[Messi]

You guys are witty! Yes, a nose is quite important

[NotExactly]

+1 for nasal spectrometer. 

Here are a few more thoughts on your questions:

Regarding excess solvent, if you have access to a high vac (i.e. a pump that pulls really hard, like 0-1 torr), leave your material on there (assuming it is not volatile) after roto-vapping for a few hours and your material will be dry.  This may be over the top but if you can, NMR is a surefire way to determine if there is still sample present.

Regarding drying agents and water washes, I'd highly recommend you wash your organic layer with brine (aqueous saturated sodium chloride) after the water wash.  The will get rid of a lot of the water in your sample and decrease the amount of drying agent you have to use.  Typically you need to add enough MgSO4 until you don't see clumps forming, but I always thought it was kinda hard to see this effect.

[helenn]

You got me with the nasal spectrometer there! I wouldn't always recommend that approach though.

I would judge the rotovap by looking to see if there is any solvent still dripping off the condenser. And then if there is a control panel you can lower the pressure to make sure the solvent is off.

With the MgSO4 I was taught that if the MgSO4 is free flowing and not clumping then you have added enough. I always add extra for good measure!

[discodermolide]

I fell into the MgSO₄ trap the first time I scaled something up in the plant! I added about 5g for a 20g reaction product in the lab. In the plant 200kg requires 50kg MgSO₄. I was told to go away and think again.
Actually the best way to dry something is a toluene azeotrope. Twice or three times and you are under 100 ppm water by Karl-Fischer.

[curiouscat]

A device for detecting very low concentrations of volatile solvents.

Spoilsport!  Delete it.

[curiouscat]

Why does a brine wash help reduce water from the organic phase? 

[curiouscat]

I fell into the MgSO₄ trap the first time I scaled something up in the plant! I added about 5g for a 20g reaction product in the lab. In the plant 200kg requires 50kg MgSO₄. I was told to go away and think again.

What's the right answer? How much to add?

[discodermolide]

Not much 0.1-0.2g if you have to, as I said azeotrope with toluene.

[curiouscat]

Not much 0.1-0.2g if you have to, as I said azeotrope with toluene.

Ah! Got it.

[fledarmus]

Why does a brine wash help reduce water from the organic phase? 

An extraction is a separation of material based on it's relative solubility in two different solvents. Normally, these two solvents are idealized to water and octanol and it is given that since these are immiscible solvents, there is no water in the octanol and no octanol in the water.

Once you start adding other components, or using more polar organic solvents such as chloroform or ethyl acetate, you do get some forces attracting water into the organic layer, and as you'e noticed if you've tried to strip one down, it can be quite a bit of water. One answer to this is to use a solvent that is even more attractive to water than plain water, and a good cheap one is a saturated sodium chloride solution. As you can tell from osmotic pressure experiments, strongly ionic solutions will pull water molecules away from pure water. It turns out they also pull water away from organic solvents that have water entrapped either due to some limited solubility of water in the solvent or to hydration of various solutes that might be present.

Adding salt to your water also does some other very useful things in extraction, like limiting the solubility of organic materials in the aqueous layer and breaking emulsions, but for that final wash with sat'd sodium chloride to remove water before using a drying agent, this is the effect in play.

[curiouscat]

Thanks @fledarmus!

Another explanation I've heard in the plant was that brine increases the density difference versus lighter organics; thereby aiding phase separation. Not sure if that's true. But I'd never thought a brine wash might help dry the organic! Live 'n learn! 

Practical problem with brine washes I faced in production was too much TDS in the effluents.

[discodermolide]

If you have too much TDS in the waste, store it and dilute it with someone else's waste. Day by day the amount will reduce itself to zero.

[curiouscat]

If you have too much TDS in the waste, store it and dilute it with someone else's waste. Day by day the amount will reduce itself to zero.

I'll see what the EPA thinks about this...

[discodermolide]

Nothing as long as you stay within the plant limits for waste disposal to the water treatment plant.

[Messi]

Another lab technique question!

So I have THF and I am told to dilute my mixture with ether because I will be doing an extraction with water there after. If I have ~15mL of THF + my product how much ether should I add to dilute my mixture?

You guys are the best!