Techniques melt manipulation

A feature of the book is the introduction of directions for the preparation of certain compounds on a very small scale. Students often acquire the habit of careless work in the laboratory practice in organic chemistry. Preparation-work on the small scale serves to counteract this effect and to develop a technique that is valuable. Such work is often necessary in the identification of unknown compounds when a small amount only of the substance is available. In many cases a crystalline derivative whose melting-point can be determined, can be prepared in a pure condition from but two or three drops of a substance. Among the examples of work of this kind which are given are the preparation of acetanilide from acetic acid, glyceryl tribenzoate from glycerol, dinitrobenzene from benzene, and dibenzalacetone from acetone. In order to facilitate such work, a section in the first chapter is devoted to a consideration of the technique used in the manipulation of small quantities of substances. 

Unlike most other salts, measurement of mixing enthalpies of molten alkali metal carbonates, due to their corrosive nature, cannot be performed in fused silica containers and be mixed by the usual break-off technique. Andersen and Kleppa (1976) showed that the Au20Pd alloy was corroded only negligibly by alkali metal carbonate melts kept under a relatively high CO2 pressure. The experimental arrangement included plunger as well as a dipper crucible that could be manipulated from the outer side of the furnace. The mass loss of the most volatile carbonate, Rb2C03, was about 0.3%. However, in spite of the relatively small vaporization losses, the attack of vapors on the fused silica liner was considerable and the lower part of the device had to be discarded after 10-15 experiments. 

There are a number of interrelated thermal analytical techniques that can be used to characterize the salts and the polymorphs of candidate drugs. The melting point of a salt can be manipulated to produce compounds with desirable physicochemical properties for specific formulation t5q)es. Of the thermal methods available for investigating polymorphism and related phenomena, DSC, TGA, and HSM are the most widely used methods. 

To prepare the aforementioned single crystals, solutions containing 0.5-1.0 M alkali biphenyl in the appropriate solvents were cooled from 30 °C at a rate of 1 °C/h. Because the anions are sensitive to air and moisture, vacuum techniques, as described in the literature (7) were applied. For the X-ray and magnetic measurements, the crystals were mounted in thin glass capillaries with the aid of a glovebox, which contains less than 1 part per million (ppm) of oxygen and water. Because of the low melting points, the manipulations were carried out at about... 

Specific studies of the vapor pressures of rare earth metals were not carried out until the late 1940 s. However, various earlier manipulations of rare earth metals, including vacuum melting of cerium and lanthanum gave a clear indication that these metals had relatively low vapor pressures, compared to the alkaline earth metals for instance. The first concerted study of the vapor pressure of a rare earth metal was that of Ahmann (1950) who used a radioactive tracer modification of the Knudsen technique to show that cerium had a vapor pressure of 10" Torr at 1735 C. Daane (1951) measured the vapor pressures of lanthanum using a direct weight loss Knudsen technique to show that lanthanum has a vapor pressure of... 

---