Melting points pure substance

Impure substances have melting points that are very dependent upon the amount of impurity present. For a few substances this is quantified as the molal freezing point depression constant. The result is that melting points can be a very useful indicator of purification efforts. As long as each purification step in a process results in a higher melting point, the substance has been made more pure. This same concept allows the quality control chemist to have a very sensitive method for detecting impurities that is lower than anticipated. 

Although the process just described is efficient, it is wasteful, and crystallization from a solvent is usually used. The substance to be purified is dissolved in a liquid from which it will separate as crystals, either on spontaneous evaporation or when the warm saturated solution cools. Separation of a pure compound from a mixture usually can be effected in this way, as the solubilities of the constituents of the mixture in the solvent will be different, and the least soluble substance will separate first. The melting point of the crystals which separate is determined. If this point is not sharp the substance is recrystallized, and the melting point redetermined. The process is repeated as long as recrystallization brings about a rise in melting point. The substance is considered pure when recrystallization does not affect the melting point. In order to obtain a substance in the pure condition quickly, it is well to select for the solvent, if possible, a liquid in which the impurity present is much more readily soluble than the substance to be purified. 

In particular, the enthalpies of invariant processes (melting of pure substances, eutectic and peritectic points, extreme phase transition temperature, etc.) can be measured with great precision. 

Substance chemically pure. This is almost invariably the cause of a sharp melting-point. 

The substance is pure, but on warming undergoes slight thermal decomposition before the melting-point is reached, and the decomposition products then act as impurities and depress the melting-point. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

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. 

If pure 3-bromo-4-aminotoluene is required, the crude base may be purified either by steam distillation or, more satisfactorily, by distillation under reduced pressure. The oil is dried with 5 g. of sodium hydroxide pellets, and distilled under reduced pressure from a Claisen fiask with a fractionating side arm a little p-tolui-dine may be present in the low boiling point fraction, and the pure substance is collected at 92-94°/3 inin. or at 120-122°/30 mm. The purified amine solidifies on cooling and melts at 17-18°. 

Fig. 2. PT diagram for a pure substance that expands on melting (not to scale). For a substance that contracts on melting, eg, water, the fusion curve. A, has a negative slope point / is a triple state point c is the gas—Hquid critical state (—) are phase boundaries representing states of two-phase equiUbrium ...

Whether or not any of these derivatives is likely to be satisfactory for the use of any particular case will depend on the degree of difference in properties, such as solubility, volatility or melting point, between the starting material, its derivative and likely impurities, as well as on the ease with which the substance can be recovered. Purification via a derivative is likely to be of most use when the quantity of pure material that is required is not too large. Where large quantities (for example, more than 50g) are available, it is usually more economical to purify the material directly (for example, in distillations and recrystallisations). 

The viscous magnesium compound formed is cautiously decomposed with dilute acetic acid (75 cc. in 300 cc. of water), the flask being cooled under the tap. Two clear layers are formed, and after separation, the aqueous layer is extracted with 100 cc. of ether, the combined ethereal solution is washed with water and dried with sodium sulfate, and the ether is distilled on the steam bath. The residue is distilled vmder reduced pressure. After a small fore-run the temperature rapidly rises to 130 at 10 mm. when the pure tricarbethoxymethane begins to distil. The yield of material collected over a five-degree interval is 204-215 g. (88-93 per cent of the theoretical amount). The product solidifies at 25°. The melting point of the pure substance is 28-29°. 

The melting point of the pure nicotinic acid salt is 180°C and the yield is 75% to 80% related to the used theophylline. The substance has a nearly neutral reaction and is very readily soluble in water. 

For a pure substance, the melting point is identical to the freezing point It represents the temperature at which solid and liquid phases are in equilibrium. Melting points are usually measured in an open container, that is, at atmospheric pressure. For most substances, the melting point at 1 atm (the normal melting point) is virtually identical with the triple-point temperature. For water, the difference is only 0.01°C. 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

The value of infrared spectra for identifying substances, for verifying purity, and for quantitative analysis rivals their usefulness in learning molecular structure. The infrared spectrum is as important as the melting point for characterizing a pure substance. Thus infrared spectroscopy has become an important addition to the many techniques used by the chemist. 

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