Heating chemicals.

Discussion in 'B&W: Film, Paper, Chemistry' started by gainer, Jan 6, 2008.

  1. gainer

    gainer Subscriber

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    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.
     
  2. Photo Engineer

    Photo Engineer Subscriber

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    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
     
  3. Ole

    Ole Moderator Staff Member Moderator

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    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.
     
  4. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    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.
     
  5. pgomena

    pgomena Member

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    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
     
  6. Trevor Crone

    Trevor Crone Member

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    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.
     
  7. Kirk Keyes

    Kirk Keyes Member

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

    Jadedoto Member

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    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.
     
  9. Kirk Keyes

    Kirk Keyes Member

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    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.
     
  10. Kirk Keyes

    Kirk Keyes Member

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    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.
     
  11. Stephen Frizza

    Stephen Frizza Member

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    OH MY GOD!!!! are you wanting to be next in line for the Darwin awards!!!!
     
  12. jacobj

    jacobj Member

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    Gainer's hypothesis isn't entirely flawed. I see one oversight though. The water heating scenario makes the incorrect assumption that microwave radiation heats similarly to convection--that is to say evenly. In fact, radiation tends to heat very unevenly, leaving hot-spots and cool-spots in the subject medium. Water tends to distribute heat very evenly through it's volume. When water is placed inside of a microwave and exposed to radiation, the exposure does not create lasting hot spots. Hot-spots ARE created, but they rapidly dissipate, especially on removal of the radiation.

    I'm sure we've all experienced putting a delectable food item into the microwave and guessing at the time. When the item is removed, you must check it to make sure that it is evenly heated, particularly with frozen items, there will be a few spots that have thawed, but others that are still frozen solid. The new microwaves with the turn tables help, but it's still an issue.

    I am not sure of the precise properties of propylene glycol, but I'm willing to bet that if one were to put it in a microwave for very long at all, hot-spots would quickly develop. Although the average temperature of the substance might not reach the flashpoint, different areas within the container would rapidly approach them.

    I would venture to say that using a microwave would be more dangerous than an open flame, with a microwave, it's much more difficult to gage whether or not the propylene glycol is about to boil over, or to get any real indication of how hot it might be. Again, even if one was to measure the temperature of the propylene glycol while it was in the microwave, one could only get an average temperature.

    I've exploded a number of things in a microwave, mainly peeps, grapes, and hot dogs. Propylene glycol would definitely be ranked low on my list of things to try.. right next to gasoline.
     
    Last edited by a moderator: Jan 7, 2008
  13. eclarke

    eclarke Member

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    Also, you can buy an electric hotplate at Walmart for $20 which will warm at gentle temperature. With a little diligence you can buy a nice heated Corning stirrer on eBay for a fair price and it is a wonderful luxury for mixing chemicals...EC
     
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  15. matti

    matti Member

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    When I mix Pyrocat-HD in PEG, I just mix in a bottle and heat in tap water. Seems like not much is needed if I just can wait a while. I swirl it around gently and don't shake it, not to introduce too much air into the liquid.

    /matti
     
  16. gainer

    gainer Subscriber

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    So your advice would be not to use the microwave to reheat a pizza or sub or to cook anything that might be flammable. I'm surprised that you do not know the definition of flashpoint. Don't tell me that having used the test apparatue to measure flashpoint shows that you know what it means. What, for instance, is the flashpoint of diesel fuel? Is that the temperature required to ignite a fuel-air mixture in the cylinder by the heat of compression?

    Heating water was not for the purpose of learning what happens to the liquid, but to show that the microwave walls, floor and ceiling were not heated in the process of heating a liquid to the flashpoint of glycol, and demonstrating it safely.

    The flashpoint of propylene glycol is close to the boiling point of water. Its autoignition point is 700 F. If there is no external ignition source, it will not ignite until it reaches that temperature. It boils at 372 F, at which point the vapor pressure at the liquid surface equals atmospheric pressure. It is a little lower here as I am at about 900 ft. altitude. That means that in order for autoignition to occur, the vapor must be at 700 F or must come in contact with an object at that temperature. Furthermore, there IS such a thing as partial pressure of glycol vapor in the atmosphere. It is dependent on the concentration of glycol vapor in air, and the ignitability of that vapor by a spark or open flame will depend on that concentration. There is a partial pressure gradient from the surface of the glycol upward. That is why the spark or open flame must be near the surface if it is to ignite the vapor at the flashpoint. The gradient is diferent in the closed cup test than in the open cup test. The situation in the microwave is that of the open cup tester. If there is a spark somewhere in the microwave, what conditions will allow it to ignite the surface of the container of glycol? The partial pressure of glycol vapor will have to be the same as it is at the surface of the glycol at its flashpoint. In order for that to happen, the glycol will have to be at a much higher temperature than the flashpoint.
     
  17. Murray@uptowngallery

    Murray@uptowngallery Member

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    Of course a microwave manual is written to avoid unsafe conditions, misuse, etc., but they usually say not to run one empty (or with paper towels). I think it may have something to do with the empty oven space functioning as a microwave 'cavity' and is part of the RF load on the magnetron.

