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Offline Messi

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Lab techniques
« on: October 05, 2012, 12:38:41 PM »
Hi guys,

How do I know if all the solvent is gone from my rotovap?

Also, I was wondering, how do I know how much drying agent (MgSO4) I need to add to my sample at the end?

Last technique question :), how much of my extraction solution do I add to my solution. For example, when I'm doing an extraction and it says wash with water. How do I know how much water to add? I usually add ~10mL to anything but I honestly have no idea how much is deemed "correct"!


Offline discodermolide

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Re: Lab techniques
« Reply #1 on: October 05, 2012, 12:42:52 PM »
You know when nothing more distills over.
Drying agent, add a couple of grams, it really depends on the scale you are working at.
Extraction, depends on how much solvent and material you have how much water you add. 10 mL seems ok, but not if you have 100mL solvent.
Remember the partition coefficients.
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Offline Arkcon

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Re: Lab techniques
« Reply #2 on: October 05, 2012, 12:53:57 PM »
In each case, you use as much as you need.  ;D Sorry, but that's the best I can do with limited information.  But let me try to give you some useful areas to consider:

Your solvent is gone from the rotovap when its all gone.  How much did you put into your reaction?  How much total volume did you lose in workup?  You can use those values to ballpark how much the rotovap is to remove.  Only you know these things.  And you're expected to keep track of them, for steps such as these, and general laboratory hygiene.  Consider:  suppose you do this process 20 times a week, or you are promoted, and you and two lab techs do this 50 times a week -- how much solvent does the lab manager need to buy?  How much solvent waste do you need to contract for disposal?  I know, you're still in school, but you should start keeping track of what  you're doing.

You need enough drying agent to remove all the moisture.  Its hard to know exactly how much there is, but if you've evaporated much of the solvent, you can only have as much moisture as is miscible in the remaining volume of solvent.

If you have to wash a product, you have to use enough water.  How much is enough depends on how much there is.  Only you know how much you added or produced.  You probably want to wash 3 times, with 1/3 the volume each time, for best extraction.

Generally, you want to be generous at each step, doing slightly more, so your ballpark figure can be as loose as you need to be to save time.  And of course, only you know if any of these procedures is detrimental to your product, in which case, it is better to do less.

Clear as mud now, right?
« Last Edit: October 05, 2012, 01:26:00 PM by Arkcon »
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline discodermolide

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Re: Lab techniques
« Reply #3 on: October 05, 2012, 12:57:22 PM »
Crystal clear Arkon, wish I had thought of that  ???
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Offline Arkcon

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Re: Lab techniques
« Reply #4 on: October 05, 2012, 12:59:53 PM »
I type slow. ;)
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline discodermolide

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Re: Lab techniques
« Reply #5 on: October 05, 2012, 01:02:48 PM »
Lol but not slower than me! Or do you write too much? >:D
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Offline Messi

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Re: Lab techniques
« Reply #6 on: October 05, 2012, 05:11:02 PM »
In each case, you use as much as you need.  ;D Sorry, but that's the best I can do with limited information.  But let me try to give you some useful areas to consider:

Your solvent is gone from the rotovap when its all gone.  How much did you put into your reaction?  How much total volume did you lose in workup?  You can use those values to ballpark how much the rotovap is to remove.  Only you know these things.  And you're expected to keep track of them, for steps such as these, and general laboratory hygiene.  Consider:  suppose you do this process 20 times a week, or you are promoted, and you and two lab techs do this 50 times a week -- how much solvent does the lab manager need to buy?  How much solvent waste do you need to contract for disposal?  I know, you're still in school, but you should start keeping track of what  you're doing.

You need enough drying agent to remove all the moisture.  Its hard to know exactly how much there is, but if you've evaporated much of the solvent, you can only have as much moisture as is miscible in the remaining volume of solvent.

If you have to wash a product, you have to use enough water.  How much is enough depends on how much there is.  Only you know how much you added or produced.  You probably want to wash 3 times, with 1/3 the volume each time, for best extraction.

Generally, you want to be generous at each step, doing slightly more, so your ballpark figure can be as loose as you need to be to save time.  And of course, only you know if any of these procedures is detrimental to your product, in which case, it is better to do less.

Clear as mud now, right?

I was hoping for an easy formula to follow :( but it seems that this doesn't seem to be the case :(

Offline Babcock_Hall

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Re: Lab techniques
« Reply #7 on: October 05, 2012, 05:44:06 PM »
I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later.  If the solvent is all gone, the mass will be about the same.

Offline Messi

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Re: Lab techniques
« Reply #8 on: October 05, 2012, 05:45:48 PM »
I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later.  If the solvent is all gone, the mass will be about the same.

Nice! I like this approach Babcock. My supervisor who is post-doc never does that though and just rotavaps his product and looks at it then removes it at will!

Offline discodermolide

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Re: Lab techniques
« Reply #9 on: October 05, 2012, 06:05:34 PM »
I used to use the Nasal spectrometer, I hope you have one in your lab.
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Offline Babcock_Hall

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Re: Lab techniques
« Reply #10 on: October 05, 2012, 06:07:50 PM »
No, we don't.  What are its advantages?

Offline discodermolide

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Re: Lab techniques
« Reply #11 on: October 05, 2012, 06:51:51 PM »
It does not require high magnetic fields and liquid helium. It is cheap and dependable, cleaning several times a day can be a disadvantage, but the sensitivity is excellent. Blockages can occur but they are easily removed by poking them out with a finger.

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Offline Messi

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Re: Lab techniques
« Reply #12 on: October 05, 2012, 07:03:19 PM »
What is a Nasal spectrometer.... I just googled it but didn't find anything  ???

Offline Arkcon

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Re: Lab techniques
« Reply #13 on: October 05, 2012, 07:09:23 PM »
What is a Nasal spectrometer.... I just googled it but didn't find anything  ???

A device for detecting very low concentrations of volatile solvents. :P
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline discodermolide

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Re: Lab techniques
« Reply #14 on: October 05, 2012, 07:20:11 PM »
What is a Nasal spectrometer.... I just googled it but didn't find anything  ???


It is right in front of your face! Every lab has several of them. Unfortunately not all are new, some can be quite old, but the reliability is still excellent.
Development Chemists do it on Scale, Research Chemists just do it!
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