bob800
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Melting point determination and DIY capillary tubes
Maybe this is common knowledge, but I thought it might be helpful to document my recent melting point setup. Without professional equipment like
spectrometers or chromatographs, melting and boiling points become essential tools for identifying and testing compounds for purity. As it turns out,
taking a melting point is a much more tedious process than that of a boiling point, though it can still be done in a home setting to a good degree of
accuracy.
Equipment:
Propane torch and bunsen burner
Soda-lime glass tubing, 6-8 mm
Thermometer, partial immersion
Thiele tube (preferred) or test tube
Lab stand and clamp
Thin rod for stirring (if using test tube)
Triangular file
1-hole stopper
Small rubber band
Pulling capillary tubes
The actual length of 6-8mm tubing is of little importance, as multiple capillary tubes may be pulled from the "scrap" remaining after pulling. Just
ensure that it is long enough to keep your hands away from the flame!
While spinning the tube with your fingers (equally with both hands), move it back and forth to heat 2-3 inches of the central region.
Continue heating until the tube begins to noticeably sag under the heat:
Remove the tube from the flame, and in one smooth and swift motion, pull the ends apart to the desired capillary thickness:
If you cannot easily pull it apart at this point, do not place the tube back in the flame and attempt to pull in the heat. This probably indicates
that you are either not heating long enough, or that your heat source is not strong enough (a regular Bunsen is generally not
sufficient).
Set the tube on a surface of low thermal conductivity (e.g. on other glass tubes), and allow it to cool.
Lean the tube off the edge of a table, and score one end of the capillary portion.
Usually the scoring alone is enough to break the capillary with a fairly clean edge:
However, if it does not break, grip the tube as shown, then pull outwards and bring your thumbs towards you (gloves are a good idea but make snapping
difficult):
Gently hold one end of the tube in a pilers, and seal the opposite in the hottest (tip of blue cone) portion of a bunsen flame. Hold the tube as
vertical as possible; it is very easy to curve the tip if you are not careful:
Repeat to polish the other end, inspecting it frequently to avoid sealing.
Packing the tube
Spread the compound to be analyzed on a watch glass or similar container, and press the open end of the tube into the powder. If you experience
difficulty, grind your compound to a finer consistency. You are aiming to fill the capillary with about 3-4mm of sample:
Stand a spare length of tubing on a table, and drop through it your capillary tube. Repeat several times until all sample falls to the bottom, then a
few more times (air pockets can affect melting points!).
Taking the melting point
Line up the capillary tube with the bulb of your thermometer; it should extend beyond the immersion marking on the thermometer. Secure with a rubber
band:
Slide a rubber stopper over the thermometer, fill a test/thiele tube with mineral oil, and setup as shown. Allow the stopper to rest upon the
test tube to allow vapors to pass, or cut a notch on the stopper:
SLOWLY begin heating, taking the stopper out from time to time and stirring (if test tube is used). When you approach 10-15 degrees of the estimated
melting point of your solid, lower the heating rate to 1 degree/min.
Record the temperature the solid first begins to melt, and then the temperature at which the entire mass has melted.
I tested some naphthalene moth balls of "99.95%" purity, and obtained a mp of 80-81 degrees (lit. 80.22), so I believe the setup is fairly accurate.
References: "Laboratory Exercise: Melting and Boiling Points", www.nmt.edu/~jaltig/MeltingBoiling.pdf
Comments, suggestions, etc? (too many pictures? )
[Edited on 15-7-2013 by bob800]
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bfesser
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Wonderful. This is very similar to pulling TLC capillaries. Any chance you plan to do a tutorial on that? Also, where did you get that awesome
burner?
(I'm going to add this to my <strong><a href="viewthread.php?tid=25000#phys_prop">Topical Compendium</a></strong>
immediately!)
[Edited on 7/15/13 by bfesser]
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Magpie
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Nice work!
FYI, my practice:
1. I place enough powder in the tube for 3-4mm.
2. I know the tube drop method is safest but I can never seem to get enough impact this way. My method is to tap the closed end against a table top
as hard as I can without breaking the tube.
3. Instead of a rubber band I use about a 3-4mm section of latex rubber tubing.
4. I mount the thermometer in a rubber stopper with a notch cut out of the side. This allows for air egress during heat up.
The single most important condition for a successful synthesis is good mixing - Nicodem
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bob800
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Quote: Originally posted by Magpie  | Nice work!
FYI, my practice:
1. I place enough powder in the tube for 3-4mm.
2. I know the tube drop method is safest but I can never seem to get enough impact this way. My method is to tap the closed end against a table top
as hard as I can without breaking the tube.
3. Instead of a rubber band I use about a 3-4mm section of latex rubber tubing.
4. I mount the thermometer in a rubber stopper with a notch cut out of the side. This allows for air egress during heat up. |
Thanks! I'll change the amount to 3-4mm; I hadn't experimented with the amount of powder but I'm sure you know better than I do. Also, I just let the
stopper rest on the tube--I should add that point.
@bfesser: Thank you. The torch (I assume you are referring to the propane torch) is a Benzomatic Surefire TS8000 from my local hardware store. I've
never done any TLC work but I've been meaning to learn about it; I might make some eventually.
[Edited on 15-7-2013 by bob800]
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bfesser
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I'm sorry, my question was ambiguous. I meant the silvery bench top (fancier-than-a-Bunsen) burner you show in the first photo (the one with the
latex tubing and needle valve).
I mastered pulling TLC capillaries in undergraduate organic lab. Once you're comfortable with it, it can be quite therapeutic.
<iframe sandbox id="benchfly-1366738442000-gHx9mivWKa" src="https://secured.benchfly.com/player/1366738442000-gHx9mivWKa/" width="640" height="360"
frameborder="0" scrolling="no" allowfullscreen webkitallowfullscreen mozallowfullscreen oallowfullscreen msallowfullscreen></iframe>
[Edited on 7/15/13 by bfesser]
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bob800
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| Quote: Originally posted by bfesser | | I'm sorry, my question was ambiguous. I meant the silvery bench top (fancier-than-a-Bunsen) burner you show in the first photo (the one with the
latex tubing and needle valve).] |
Ooh, I purchased it from Avogadro's Lab Supply: http://www.avogadro-lab-supply.com/item.php?item_id=470. I believe their stock photo still included the blue base when I bought it, but they sent
me a silvery base instead.
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amazingchemistry
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One tip from Zubricks "Organic Chem Lab Survival Manual" (Carried religiously during my undergraduate days) is this: "If you can see it, you can see
it melt." Bigger samples melt unevenly, causing errors. Perhaps you can look into the tube though a magnifying glass? Most melting point apparatuses
(apparati?) have some sort of magnifying device. I was wondering if heating a beaker on a stir/hotplate would work as well (or maybe better) than a
thiele tube.
Always remember to check your pride at the door and ask that "stupid" question. Learning comes from having your "stupid" questions answered.
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roXefeller
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I was at my coffee table today doing some soldering on some surface mount components. I had my Chinese Kendal SMD rework station with iron. As I was
working I had a thought about using its digital temperature control and high airflow to determine melting points. So I looked for some pure reagents
to fill my capillary tube with. At first I found that the temp readout was wrong by 20-30 degrees. Opening up the enclosure I found only three trim
pots. So I tried each one and found the one that brought the temp down. So I dialed in my standard melt point and kept adjusting until it was
calibrated. I like it because the high airflow allows me to maintain a gentle heat up that I can turn up or down quickly so it's more agile than a
tube of oil.
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