Does anyone know what is wrong if a relatively pure substance gives extremely spiky peaks in a UV-vis spectrum?
(Long) Backstory:
At work, I am trying to develop a method for analyzing a particular process chemical. It is the sodium salt of a weak organic acid; I will call it "X" for now. Unfortunately, compound X decomposes rather readily (it's somewhat light, heat, and air sensitive; it is also hygroscopic). I don't have the reagents or the equipment to synthesize the stuff, so I need to purify the technical grade compound.
According to the literature, the compound can be "readily recrystallized from absolute ethanol or acetone" to give >99% purity. Nobody seems to have more elaborate instructions than that. Well, recrystallizations are easy, right? Heat solvent, add just enough solvent to dissolve sample, let cool to precipitate pure crystals, filter, wash. Even someone organically-illiterate like me can do that. So I did.
The first time I tried heating acetone, it superheated and popped right out of the beaker. It was kind of interesting to watch, actually...one minute there's acetone in a beaker, the next minute there isn't. What happens when using high-purity acetone without boiling stones, I guess (no boiling stones in the lab.)
Anyway, eventually I got the acetone heated up, and dumped some onto my technical grade compound. The technical grade stuff is pale-ish yellow. From memory of an MSDS somewhere, the pure stuff is also pale yellow. But when I poured the acetone in, the mixture was a deep orange slurry, heavy with orange precipitate. Not really a solution at all. Adding more acetone does not help dissolve the orange precipitate. Nor does heating the slurry.
My thoughts:
Well, I figured I'll filter the orange precipitate from the hot solution, and see if anything precipitates from the filtrate. Then I can run a UV-vis spectrum on the orange residue, any precipitate from the filtrate, and the technical grade compound, to see which is which. That's nice...in theory. The procedure reminded me just how tedious gravity filtration is. The precipitate clogged up the filter paper very badly, and I have a feeling the acetone evaporation rate is potentially faster than the filtration rate. That's just sad. The only good thing is that the filtrate is yellow. That's a good sign (I hope), and at this stage I need all the good signs I can get.
Odder yet, there are multiple "species" that ended up on the filter paper. The majority is the orange precipitate. However, there's yellow stuff near the top of the filter paper, and some fine white powdery stuff as well. Hmm.
Also, nothing crystallized out of the filtrate. I ended up boiling some filtrate to start the crystallization process. Alas, acetone being acetone, most of the solvent evaporated by the time the boiled filtrate has cooled down to room temperature, leaving a paste-like solid mass. Probably not pure.
So, in the end, I dissolved a bit of everything (separately) in some distilled water, and tried to run a UV-vis spectrum of the samples. But the UV-vis was stubborn and gave oddly spiky peaks, not one of which appeared at where the maximum absorbance is supposed to be. I mean, I'll accept spiky peaks for my botched recrystallization samples, but another coworker managed to get a very nice peak for the technical grade compound which isn't happening now. Unfortunately, the coworker is on holiday at the moment and thus unavailable. The help file wasn't helpful.
In the end, I was getting kicked out of the lab (it's the end of the work day), so I covered everything with parafilm, ineffectually tried to keep them in the dark with paper towels, and left it at that.
What to do, what to do? I can't progress on this project without pure analyte! (Among other things...I can't calibrate the machine with technical grade compound.)
Saturday, February 16, 2008
Spiky Peaks and Underestimating Simple Tasks
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1 comment:
The best way to recrystallize anything is to start with a small amount of solvent and heat to reflux (~20 degrees above boiling point usually works). Once that's ready, dump in the stuff you want recrystallized and proceed to patiently add small amounts of solvent until it is all dissolved (or close enough that you get no change upon adding more solvent, if you suspect some insoluble material might be an impurity of sorts--sounds like this is the case for your stuff). Wait until the entire mixture is nice and toasty, then QUICKLY transfer to a hot frit (one you've had in the oven or hit with a heat gun) and vacuum filter. Gravity filtration is no good for this. Allow the solution to cool to room temperature without disturbance. Then you can throw it in the fridge (for at least several hours--overnight is preferable) and gravity or vacuum filter when it's ready. Oil baths and round bottoms are the way to go for this, IMO--the heating is nice and even. I'd only use a naked hotplate and a beaker if I were recrystallizing a very small amount of something. Never a heat gun--bad experience!--you're likely to burn the hell out of the stuff you want to purify. (I know you didn't ask for suggestions on this, but the technique has been described as a lost art by some, and it's one I use on a regular basis. I'd have used ethanol rather than acetone, too, but that's more a personal preference.) Parafilm and acetone or DCM...generally not a great combination as the solvent will slowly dissolve the parafilm. Foil may be better. Or not. Up to you.
Spiky peaks sound weird--something you really never see in a solution or thin film UV. Maybe the lamp wasn't warmed up? That's all I could really think of that might cause it.
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