Sciencemadness Discussion Board

Tips learned through experience

Melgar - 10-11-2017 at 10:53

We've all learned time-saving shortcuts, useful rules of thumb, and ways of doing things that are more conducive to amateur chemistry than what's taught in books. So let's share some of that?

A few things I've learned:

To generate ammonia gas, thermal decomposition of urea is superior to just about everything else I've tried. You only need 200C temperatures or so, there is a lot less water than other ways of generating it, and urea is very cheap.

To generate HCl gas, I just have a stopper with a piece of glass tubing in it, that bends in a v-shape. It comes to a point like an eyedropper does. I have a lot of MgCl2 that I bought in the form of driveway salt because it was cheap, and it comes as prills. Anyway, I just put some of this stuff in a flask, add a few mL of H2SO4, and put the stopper in the neck as quickly as I can. To stop the HCl generation, just add water. This method is also anhydrous, incidentally, due to H2SO4's strong affinity for water. You can also use NaCl, preferably as rock salt so that the sulfuric acid can reach the bottom, and also so that it goes more slowly. Rate can be increased by applying heat. Don't use CaCl2 though, because it's hard to clean out. For generating larger amounts of HCl gas, like for saturating a methanol solution, you can use a filter flask. Attach a dropper funnel to the neck with a stopper, then run tubing from the vacuum connector to your solution. Put H2SO4 in the dropper funnel, then drip that onto your chloride salt. Use a suckback trap if you think you need one (you probably do).

If you accidentally add too much sodium bicarbonate to a flask, and you're watching the foam rapidly approach the top of the flask, knowing it's going to erupt all over your workspace, there actually IS something you can do about it! Just clamp your hand over the opening. (You are wearing gloves, right?) CO2 doesn't really generate that much vapor pressure when coming out of an aqueous solution, and you should have no trouble maintaining pressure until the evolution of gas slows down. The foam will gradually recede as the bubbles pop. Just release pressure gradually until it subsides.

If you're drying out a hygroscopic salt that keeps forming a skin on the surface and preventing evaporation, you can add isopropanol to crash out a lot of the salt and eliminate the skin. Isopropanol also forms an 85% azeotrope with water, which is really useful for removing the last traces of water without heating the salt up too much. Just make sure the salt doesn't react with isopropanol, obviously.

Silicone oil is a good anti-foaming agent. So is hair conditioner, which contains cetyl alcohol as a main ingredient.

14/20 glassware breaks far less frequently when it's dropped than 24/40. Really, the bigger a piece of glassware is, the more likely it'll break when dropped. Over the course of a year, I probably saved several hundred dollars worth of glassware that would have broken when I dropped it if it was 24/40. Sometimes your gloves have slippery stuff on them and you don't realize it. And while beakers and the like are cheap, glassware with ground glass joints is not. I've also saved a lot of money on chemicals too, because reactions can be done on a smaller scale. The only annoying thing is that it's really hard to find 14/20 flat-bottomed ANYTHING, and most of the flasks that are available are 50 mL or less. 250 mL and 125 mL can be found if you look hard enough, but they're always round-bottom, and I'd rather just be able to set my flasks down on a flat surface than try to figure out the mechanics of setting up an oil bath or whatever. Usually what I do is just use a 24/40 flat-bottom flask with an adapter.

What about you guys?

LearnedAmateur - 10-11-2017 at 11:49

When I'm distilling something or bubbling generated gas into a solvent, I do so into a borosilicate measuring cylinder using a long tube distillate receiver. It gives a quick indication on how much distillate has come over thus when the reaction is nearly complete, and in the event of suckback, the low volume to cylinder height means that the surface level decreases quickly and pressure is equalised before it can rush back through the condenser. Plus, it picks up the last few drops of distillate trapped in the apparatus. This probably won't be helpful for large scale operations but it's definitely useful for small scale (<50mL in my case but up to 100mL) procedures.

Vosoryx - 10-11-2017 at 15:01

When extracting DCM from the heavy duty stripper, distill right out of the can it comes in. Saves you hours in clean up. (Just add teflon tape around the stillhead to bring up the diameter so it'll fit.)

No fractionating column? Use a spare uncooled condenser, at a slight decrease in efficiency.

Ill come up with more in time, im sure. Just two off the top of my head.

EDIT: mechanical change because i hate writing this on my phone.

[Edited on 10-11-2017 by Vosoryx]

NEMO-Chemistry - 10-11-2017 at 15:16

When working with Nitric acid I prefer no gloves, most cheap types that dont react are slippery. Most common types used generally, react with Nitric, also i tend to be more careful generally when glove free with acids.

Use silicon grease with all glass stopcocks, i dont like leaving them out cos i loose them.

Dont boil sulphuric acid then place on a cold surface.........

Crowfjord - 10-11-2017 at 15:24

There is a similar thread to this one: Laboratory tips and tricks.

I have found that in a pinch, acetone will substitute fairly nicely for ethyl acetate (mixed with petroleum ether or ligroin) in silica gel TLC. That is, instead of 1:1 ethyl acetate: pet ether, substitute 1:1 acetone: pet ether. I only tested this with substituted aromatics, but the separation was on par with what I would normally get with ethyl acetate.