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Amazing video abstract wow so good.webmIntro
Back in eight grade physical science at Milton, one of the final projects of the year was called the “sludge lab.” Each lab group was presented with a mixture of various unknown chemicals, a list of what substances could possibly be in them, and tasked to sort out what they had. Using processes like distillation, differences in solubility etc. we could separate our substances, and then compare their characteristics: densities or boiling points for example, to the characteristics of the chemicals in the list provided to determine the identities of the chemicals in our sludge. Unfortunately the lab was a complete failure, I don't think a single group got more than half of their predictions correct. I walked away from that lab questioning the efficacy of the methods we were taught, and when searching for a way to redeem myself through this DYO I found thin-layer chromatography (TLC).
Intro (cont.)
A TLC plate coats some inert substance, in my case aluminum, with an absorbent one, generally silica gel. The substance you hope to separate is applied to the bottom of the plate using a capillary tube and a beaker is prepared with a folded over sheet of filter paper in the back, and a few millimeters of solvent at the bottom (I used 3mm). The filter paper enables some solvent to surround the entire beaker as a vapor, which generally produces more consistent results. Finally, the strip itself is placed inside the beaker. The solvent will travel up the plate through capillary action, the tendency for a liquid to flow upwards in a tight space, and displace chemicals on the plate. Since silica gel is polar, very polar substances will exhibit very strong attraction to the silica gel, and so when the solvent travels upwards and attempts to displace the substance on the silica gel, more polar chemicals will move less, and non-polar chemicals will move more, separating them. I chose to use ethyl acetate for my solvent, as its polar enough to ensure that the substances actually travel upwards and seperate, but not so polar that all the substances would just be pushed all the way to the top. You cant see these distances with the naked eye, but instead have to use a UV lamp (Fig 2). Planning to use this technique, I devised an experiment to determine how consistently I could identify substances in solutions.
Procedure
The 5 substances I chose to use were acetylsalicylic acid, benzophenone, benzoic acid, caffeine, and acetaminophen, all of which were dissolved in ethyl acetate. I made .1M solutions for each individual substance, and then made my mystery solutions. Through hiding the labels of each substance and mixing them up, I could make mixtures completely blindly, labeling the beakers by the numerical order they were in instead of the substance inside them. I then actually made my solutions: one using only beaker 4, one using beakers 3+1, one using 4+2, one using 5+3+2, another using 5+4+1, giving me a wide variety of in interactions between substances and in the amount of substances.
Identification/Data
Solution 4: This was the most simple task as it was just figuring out which one of five solutions was in my beaker. I found sticking all six solutions on a single test strip to be challenging, and so I opted to run two strips in a single beaker. For the most part it worked pretty well, and I just ran one last test of solution 4 versus the two closest chemicals: benzoic acid and acetylsalicylic acid, and confirmed it was the latter.
Solutions 2+4 and 3+1: To try and be more efficient, I tested both of these solutions on a single test strip, and tried to compare them to a strip I had previously done that simply used all 5 chemicals. As a side note, I ignored previous identifications and generally ended up completely forgetting, which made the process more realistic as I would be less informed. Solution 3+1 looked like it only had a single dark spot, and I was kind of worried that I had only put a single chemical in the solution. However, after testing it a couple more times, I noticed that the spot sometimes looked abnormally long. So I tested the mixture against the two chemicals that traveled the furthest in my prelim: benzophenone and benzoic acid, and sure enough in Fig 5.2 they were on top of each other, teaching me that I could never assume a spot has only one chemical, even if its extremely convincing like in figure 5.1. Solution 2+4 ended up confusing as well, at first I was sure that one was caffeine and the other was either benzophenone or benzoic acid, as the upper blob in Fig 5.1 was aligned very similarly to those two chemicals in Fig 5.3: very close to the top. However, when I actually tested it against those two chemicals, neither lined up, and in the end I figured out it was caffeine and acetylsalicylic acid in Fig 5.5. This taught me another good lesson going into the more challenging solutions with three different chemicals: while the order of chemicals stayed consistent between plates, and distance traveled could be misleading, as time is an important factor with this technique.
Lastly, I tested solutions 5+3+2 and 5+4+1. For solution 5+4+1 I first saw that it definitely did not have caffeine, which always moved very little in prior trials, and so I then simply tested it against all the other chemicals in Fig 7.2. Even just the 5 comparisons were very crowded, justifying my decision to not just compare each unknown solution to all the other chemicals in one go, but nonetheless it was pretty clear that it lined up with benzophenone, acetylsalicylic acid, and acetaminophen. For solution 5+3+2 I needed to eliminate a candidate so that everything was on one plate, so I did an extra test with acetylsalicylic acid and acetaminophen and determined the former was not in the solution in figure 7.3. So lastly, I tested all the other 4 chemicals against solution 5+3+2 and came to the ultimate conclusion that it had caffeine, and benzoic acid, and acetaminophen. At that point, I looked at the photograph in the procedure section for the very first time, and was able to confirm that all my predictions were correct.
What I Learned and Why it Matters
While the framework of this lab was set up around the idea of identifying random chemicals, this success is not as valuable as the process I learned throughout the experiment. The entire ordeal lead to me continually develop and refine my method of testing a new substance to achieve a far more efficient and accurate procedure than I started out with. Being able to pick out which one of five compounds is in a solution is not super impressive on its own, but I believe the experience developed here would allow me to tackle a much more difficult project, like having a solution with completely unknown chemicals, whereas before I would likely have struggled or been extremely wasteful of materials and time (and possibly not even end up accurate!). And a technique like thin layer chromatography is a super helpful thing to know, the applications for it are extremely broad. It's not hard to imagine many use opportunities where being able to just drop some solution on a pad and figure out whats its polarity and/or identity is would be very helpful in future experiments where I may be reacting something without knowing the product, or needing to confirm it.
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