Tuesday, June 16, 2009

Temperature solubility curves using sequential precipitation

We were going to wait for a while to tackle temperature related solubility because it wasn't clear that our NMR technique was applicable. The main problem is that separating out the supernatant at a given temperature can be tricky for both filtration and centrifugation with our available equipment.

However, while discussing the issue with Marshall last week it occurred to me that we might be able to get the data by sequential crystallization. The idea is to prepare several solutions at known concentrations at a high temperature then let the bath cool down and note the temperature when precipitates are first observed. We can then intrapolate to report the solubility at room temperature if necessary.

Khalid and I made an attempt on Friday for pyrene in acetonitrile (EXP109) and obtained a value of 0.16 M at 22C, about twice the value found by Marshall in EXP108 using NMR. Part of the discrepancy is probably related to longer relaxation times of aromatic protons but we have to keep in mind that NMR is not particularly precise at such low concentrations. Hopefully we will soon figure out how to run our NMR experiments to allow full relaxation of all protons and address this issue more conclusively.

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