Working with a Mixed-Solvent System— The Good Part

Every year I run a chem lab and when someone is doing a vacuum filtration, suddenly I ll hear a scream and a moan of anguish, as water backs up into someone s filtration system. Usually there s not much damage, since the filtrate in the suction flask is generally thrown out. For vacuum distillations, however, this suck-back is disaster. It happens whenever there s a pressure drop on the water line big enough to cause the flow to decrease so that there is a greater vacuum in the system than in the aspirator. Water, being water, flows into the system. Disaster. 

for your own protection, make up a water trap from some stoppers, rubber tubing, a thick-walled Erlenmeyer or filter flask, and a screw clamp (Fig. 48). Do not use garden variety Erlenmeyers they may implode without warning. Two versions are shown. I think the setup using the filter flask is more flexible. The screw clamp allows you to let air into your setup at a controlled rate. You might clamp the water trap to a ringstand when you use it. The connecting hoses have been known to flip unsecured flasks, two out of three times. 

WORKING WITH A MIXED-SOLVENT SYSTEM —THE GOOD PART 

after sufficient agony, you cannot find a single solvent to recrystallize your product from, you may just give up and try a mixed-solvent system. Yes, it does mean you mix more than one solvent, and recrystallize using the mixture. It should only be so easy. Sometimes, you are told what the mixture is and the correct proportions. Then it is easy. 

For an example, I could use solvent 1 and solvent 2, but that s clumsy. So I ll use a rethe ethanol - water system and point out the interesting stuff as I go along. 

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Working With a Mixed-Solvent System—The Good Part 129... 

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