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*Columns Week meets #RealTimeChem*

FC - Silica Stirring

This is an issue that crops up quite a bit whenever methanol-based columns are mentioned (ex: here, here and in the comments thread of yesterday’s post).  Moving above 10% MeOH in DCM is said to dissolve standard flash grade silica, causing large quantities of amorphous solid to form in the test tubes.  Against doom-saying comes a relatively convoluted theory, put forward by Milkshake and others, which suggests that the relatively high heat of solubilization for silica/methanol leads to a wave of expansion in the column, forcing smaller silica particles through standard coarse frits.

In the last few years the majority of my columns have used methanol in one form or another, either as MeOH/DCM mixtures or as part of reverse phase columns.  I hadn’t noticed any issues with the standard flash silica (perhaps because I use sand instead of glass frits), but flushing my glass-fritted reverse phase column with pure methanol often–but not always–lead to the buildup of silica mentioned by others.

What I haven’t found online was anyone actually testing the solubility of silica gel in methanol.  So, I did the legwork.  1.0040 g of Silicycle P60 grade silica gel was suspended in 100 mL of pure methanol, and left stirring for 48 hours.  The solvent was then filtered through Celite [1], concentrated, and the residue weighed.

Silica results.

Against expectations, the RBF did contain a measurable amount of silica (see above).  However, the mass clocked in a just under 4 mg.  The amount of silica observed following some reverse phase columns was closer to several grams, and given the conditions of this experiment I think we can safely discount dissolution as a significant factor to test tube/silica contamination.

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[1]  Celite was added to the filter as a slurry in water, rinsed 5x with distilled water, and then rinsed at least 3x with methanol.  The collected solids were washed 3x with methanol as well.

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