Freezing-point apparatus

Nitrobenzene. Usually rather a troublesome solvent, as it is markedly hygroscopic and the freezing-point of the solvent itself tends to fall steadily moreover it is, like benzene, an associating solvent. For a modification of the above freezing-point apparatus, by which the determination may be made in the absence of water-vapour, the student should consult an advanced textbook of practical physical chemistry. 

A Beckmann Freezing-point Apparatus.—The form of apparatus is shown in the accompanying Fig. 30. It consists of a glass jar standing on a metal tray and furnished with a stirrer. The cover of the jar has a wide slit to admit the stirrer, and a circular aperture with clips to hold a wide test-tube. 

The freezing point apparatus allowed an easy, reliable and reproducible determination of freezing points under gas pressure. The experiments resulted in remarkable effects of different gases to the melting points of the examined mixtures due to solubility of the gases in the melts. 

Figure 4.2. Freezing point apparatus for use in the depression of the freezing point of benzene test...

FIGURE 11.2-9 Typical temperature-time trace from freezing-point apparatus. 

The purity of several highly reactive substances, such as titanium(lV) chloride, was obtained in a special freezing-point apparatus developed by Glasgow and Tenenbaum (28). The freezing point of titaniumUV) tetrachloride under saturation pressure with zero impurity was calculated to be -24.10 0.0 l°C. 

Freezing-point apparatus showing structure of stirrer... 

Glacial Acetic Acid. Acetic acid is very hygroscopic, and its freezing-point is considerably affected by the water absorbed. Satisfactory results cannot be obtained with the apparatus shown in Fig. 79, p. 433, and a specially modified apparatus must be employed (see Nitrobenzene). 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

From the ventilation point of view, the fixed points -38.83 °C (triple-point of mercury), 0.010 °C (triple-point of water), 29.76 °C (melting point of gallium), and 156.60 °C (freezing point of indium) are of relevance. The triple-point of water is relatively simple to achieve and maintain with a triple-point apparatus. Some freezing point cells are covered in standards. In practical temperature calibration of measuring instruments, the lTS-90 fixed points are not used directly. 

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