Student Comments Fall 2008
Melting Point
When attaching the capillary tubes, slide the rubber band down to the bottom of the thermometer first. Do not force the capillary tube underneath the band while it is around the thermometer. This can snap the tube, causing a delay. MSub
During the procedure, one of my unknowns had the same melting point as one of my already identified unknowns. Don't be alarmed if this happens because your 2 unknowns are isolated from the same substance. It turns out that it was 'masked' by the other unknown present in the substance and as a result, the proper melting point of the other unknown substance was not able to be seen. SF
Some things to keep in mind: the physical properties of the compounds researched in your prelab keep lists melting point ranges in either Chemfinder or Sigma-Aldrich are the melting points of compounds as they are solid. Also, If the compounds are slightly impure the melting point range will reflect this fact; since the melting point listed in the
Merck Index or MSDS are the melting point of pure compounds. NMR
In order to get the solids into the capillary tube put a little bit of the chemical on the center of a piece of filter paper. Then fold the paper into a half circle with the chemical still in the middle. Pinch the filter paper in the center so that the solid sits on one side of your fingers inside the filter paper. Then take the open end of your capillary tube and twist it on the solid chemical. Your fingers will keep the chemical from sliding away from your tube and filling the tubes will go much faster using this technique. CA
When determining melting point range, be careful that you do not pull out your capillary tube before your sample has actually melted. It's easy to think that your sample has melted because the condensation on the inside of the tube makes it difficult to see. This is why a melting point range is helpful and could be defined as the temperature at which liquid appeared containing solid crystals to the temperature at which the entire sample in the capillary tube was a just a translucent liquid. AM
Some solids possess a rock-like texture therefore before you can get them into a capillary tube you might need to crush them. BL
Also, for the final stage of this experiment (comparing your unknowns to known compounds) be sure to use the same amounts of the compounds (unknown and known) in both capillary tubes otherwise your melting ranges might be off and this will make it harder to determine your unknown. BL
To quickly and properly fill the capillary tubes tap your tubes over the desired compound.
Once you've done this place the tube in between your thumb and index finger and let the tube slide in between them. The tube will bounce off of the flat surface and slowly but surely pack the compound into the tube. With enough practice you can bounce the tube repeatedly. HI
If your substance is hard to break down, try crushing it and then gently trying to insert it into the capillary tube but be careful because you don't want to break the capillary tube!
NNM

Heat SLOWLY. By heating slowly -- VERY slowly -- you will be able to get accurate m.p. ranges. You will be surprised how quickly the solid melts and you may have your head down writing the initial m.p. starting temperature and by the time you look up the solid has melted and you don't know where the end of the m.p range was. Take the extra time in lab to heat extra slowly and you won't end up repeating trials because you missed the range. JW
It is definitely wise to heat it slowly because I had trouble on the first part because I didn't think it melted and i ended up waiting until it reached too high of a range for the melting point. Make sure you do watch it right away or you will be spending too much time on the lab. NT
Make sure that the capillary tubes are not wet inside or around the edges when trying to fill them with your substance because then it will get stuck and not fill all the way and you will be sitting there trying to smash it in NT
I was able to use the melting point apparatus and the thermometer and capillary tube method. Although, the thermometer and capillary method took more time I was able to control the amount of heat to get a more accurate reading. When I decreased or increase the heat it went into affect faster. With the melting point apparatus if my heat was too high it took longer for the temperature to decrease. LC
If you get inconclusive results for the melting point of one of your substances do not be afraid to heat a fourth or fifth time. Using more trials gives you more accurate results when gathering the average melting point. JS
The melting point apparatus is very sensitive. There may be a tendency for the temperature to rise quickly and then stabilize when the compound is heating. When the compound is mostly gone, the temperature can either jump up or start to decline. Both are normal. Just note what happens. BES
When completing this it is very important to pay close attention to your solid. Taking the time not only to heat slowly but notice the time difference between when the solid begins to melt and has completely melted is very helpful. The more ambiguity you can avoid the better the results. DF
I found the easiest way to pack the capillary tubes was to use rubber gloves and place a small amount on your finger. You can then press the tube gently against your finger to get the desired amount. Also I found it to be useful to heat as slow as possible. You will get much better data that way. BR
Make sure you record the initial and final temperatures. This will help you determine the melting point of your compound. JVil
Make sure the sample size in the capillary tube is small otherwise your results will be skewed. Too much can increased the time it takes or temperature to melt. Knowing the literature melting points of the compounds and also testing them with rapid and slow readings in the Thiele tube helps narrow done the samples and is a faster and cheaper method of determining compounds. A range of greater than 5 degrees Celsius will indicate that a compound is impure. KM

Be sure to pay very close attention to your capillary tube filled with the sample, because it is very easy to remove the capillary tube before the sample has completely melted.
The more patient that you are with this experiment, the more accurate your results will be. KJ
The use of thermometers and capillary tubes attached to them in the oil is not very productive because it is very hard to see the compound actually melting down, and recording the actual range of m.p. since there is a lot of bubbles coming from the oil boiling and the tube gets misted. Using Thiele tubes is a lot easier and more accurate.
OP
I would suggest doing more trials of the melting point range. This would allow for a better recording if you heated the product too quickly the first time. JVar
Make sure to cool down the Thiele tube between trials. A compound might have a melting point that is less than the thiele tube temperature. LM
Make sure you record "ranges" of temperatures from the starting m.p to the ending m.p. so that you get more accurate results when comparing to other compounds. PO
Do not clamp the Thiele tube too tightly so that it will remain in place, it will break!
It is a good rule of thumb to not clamp ANY glassware too tightly. RR
I also recommend not to look away when you are trying to get your first melting point range. You may miss the melting point and the step would have to be repeated. AJohn
When setting up the apparatus if your oil does not begin to get hot quickly or seems to have a constant temperature there might be water in it preventing the temperature from rising anymore KB
During the experiment, I had to use the electric heating chamber (instead of the hot oil bath). If equipment is running short or has been broken, likely someone will have to use the electric heating chamber again next year. One thing to remember is that the chamber heats up more quickly than the hot oil, but it also takes much longer to cool down. Keep this in mind when managing your lab time. You need to make sure the chamber has cooled to a temperature lower than any of the potential melting points before starting the next trial or your results will not be accurate. KV