it almost was! At 11:00 on Monday night, my failure was thick. That’s when I decided to look back at ISE manual. Lo and behold, I noticed something. It said NOT to use the solution shipped with the reference electrode, and instead to use solution #900011. I could have cried. One blooming number was ruining the readout. I had been using solution #900001! I rinsed the electrodes and filled them with the new solution.
Then I held my breath and made another measurement.
It was awful. I don’t think it had improved at all.
That’s when my list of suspects grew. If the reference electrodes weren’t changing the readout, then the culprit must be the ISE.
4) No ISE tip.
Then strange and wonderful gyrations started occuring. Several statements in the lab manual hadn’t made sense. Wasn’t there something about a grey/black boundary? There was only black on this electrode! I had rationalized this before, because there were no markings on the electrode identifiying it. I wasn’t sure that the electrode I was holding went with the instruction manual I’d found in the drawer. The second instruction that didn’t make sense was something like “don’t touch the membrane.” I actually penciled “What membrane” into my lab manual.
When I started groping around in one of the lab drawers, I found buried treasure: a tip. A tip that matched the picture in the book. The picture I hadn’t quite analyzed closely enough. You were supposed to let the thing soak for 1-2 hours, but I let it soak a healthy 1-2 minutes.
What I found was that the measurements weren’t improved in the least.
I decided to soak the tip for the prescribed 2 hours.
I found a second tip (I actually found a few of them), and let it soak in a separate, tiny beaker as well. Then I threw in the towel for the night.
Tuesday morning dawned light and fair, but I had to solve Exp. 8.
All I could think of was that – all those kids were doing their activity with HALF an electrode last week! HA!
As it turned out, I had 5) Used an old tip.
The new tip gave solid measurements (readings that didn’t trip up or down by a few hundred mV).
I took the measurements I needed and proceeded on to make interfering ion solutions (more solutions for the experiment that the students would make for themselves from the stock solutions I had already finished).
Lab started at 1:00. At 12:45 I was almost done preparing the interfering iron solutions. I realized there was no way I could take 5 measurements, letting the ISE soak for 3 minutes between each measurment (as the lab manual preached).
I left the interfering ion solutions, and gave the kids strict instructions NOT to dump them out!
I mentioned to the other TA that I had had the kids working with only half the electrode. He nodded his head and that was that. I guess I was expecting a bigger reaction. At least he didn’t report me.
So after lab was finished for the students, I finished taking my measurements.
Today when I graphed my concentrations versus potentials to obtain a working curve, I had a rude shock. The graph looked like a preschooler had dotted the graph at leisure. There was no straight line here, even if you examined the graph with a greased imagination.
I was a failure, and my work was for naught.
6) Wrong scale.
That’s the final mistake – for now. The original graph I was looking at was logarithmic, but I didn't recognize that!
It turns out (I cheated and looked at what older TA’s from past years had done on this lab), that if you graph the LOG of the concentration versus the potential, you obtain a BEAUTIFUL linear graph. And I mean BEAUTIFUL. I have never been so happy to see a line in all my born days!
I’ve saved that graph. I think it’s a work of art.
(Note: these two graphs are representative of the shape of my graphs, but the actual values were different.)
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