Thursday, February 07, 2008

Recycling Silica Gel

Insert usual excuse for infrequent posting here: .....
I recently received the following email regarding recycling of silica gel:
"In the past we used to regenerate SiGel with fuming nitric acid, much as you do here (refers to previous post - D!) .Place the coloured, used SiGel (which of course must be uncoated and completely dry) in a large beaker. Approximately a third full. Place it inthe fume cupboard and pour in the nitric acid so the SiGel is completely moistened. If necessary, stir it. After a possible initial fuming has stopped, heat it on a steam bath for 10 minutes - leave it overnight. Next day, fill up with water, stir and let it settle. Wash 3 more times. At this point, it should be colourless, if not, repeat with more nitric acid. Then wash with satd. sodium bicarbonate until neutral and filter on a Buchner funnel. Wash with water, methanol and acetone and let it suck completely dry. Finally, activate by heating it in an oven overnight at 100-120 C.This way, it is as good as new for most purposes."
This may be of use to some chemists working at institutions where money is very scarce. However, considering the amount of time and large volumes of solvent required to do this I think it can be classified as historically interesting but not practically useful to most of us.
Nevertheless thank you for the email. I have wondered exactly how you would go about recycling your silica gel.
Speaking of silica gel and techniques that are disappearing from the chemists hood check one of the most recent posts at In the Pipeline out. Derek seems to think that TLCs are on their way out as LC-MS is becoming more and more common. I believe that it will take several decades before Derek's predictions come true (especially in academia) but he does have some very good points. D!


milkshake said...

When I was in Prague in early 90s we would recycle silica from large-scale columns, we would scrape off the top ugly dark band and load the rest into a giant Soxhlet with methanol+acetone mixture, let it extract overnight, then dry the stuff and have it re-activated in the oven. Silica for chromatography is expensive - but it takes lots of work and solvents to recycle it. And even if you extract all extractible stuff you always have a problem of variable separation power, depending on how you dry it, and accumulation of salts and basic impurities that affect the separation.

Recycled silica is like when you get a toillet papr with note "Made with 60% of post-consumer dietary fibre"

mevans said...

TLC on its way out? This is certainly good news!

Anonymous said...

I was being shown around a big Chem department in the US the Prof had two Chinese post-docs stirring a huge vat of what I later found out was silica and methanol.

It must have weighed half a ton!

Henrik T, Simonsen said...

Due to the very speed and the very very easy technique, TLC will never disappear. As long as you do prep. chromatography like Flash or other non/very little pressure coloumns TLC will be very handy. And there are several compounds that are very difficult to "fly" in the MS and that are not detected in the UV.
So TLC will stay in academia and real reseach groups not only for decades but for the next century.

Daniel Sejer said...

Henrik, I agree that non-UV active compounds are tricky so in that particular area I would agree that TLC has a place for years to come. However, as automated flash chromatography becomes more affordable and common this will alleviate the need for TLCs. These systems are fitted with a UV detector that does the job. D!

mamid said...

The procedure mentioned by milkshake is found here: DOI: 10.1021/ed067p511.

An alternative from the same journal: DOI: 10.1021/ed065p891. I like this one better.