An unknown D found to melt at 150°C-151°C can be suspected of being identical with one or the other substance A or C; observation that the mixture A and D shows a melting point depression would exclude identity with A, and failure of C to depression the melting point of D would prove C and D identical. If a substance melts at 150°C or higher, the thread of mercury at the upper part of the thermometer is cooler than that in the bulb and hence the temperature recorded is a little lower than the actual bath temperature. The extent of the error due to stem exposure depends upon the design of the temperature and the type of heating bath used; it may amount to 2°C-5°C at 200°C, 3°C-10°C at 250°C. an approximate correction for stem exposure can be calculated from the formula:
Stem correction (°C = 0.000154 (t-t’) N)
Where the fraction represents the difference in the coefficients of expansion of glass of mercury, t is the temperature read, t’ is the average temperature of the exposed column of mercury (determined approximately by reading the temperature of a second thermometer whose bulb is placed midway between the bath and the point on the first thermometer corresponding to t), and N represents the length, measured in degrees, of the thread exposed between the top of the heating liquid and the point t. the convention for reporting a corrected melting point is: m.p. 283.5°C-284.5°C (corr.). The error can be eliminated by use of a heating bath designed to accommodate a series of short thermometers that can be totally immersed.
Since it is frequently difficult to determine what type of thermometer is being used, it is often advantageous to compare the thermometer against known pure compounds and to prepare a calibration curve such as that shown in the drawing. Compounds suitable for such standardization are:
The corrected melting points obtained in this manner will correspond exactly to the corrected melting points in handbooks and the literature.
APPARATUS AND MATERIALS:
Ice, benzoic acid, urea, unknown compound (X), thermometer.
EXPERIMENTAL PROCEDURES:
Part 1: Melting points of the standard compounds.
The most efficient procedure is to range all the samples in order of increasing melting point and determine the apparent melting point of each, beginning with the sample of lowest melting point. The apparent melting point of ice was determined by inserting the bulb of the thermometer into ice slurry. The observed melting points and corrected melting points of the standard compounds as listed earlier were recorded. A correction chart was constructed by plotting the observed melting points against the corrected melting points. The resulting points were connected, and that is the calibration curve of your thermometer.
Part 2: Melting points of known compounds and mixture
- Benzoic acid
- Urea
- 10 mol per cent benzoic acid and 90 mol per cent urea
- 90 mol per cent benzoic acid and 10 mol per cent urea
Part 3: Identification of an unknown compound by the mixed melting point method
Six of the compounds listed below were obtained from the instructor, which will be designated by number only. Their melting points were determined and the values found were compared with those given in the table. The probable identities of the unknowns were decided and were tested in the following way: a small sample of the reference compound was secured from the shelf and its melting point was determined. This observed melting point might not be exactly the same as the recorded ones. A small portion of the compound was then mixed intimately with an equal portion of the reference compound and the melting points of the mixture were determined.
DATA/RESULTS:
Part 1: Melting points of the standard compounds.
Ice : 0.5°C – 0.8°C
Benzoic acid: 122°C - 125°C
Salicylic acid : 160°C - 167°C
Part 2: Melting points of known compounds and mixture
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Benzoic acid : 122°C - 125°C
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Urea : 134°C - 145°C
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10 mol per cent benzoic acid and 90 mol per cent urea : 128°C - 134°C
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90 mol per cent benzoic acid and 10 mol per cent urea : 120°C - 127°C
Part 3: Identification of an unknown compound by the mixed melting point method
Table 1.0: The melting points of organic compounds.
DISCUSSION:
The melting point of an organic solid is probably the most widely used physical constant. If you want a quick, easy, and cheap way to characterize an unknown solid, determining the melting point is your ticket to success. A melting point of a solid is the temperature at which the first crystal just starts to melt until the temperature at which the last crystal just disappears. Thus, the melting point (m.p.) is actually a melting range. For a pure compound the melting point is quite sharp (occurs over a 0.5 -1°C temperature range). A melting point range of greater than 5° C usually indicates an impure compound or poor technique. The melting points are characteristic of a compound but the melting point itself is not a unique characteristic of the compound. That is, two different compounds could have the same melting point, but two substances of differing melting point are unlikely to be the same compounds. You can compare an unknown compound with a known compound by thoroughly mixing the two and taking the melting point. If the mixture melts at a lower temperature than the known compound or over a broad range, your unknown is not the same compound. If the mixture melts at the same temperature and same range as the known compound it's a good bet that it is the same compound.
Measurements of the melting point of a solid will provide you with information about the purity of the substance. Pure, crystalline solids melt over a very narrow range (melting range) of temperatures, whereas mixtures melt over a broad temperature range. Mixtures also tend to melt at temperatures below the melting points of the pure solids.
Many solid substances prepared in the organic laboratory are initially impure. These impurities affect the melting point of a substance. In a sample that contains a mixture of two compounds, each component usually depresses the melting point of the other, giving an observed melting point range that is lower and broader than the melting point of either component.
CONCLUSION:
When a solid substance is heated, typically it will melt; that is to say, at some temperature the solid will begin to liquefy and by some slightly higher temperature all of the solid will have become liquid. The melting point (actually melting point range) of a compound is then defined as the temperature at which an observer can first see liquid forming from the solid to the temperature where the last particle of solid has become liquid. For example, the melting point of pure sucrose (table sugar) is 185o-186oC. This means that as a small sample of sucrose is slowly heated some of the crystals begin to liquefy at 185°C and all of the crystals have become liquid by 186oC. Sometimes only the second number (completely melted) is reported as the melting point. In general, this is not a good idea and should be avoided. Melting points are usually determined by placing one or two milligrams of the material to be tested into a melting point capillary, and heating the capillary and a thermometer together, and observing over what temperature range the material melts. The melting point capillary is a thin-walled glass tube, about 100 mm in length and not more than 2 mm in outside diameter, sealed at one end.
REFERENCES:
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- http://jan.ucc.nau.edu/~jkn/235L1-MeltingPoint.htm, date accessed: 28/07/09
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