Sciencemadness Discussion Board

Microwaves for evaporation

wonderboy - 26-2-2016 at 17:20

I'm kinda a newbie here, but I was wondering if it would be safe to microwave a solution to evaporate it.

I need to evaporate some potassium carbonate solution to concentrate it.

Thanks!

UC235 - 26-2-2016 at 17:25

I would advise against doing so. While microwaves are excellent for heating polar liquids, and even better for salt solutions, they do not heat evenly and solid salts seem to preferentially suck up microwave power in many cases. With conventional heating, spatters of solution at the top of a beaker are not a problem, but in a microwave I find that they often become hot spots if the beaker is not covered. I have also had the misfortune to melt a glass container in the microwave when attempting to dry solid calcium chloride to anhydrous.

wonderboy - 26-2-2016 at 17:28

Would pyrex be able to stand up to a direct flame without shattering? Or would there be some better way to evaporate it?
(Sorry I am new to chemistry experiments.)

TheAlchemistPirate - 26-2-2016 at 17:36

Maybe a small stove top could be used with your pyrex glassware? Flames are generally bad for larger glassware unless you make some sort of sand bath.

wonderboy - 26-2-2016 at 17:39

I have heard that pyrex can shatter on stove tops. Is that true?

TheAlchemistPirate - 26-2-2016 at 18:13

To be honest I assumed that a stove hotplate and a lab hotplate would act the same with glassware. Recently I made R-candy in a large 1L beaker on a stove top with no problems, though that doesn't mean I could always get away with it. A quick google search showed that "Pyrex" can refer to either a crappy soda lime glass which will crack in direct heat, or the laboratory-grade borosilicate glass which won't break in most of these situations. It seems they don't make Pyrex cooking glassware which can be used with stoves anymore. So don't use your cooking Pyrex measuring cup! Maybe this will explain it better https://www.quora.com/Why-did-my-Pyrex-cup-explode-when-I-bo... . I'll make no promises but it seems that if you can use your glassware on a lab hotplate, it should work with a stove hotplate.

gdflp - 26-2-2016 at 18:53

Hot spots are definitely an issue with microwaves. I've experienced this in the past when I attempted to dry a potassium acetate solution that was refusing to crystallize by evaporation. Upon taking the beaker out of the microwave, the bottom was glowing red hot and the potassium acetate had charred leaving a positively foul smelling black goo. To add insult to injury, the 1L beaker cracked upon cooling as well.

TinSandwich - 26-2-2016 at 22:27

Pirex isn't supposed to shatter when heated, that being said I always use beakers for heating stuff.
If you want to be safe use a sand/oil/water bath depending on how hot you need it to be, if you want to take the risk of breaking your container you could heat it over a gas stove using a wire gauze wich you can buy from the dollar store or simply use an electric stove wich should distribute the heat more evenly.

[Edited on 27-2-2016 by TinSandwich]

[Edited on 27-2-2016 by TinSandwich]

Great - 26-2-2016 at 22:54

I use a microwave all the time whenever I'm doing non toxic stuff. The hot spots are never a problem if you watch and stir it. Just DONT MICROWAVE UREA! The hot/ cold spots from it dissolving will severely fuck your glassware with the long dick of science. I've lost 2 pieces of glass from evaporating and dissolving stuff carelessly.

Sulaiman - 27-2-2016 at 02:02

I believe that "pyrex" is toughened glass and "Pyrex" is borosilicate
kitchenware is thick to be robust so heating and cooling should be gentle and even.
Thin laboratory flasks can tolerate more rapid and/or uneven temperature changes/distribution.
(minimal contact with hotplate, standing wave hotspots in microwave ovens, hotter/cooler areas with flame heating ....)
some info here (manufacturer of glass tubing that I have bought)
http://www.kavalier.cz/en/section/32-simax-glass-mass.html

glass is not a particularly good conductor of heat (e.g. iron has about 50x the thermal conductivity)
so temperature gradients (hot spots) develop much more easily.
it's good but not perfect, think even and gentle.

Great - 27-2-2016 at 09:35

Quote: Originally posted by Sulaiman  
I believe that "pyrex" is toughened glass and "Pyrex" is borosilicate
kitchenware is thick to be robust so heating and cooling should be gentle and even.
Thin laboratory flasks can tolerate more rapid and/or uneven temperature changes/distribution.
(minimal contact with hotplate, standing wave hotspots in microwave ovens, hotter/cooler areas with flame heating ....)
some info here (manufacturer of glass tubing that I have bought)
http://www.kavalier.cz/en/section/32-simax-glass-mass.html

glass is not a particularly good conductor of heat (e.g. iron has about 50x the thermal conductivity)
so temperature gradients (hot spots) develop much more easily.
it's good but not perfect, think even and gentle.


The brand name pyrex has become synonymous to borosilicate, make sure that what you're using actually is. I've found 'Pyrex' dishes and kitchen type beakers made of toughened glass so the only real way to tell is to search the product online or whatnot.

BromicAcid - 27-2-2016 at 09:58

After recently looking for cookware for the kitchen I found that in the US the name Pyrex has become decoupled with borosilicate, nearly anything Pyrex you see for the home in the states is instead a tempered glass. However, in the European countries they were unable to decouple the two for legal reasons and so the Pyrex labeled homeware you purchase there is borosilicate. At least as far as I have learned.