    A friend (now an electrical engineer) once heated a jar of caramel ice cream topping and didn't realze it had a foil label. He came back to find a shattered jar & a mess to clean up.

    Certain he was smarter than the average microwave, he soaked the next jar in water to remove the label. This one he observed thru the glass door. It started to boil over, then developed a purple corona then shattered again.

    I'm reminded of the fun we used to have experimenting at school with the student union microwaves.

    We submitted a variety of metallic and food items to irradiation endurance tests.

    We found that metallic items close to the walls didn't create fireworks (forks, thin flat foil ashtrays), but all hell broke loose with larger items close to the center of the the cavity (cafeteria napkin holder).

    Something to do with standing wave behavior at the walls. Not sure how many conclusions can be drawn from heating behavior at the bottom of the microwave.

    Canada Mints were fun (kind of like those 4th of July 'snakes') and relish packets would clear the building.

    Regarding glass, ceramic molecule size, I always associated microwave 'safeness' of vessels to be an issue with how 'lossy' a material they were made of. I don't know if this is related to molecule size or something else. I have had some ceramics that said "microwave safe", but the contents didn't heat as well and the ceramic came out hotter than other vessels. (Clay mineral content?)

    Similarly, some polymers are terrible insulators at certain frequencies and melt in certain installations (nylon at vhf and higher for example, in antenna structures). Is a nylon molecule smaller than a Teflon one...maybe. Does the polymer chain length matter?
     
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  18. pesphoto

    pesphoto Member

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    Peeps in the microwave are a thing to behold.
     
  19. Photo Engineer

    Photo Engineer Subscriber

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    Patrick;

    If the vapour comes in contact with an object at or above its flashpoint, it can ignite. So, your statement above that "the vapour must be at 700F or must come in contact with an object at that temperature" is in error as both Kirk and I have repeatedly pointed out to you in posts.

    The lightbulb in my microwave could reach the flash point temperature during the heating process! IDK, but I don't want to run that test.

    I have boiled most organic solvents over my years in the lab, and I always used a steam bath or insulated electric hot cup to keep from igniting vapors. I also had a condenser over the flask in most cases to insure that there was no vapour escaping from the flask or container while heating. In spite of all of this, I have had at least two serious fires in the lab. I don't recommend that experience for anyone.

    PE
     
  20. Photo Engineer

    Photo Engineer Subscriber

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    PEEPS?

    PE
     
  21. pesphoto

    pesphoto Member

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    peeps
     

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  22. gainer

    gainer Subscriber

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    The disipation of hot spots is due to convection as well as the rotation of the turntable. Glycol, or for that matter any liquid, is subject to those forces. Some microwave ovens have temperature probes that allow heating to a set temperature. You also seem not to have studied about the difference between flashpoint, boiling point, autoignition and other characteristic temperatures.

    One does not live my 80 years without learning some precautions. Microwaves have timers for a reason. If you don't know how a substance will behave, try a short time, like 10 or 15 seconds and measure the temperature rise. As for using a hot plate, check the temperature of the plate. There must be a temperature gradient between the plate and the liquid you are heating, or heating will not take place.

    I doubt that anyone can prove that glycol vapor from a surface at the flashpoint (~210 F) will rise above the side of the container and fall onto a surface that is hotter than the one it came from, but if that is all that is required to ignite the glycol, then the hot plate is no better than the microwave, and may be worse since its surface is necessarily hotter than that of the microwave. A hot water bath heated by a submerged element might be the safest.
     
  23. jnanian

    jnanian Advertiser Advertiser

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    :tongue:


    reminds me of a guy who asks people to send him their OLD HARD peeps.
    he sets them up on the street in a little box and makes dioramas ...
    has a little hole in the box to look in, he calls it a PEEP SHOW ...
    some people are not amused ...

    but nuking-a-peep, THAT sounds like quite the peep show!

    -john
     
  24. pesphoto

    pesphoto Member

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    hehe..a peep show!

    They slowly expand to about 10 times their normal size and then...........
     
  25. Photo Engineer

    Photo Engineer Subscriber

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    Convection in water is more rapid than PG due to viscosity. Therefore, I would believe that PG would develop hot spots more easily than water would. Also, since water has a very high specific heat, it is harder to drive it into boiling with a given amount of energy. IDK the factors for PG, but I can say that due to many physical constants associated with these two liquids, their behavior would be vastly different in all situations where heat is applied.

    PE
     
  26. gainer

    gainer Subscriber

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    Not by any definition of flashpoint I have ever seen, except yours. Not by any test of ignition I have ever seen, either. You have funny ideas about fluid dynamics. Heat a screwdriver to 250 or 300 F. You can do that in a deep fryer. Now hold it near the surface of a small amount of propylene glycol heated to 212 F. It will not ignite the glycol. Heck, plunge the screwdriver into the glycol. Time out while I go check the flashpoint of gasoline to see if I dare open my gastank.