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.
Step 1: Heating Block Construction
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.
Step 2: PARTS LAYOUT & WIRING
This equipment operates at mains voltages; be sure that you understand how to do mains wiring safely or get someone who is qualified to do it to help you. Mistakes can be FATAL !
The photo above shows the parts ( refer to the parts list for identification ). These were laid out inside the case as in the next photo. All parts except the heater switch, SCR controller, magnifying lens and LED were mounted on the base, using metal brackets and screws. A 10 mm diameter hole was drilled in the base for the LED tube, and a 25 mm diameter hole cut in the lid to allow viewing of the heating block with a magnifying lens. The remaining parts were mounted on the lid. The case I used had an opening along the top edge allowing access for the capillary tubes as well as providing ventilation for the heated components. If you are using a closed case, an opening would need to be cut for this purpose, and a number of ventilation holes drilled above the soldering iron. After mounting the parts, the wiring was completed as in the diagram above.
The LED and its dimmer control are mounted on the 10 mm diameter aluminium tube extending from the back of the instrument. A small bracket was used to attach the tube to the back of the case. At the front, the magnifying lens is mounted above the hole in the heating block so that the capillary tubes are clearly focused when inserted in the block. Some epoxy adhesive can be used to fix the magnifier in place. On the bottom of the case can be seen the wire bail arm used to hold the apparatus at a comfortable viewing angle. This can be made using the wire from a coat hanger.
Step 3: CALIBRATION OF THE INSTRUMENT
CALIBRATION OF THE INSTRUMENT
Before measuring sample melting points, it will be useful to calibrate the SCR regulator settings vs. heatup rate ( oC per minute ). This can be done by recording the temperature at 2 minute intervals at each setting of the regulator and finding the average slope of the temperature-time graph. ( see graph above )
The thermometer reading may be calibrated by observing the melting points of at least two known pure substances. Two readily available substances are naphthalene (lit. mp 82 oC †) and urea ( Lit. mp 133 oC †). The determinations should be carried out in duplicate with fresh samples at a slow heatup rate ( 1.0 oC per minute maximum ) to avoid thermal lag effects. [ See Step 4 for details of method ].
† These are final melting points, the temperature at which the sample is completely liquid.
A calibration graph can then be made ( see above ) showing any correction to be applied to the thermometer reading. Alternatively, the corrected temperature can be calculated from the following equation :
Ts= TA + [(ts- tA )*(TB - TA)/(tB - tA) ]
Ts= Corrected temperature for sample tA= indicated m.pt. for standard A ;
t s= Indicated m.pt. for sample ;
T A = Literature m.pt for standard A;
tB = indicated m.pt. for standard B ;
TB= Literature m.pt for standard B;
This equation reduces to the form Ts = a * ts + b where a & b are constants.
The Excel spreadsheet CalibrationGraphA.xls below can be used to generate the graph and compute the coefficients a & b.
Step 4: DETERMINATION OF MELTING POINTS
Pure crystalline substances have a fixed and sharply defined temperature at which they change from solid to liquid. Since the melting process requires thermal energy input to disrupt the crystal lattice, the temperature remains constant while melting takes place, provided that the two phases are in equilibrium. In a laboratory melting point determination, this condition is not usually achieved and the melting of the sample is observed to occur over a small temperature range, typically 1 – 2 degrees Celsius. The situation changes when the sample is not completely pure, and the presence of impurities in the sample will cause melting to occur at a lower temperature and over a broader range. This depression of the melting temperature increases as the amount of impurity increases, making the observation of a sample's melting range a useful indicator of its purity. ( This does not apply to some substances which decompose before reaching their melting point ). The melting point depression phenomenon can be applied to testing two samples to confirm whether they are the same substance. If a mixture of approximately equal parts of the two samples melts at the same temperature as the two individual samples, then it is almost certain that they are identical. ( There are a few rare exceptions, but these can be eliminated by testing further mixtures of different proportions. )
In practice, the melting point test is carried out on a small amount of finely powdered substance contained in a length of 1.5 mm diameter capillary tubing which has been sealed at one end by melting in a gas flame. The depth of the sample in the tube should be 2 – 3 mm after packing the sample in the sealed end by tapping the tube on the bench. It is essential that the sample is completely dry and free from residual solvents. The tube containing the sample is then slowly heated in a suitable apparatus while closely observing the sample for changes. As the temperature nears the melting point, the tiniest crystals adhering to the wall of the capillary tube will be seen to melt first, followed shortly afterwards by the bulk of the sample beginning to shrink, collapse and liquefy. This temperature is recorded as the INITIAL MELTING POINT. Soon a meniscus forms at the top of the sample, while some solid remains in the lower portion. The solid portion rapidly decreases until the moment when the sample becomes entirely liquid. This is recorded as the FINAL MELTING POINT.
It is normal to carry out a preliminary test at a fairly rapid heatup rate, and then repeat the test approaching the melting point more slowly for a more accurate result. Once a sample has been melted, it can not be re-used. A fresh sample must be prepared for a duplicate test.
It is also possible with this apparatus to eliminate the capillary tubes and place the test specimens between two microscope slide cover slips. These are small squares of glass, about 18 mm square and 0.2 mm thick. The glass "sandwich" is placed directly on top of the heater block. To utilise this method, the top of the block needs to be filed, sanded and polished with fine abrasive paper to obtain a smooth surface. The test specimen is observed while the temperature is increased slowly, and the temperature range over which the sample undergoes melting is recorded. This method is less accurate than the capillary tube method, but when suitably calibrated against known standard compounds will still give good results. It has the advantage that the tedious process of preparing and filling capillary tubes is avoided.
Step 5: IMPROVEMENTS
The following ideas for improvement are suggested. In order to meet the deadline for the Lab Equipment contest, I have not had time to develop these.
~ A webcam could be attached to display an image of the melting point capillaries and the temperature readout on a computer monitor, and record the operation.
~ Suitable image processing software could detect the changes in the optical properties of the tube contents and record the melting temperatures automatically.
With these improvements, the equipment would perform comparably to commercial equivalents costing $ 2500 or more.