A DIGITAL MELTING POINT APPARATUS
A very frequently performed operation in organic chemistry laboratories is the determination of the melting point of a solid specimen. The temperature at which a sample melts is indicative of the purity and identity of the substance. Originally the sample, contained in a 1.5 mm diameter glass capillary tube, was heated in a bath filled with mineral oil ( or sometimes concentrated sulphuric acid ) with a Bunsen burner until melting occurred, and the bath temperature read from a thermometer. This method involves some hazards from the use of an open flame and the chance of spilling the hot liquid while the operator has their face next to the bath observing the sample. Electrically heated melting point devices were introduced to reduce these risks, and there are now several designs on the market, ranging from a simple electrically heated metal block up to computer controlled automatic instruments which record the entire operation on a video camera. Prices for these items range from a few hundred up to several thousand dollars.
Here I describe how to build an apparatus which will do the job at very reasonable cost, while providing many of the features of the more expensive commercial versions. The following items are required ( approximate prices as found on eBay )
[A] Aluminium Heating Block, 30 mm x 20 mm x 20 mm $ 5.00
[B] Soldering iron 220 V 60 W Temperature Controlled ~400 C $ 18.00
[C] Magnifier loupe, 3 X $ 2.50
[D] 2 x Computer Cooling Fans, 40 mm ø ,12 V DC $ 4.00
[E] Rocker switch, SPST 220 V + SPST Toggle Switch $ 4.00
[F] Digital Thermometer 0- 1300 oC x 0.1 oC Type K Thermocouple § $ 20.00
[G] White LED 6000 mcd 5 mm ø Dimmer control$ 2.00
[H] SCR Energy Regulator 220 V 500 W $ 5.00
[J] 12 V DC 300 mA Plug Pack $ 4.00
[K] Aluminium Tubing, 10 mm ø x 150 mm $ 1.00
[L] Metal case* , assorted screws & nuts, brackets, electrical terminals & wire , assorted screws$ 20.00
[M] 6 mm ø x 75 mm bolt, head removed
Total $ 88 approx.
* If you can use an existing case or make one, this will save some money. I salvaged the case from an old compact disc player, cut and shut to a suitable size ( 260 mm x 170 mm x 75 mm ).
§ This model gives readings to ± 0.1 oC, and has a DATA HOLD feature which is very convenient. A cheaper model ( $ 6.00 ) which reads to ± 1 oC can be substituted if less accuracy is accepted. A digital meat thermometer as shown above can also be used.
BRIEF OUTLINE OF THE APPARATUS
The soldering iron is the basis of the heating system, which is controlled by an SCR energy regulator. The soldering tip is removed and an aluminium block is attached in its place, connected by abolt which has its head cut off. Holes are drilled in the block for the capillary tubes and the thermocouple sensor. The white LED illuminates the sample from the back, and the magnifier shows an enlarged view of the samples. Two cooling fans provide rapid cooling between measurements, and a digital thermometer indicates the sample temperature.
The heating block was formed from a stack of aluminium plates as shown above. These were cut from 20 mm x 5 mm aluminium bar stock. There are 6 pieces required . The components are held together by 4 small bolts. A hole 5 mm diameter was drilled through parallel to the bolts. Four 2 mm diameter holes were drilled down from the top for the capillary tubes and thermocouple probe. When using small drills like this, it is vital to lubricate the work while drilling, otherwise the drills will snap.