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]

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.

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).]