Poly Who?

My titration lab apparatus at the end of the titration. Notice the MANY data points recorded!

Last week I got the chance to titrate a polyprotic acid. I found this a refreshing diversion from the annual acetic acid titration I do with my honors chemistry class. Not only was I interested in titrating a “new” acid with two acidic protons, but this was the first time I performed a lab as a student for many (read very many) years. I experienced a wide range of emotions during the lab period that included fear, anxiety, frustration, confidence, excitement, and even wonder. Looking at this titration through the eyes of a student was such an amazing opportunity for me. I found myself spending as much time observing the classroom dynamics as I did performing the experiment. The undergraduates in the class are terrified of me. Can you imagine the strange looks I got when I started taking pictures of the TF as she demonstrated the proper use of volumetric pipet? The guy across the bench watched me suspiciously as I photographed my lab apparatus from different angles. When I tried to explain to him that I write a chemistry teaching blog, things just got worse. Anyway, this alien life form (a person way beyond twenty-five years of age) has landed in their lives to make the summer lab course a bit more entertaining.

Katherine demonstrating the use of a volumetric pipette

The first thing that happened during the lab period was a flood in our lab. The TF’s had to scramble around transferring all the chemicals and equipment from our lab to another teaching lab for the day. Each of the students had to go in shifts to get equipment from our lockers for the experiment. This was such a perfect example of why teaching a lab course is challenging! Things like this happen all the time; as the teacher you have to be ready to adapt and move forward. Without much fanfare, the lab started right on time in the new location and the TFs quickly got everyone up and running in the new space.

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.

Everyone is very shy during the first lab. The guy in the back can't see anything, I'm sure!

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.

Buffer solutions that we used to calibrate the pH meter.

Calibrating my pH meter proved to be difficult. I hailed Katherine at least three times for help because my meter was not cooperating. Getting used to new equipment requires patience and multiple attempts, something I usually don’t have to draw upon in my own teaching lab. My pH meter would not calibrate, even after several cycles through the three buffer solutions. Katherine gave me a different meter to use, luckily we only have fifteen students in the class; the new one worked well and I was soon calibrated and ready to titrate. Katherine and Nick hustled around the lab helping everyone get their meters calibrated and trouble shooting the equipment. Once everything was in place and ready to go, I stopped to appreciate the student experience that requires learning the correct use of the lab equipment, doing it wrong, trying again, replacing a faulty meter, and repeating the process. Making mistakes does not come naturally to these talented young science students. Yet, here they are in a lab course that forces them to open up the possibility of not only making mistakes but also fixing them.

Nick and Katherine, our wonderful TFs for the class.

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.

Starting a second trial, notice the pH of the original acid solution.

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.

Graph of my titration of glycine hydrochloride with NaOH.

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 pK_a of the first acidic proton. I decided to use this data point to derive the second pK_a 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.