|My titration lab apparatus at the end of the titration. Notice the MANY data points recorded!|
|Katherine demonstrating the use of a volumetric pipette|
Every lab starts with verbal instructions from the teacher. Today was no different. Katherine gave a review of the safety considerations, pointed out the location of chemicals and special equipment for the day, and she demonstrated how to use a volumetric pipet. The students stood in their spots around the lab, afraid to move closer to get a better look. I know that the students in the back couldn’t see what she was doing, but they held steady at their lab stations.
I appreciate that she did not assume we all knew how to
use the volumetric pipets. Something that seems so obvious to the instructor
can set the students back during an experiment. She also pointed out the three
buffer solutions to use for calibrating the pH meter along with the
instructions. Once the pre-lab instructions were complete, everyone began to
gather equipment and get organized.
|Everyone is very shy during the first lab. The guy in the back can't see anything, I'm sure!|
|Buffer solutions that we used to calibrate the pH meter.|
It wasn’t long after I finished calibrating my pH meter that I was ready to titrate my sample of glycine hydrochloride. That competitive side of me came out after the first data point. Naturally I wanted to get the best results in the class. Why wouldn’t I, I’m a chemistry teacher after all! I tried to listen to my own advice, which I wrote in my lab notebook before the lab: “go slow and be patient so you get good results”. I took MANY data points until I found the first equivalence point. My heart raced a bit when the pH started to climb dramatically after each additional drop of NaOH. I got obsessed with trying to get the indicator to turn pick after an addition of a single drop. Why not geek out on the lab? I had four hours to do it and no reason to rush! Even after all these years of using burets, I still made the mistake of turning the stopcock the wrong direction at least twice. Once I passed the first equivalence point, my attention waned a bit, and I actually forgot about the second equivalence point. Many years of titrating vinegar has emblazoned a single spike into my brain. Luckily the students across the way were discussing the pH of their second equivalence point, and I snapped back into focus. The resolution of my second equivalence point is not that good because I was adding too much base with each data point. So much for getting the best looking graph in the room.
We stayed after the lab to analyze the data with Katherine and Nick at the ready to help us plot graphs of the data. I have been terribly spoiled by the logger pro interface that we use in my lab. No need to plot graphs or calculate first or second derivatives with logger pro, it all happens within the program if you know how to ask the right questions. For this experiment, I was grateful for the refresher course in calculating and plotting the first and second derivative graphs for my titration data. Doing the graphs in Excel is very easy, the only laborious part is entering all the data (which I had plenty of, at least until about 25 mL of NaOH added). I am a big advocate for teaching my students important skills through the context of chemistry class; Excel is top on my list of tools I want my students to master in my class. I am so excited to see how it is taught and applied to a college course. Now I will have a better understanding of how best to prepare my students for advanced study in chemistry.
As you can see from my graph, I got a really nice first equivalence point and a pathetic blip for the second equivalence point. Upon further analysis, I determined with confidence that my first equivalence point happened after 20.68 mL of NaOH, which allowed me to determine the pKa of the first acidic proton. I decided to use this data point to derive the second pKa value rather than rely on the graph because the transition for the second equivalence point was not distinct. When I compare my results to the literature values, well, let’s just say that I didn’t have the best results in the room. Enthusiastic science teacher: yes, great lab results every time: not a guarantee.