Heating chemicals.

[gainer]

For those concerned about using a microwave oven to heat chemicals such as propylene glycol, I did the following experiment.

The concern about heating flammable materials is that when the material is heated above its flashpoint, the heated vapor may rise over the side of the containing vessel, then flow down to the source of heat and ignite the liquid in the container. But what if the source of heat is a microwave oven? I heated 8 ounces of water to boiling in a Pyrex measuring cup, which took about 2 minutes, then removed the cup from the oven and felt various surfaces in the oven with my bare hand. There was no surface that was even warm to the touch. I ran the oven for 30 seconds with nothing in it but the plate that rotates. As long as the plate is clean, it does not heat. If it is wet or has something on it from a previous spill, it will get hot, so clean it and dry it before doing this test.

The source of heat in a microwave oven is in fact the material that is being heated by microwave radiation. The surface on which the supposed vapors would fall is much cooler than the surface of the liquid from which the vapors came.

I did not question the wisdom of the assumption that the vapors might rise due to heat in the vessel, then fall in spite of the heat outside the vessel. In any case, if you think the heating might proceed beyond the flashpoint of the material being heated, it appears that it is safer to do the heating in a microwave oven than over a flame or electric hotplate.

[Photo Engineer]

Patrick;

You do what you wish, and advise people as you wish. In this type of work, all it takes is one big accident. So, you will have to bear all of the responsibility, not me if anything happens. I urge everyone to caution and err on the side of caution to prevent accidents.

You have chosen to ignore the advice of several chemist such as Kirk Keyes. Good luck to you.

PE

[Ole]

Heating water in a microwave oven tells you exactly nothing about how other liquids will behave.

I would urge anyone even thinking of heating anything potentially flammable in a microwave oven to think twice, then twice more.

If you still think it might be a good idea, place the microwave oven in an explosion-proof enclosure and stand well back. Next county might be the best place.

[David A. Goldfarb]

I'd be concerned that vapors might be set off by a spark somewhere in the microwave oven, or that the liquid might heat unevenly from the center out or bottom up and the vapor could cause the liquid to erupt all over the interior of the oven (as it might if one were to boil a container of water with oil floating on top), messing it up for the purposes of drying test strips and warming coffee.

[pgomena]

I wouldn't think of doing this. I have seen a cup of coffee flash to a boil and spill over the sides of the cup many times.
Peter Gomena

[Trevor Crone]

Be very careful. Having been a laboratory technician all my working life we were only allowed to heat volatile chemicals in thermostatically controlled water/oil baths.

As I'm sure you are aware it only needs a spark. Very little is full proof but we must reduce the risks to an absolute minimum.

[Kirk Keyes]

For those concerned about using a microwave oven to heat chemicals such as propylene glycol, I did the following experiment.

Patrick, I expect better engineering from you than this test demonstrates. You need to think outside the box more...

[Jadedoto]

I would not heat chemicals up in a microwave.

A little about the microwave, IIRC, is that the glass doesn't get warm because the reason a microwave works is that the wavelength of the EMR is approximately the size of a water molecule. This causes the water molecules to vibrate as the waves pass through them to match polarity with the waves. The glass' molecules are far too large and compact to be vibrated in such a fashion.

[Kirk Keyes]

They make explosion proof microwaves for use in laboratories, and I am quite certain that a microwave for home use does not meet this qualification. In fact, they even have explosion proof refrigerators and freezers for lab use. And there is no heating of solvents going on in those.

Also, you don't know if the person trying this at home has a lit cigarette in their hand or mouth. It only takes a small ignition source, not a big, giant flame.

At no point has anyone involved in debating with you about the safety of heating flammable liquids in a microwave ever suggested that the walls of the microwave was getting hot and could start a fire. There are much more realistic potential sources of heat than that to be concerned about.

[Kirk Keyes]

In any case, if you think the heating might proceed beyond the flashpoint of the material being heated, it appears that it is safer to do the heating in a microwave oven than over a flame or electric hotplate.

Heating flammable liquids to near or above the flashpoint should not be done unless it is in a ventilation hood as is found in a lab. Not something most of use have in a home photo lab.