Melting-point, Determination

Identificatioii by Mixed Melting-points. It will be clear that melting-point determinations afford a ready method of identifying minute quantities of a solid compound, if the probable identity of this compound is already suspected. Thus if there is reason to believe that a particular substance is, for example. 

Checking the Purification. The purity of the dry re-crystallised material must now be determined, as it is possible that repeated recrystallisation may be necessary to obtain the pure material. The purity is therefore checked by a melting-point determination, and the recrystallisation must be repeated until a sharp melting-point is obtained. Should the compound have no well-defined melting-point e.g.y the salt of an organic acid or base), it must be analysed for one suitable component element, until its analysis agrees closely with that theoretically required. 

Reagent A is particularly useful for the treatment of the lower aliphatic aldehydes and ketones which are soluble in water cf. acetaldehyde, p. 342 acetone, p. 346). The Recent is a very dilute solution of the dinitrophenylhydrazine, and therefore is used more to detect the presence of a carbonyl group in a compound than to isolate sufficient of the hydrazone for effective recrystallisation and melting-point determination. 

The capillary tubes used for the melting-point determinations should be both longer and wider than those normally used they should be at least lo cm. long (to decrease the chance of slipping off the thermometer during the determination) and 2 -3 mm. in diameter (in order to have sufficient semi molten material in the tube to observe accurately the process of melting). 

The comparatively inexpensive long-scale thermometer, widely used by students, is usually calibrated for complete immersion of the mercury column in the vapour or liquid. As generally employed for boiling point or melting point determinations, the entire column is neither surrounded by the vapour nor completely immersed in the liquid. The part of the mercury column exposed to the cooler air of the laboratory is obviously not expanded as much as the bulk of the mercury and hence the reading will be lower than the true temperature. The error thus introduced is not appreciable up to about 100°, but it may amount to 3-5° at 200° and 6-10° at 250°. The error due to the column of mercury exposed above the heating bath can be corrected by adding a stem correction, calculated by the formula ... 

In addition to the use of a melting point determination as a criterion of purity, an equally valuable application is for the identification of oiganic compounds. If the melting point is known within one degree, the major proportion of possible substances is immediately eliminated from consideration. The study of the general chemical properties of the compound and a mixed melting point determination (Section 1,17) will largely establish the identity of the compound. 

If the filtrate is of value, it should be transferred to another vessel immediately the crystals have been drained. Frequently, the mother liquor may be con centra ted (suitable precautions being, of course, taken if it is inflammable), and a further crop of crystals obtained. Occasionally, yet another crop may be produced. The crops thus isolated are generally less pure than the first crystals which separate, and should be recrystaUised from fresh solvent the purity is checked by a melting point determination. 

Before attempting a melting point determination as a check on the purity, care must be taken to ensure a perfectly dry sample of the compound since traces of solvent may lower the melting point appreciably. 

The application of mixed melting point determinations to the identification of organic compounds has been described in Section 1,17. In order to gain experience the student should carry out the following simple experiment. 

Determine the melting point of pure cinnamic acid (133°) and pure urea (133°). Intimately mix approximately equal weights (ca. 01 g.) of the two finely-powdered compounds and determine the melting point a considerable depression of melting point will be observed. Obtain an unknown substance from the demonstrator and, by means of a mixed melting point determination, discover whether it is identical with urea or cinnamic acid. 

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

N-tert-Butyl urea [1118-12-3] M 116.2, m 182°, 185°(dec). Possible impurity is A,A -di-rcr/-butyl urea which is quite insol in H2O. Recrystd from hot H2O, filter off insol material, and cool to 0° to -5° with stirring. Dry in vac at room temp over KOH or H2SO4. If dried at higher temperatures it sublimes slowly. It can be recrystd from EtOH as long white needles or from 95% aq EtOH as plates. During melting point determination the bath temp has to be raised rapidly as the urea sublimes slowly above 100° at 760mm. [Org Synth Coll Vol III 151 1955.]... 

Schmelzpunkt, m. melting point, fusing point, -bestimmung,/. melting-point determination, -bestimmungsrohr, n. (or-rdhre,/.), melting-point tube-... 

B. Succinic acid (CH2COOH)2 R.M.M. = 118.09. The pure commercial product should be recrystallised from pure acetone and dried in a vacuum desiccator. The purity is checked by means of a melting-point determination (185-185.5 °C). The acid is fairly soluble in water phenolphthalein is a suitable indicator. 

Maquenne Block. See under Melting Point Determinations... 

Melting Point, Determination. Most methods for the detn of mp are microprocedures, and may be conveniently classified on the basis of the type of app used for the detn (1) the capillary tube method. (2) the heating bar method, and (3) the microscope heating stage (micro hot stage) method. 

For the melting point determination the capillary is placed in the apparatus at 60°, and the temperature is raised at 4°/minute. 

Triethanolamine salts of alcohol sulfates form white crystals when obtained in pure form after recrystallization. At their melting point they are semisolid with gelatinous appearance and the transition is difficult to detect. Melting points, determined through thermograms obtained by differential scanning calorimetry, gave 72, 76, 80, and 86°C for dodecyl, tetradecyl, hexadecyl, and octadecyl sulfates, respectively [63]. 

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