A MELTING-POINT BATH HAROLD P. BROWN University of Kansas City, Kansas City, Missouri
THE hazards of sulfuric acid baths and the unpleasantness and inadequacies of oil baths for the determination of melting points and boiling points are well known. It is surprising that more satisfactory materials have not supplanted these substances which are commonly employed in laboratory practice. For the needs of the chemist, and particularly of the student, dibutyl phthalate offers many advantages. The high boiling point of the commercially available product* of 340°C. a t 760 mm.facilitates the duplication of virtually all melting points and boiling points above room temperature recorded in the literature for organic compounds. The material has the decided value of being liquid a t room temperatures. Although the flash point of 180°C. limits the utility of dibutyl phthalate as a general purpose high-temperature bath, melting points as high as 300°C. can safely and readily be taken by using a Thiele tube or similar device exposing a small area of liquid surface in a relatively tall container. The rapidity of acquisition and loss of heat, the fluidity, the -
* Data given are for the product manufacturedby Carbide and Carbon Chemicals Corporation.
absence of corrosive action, the absence of highly irritating fumes, inexpensiveness, and clarity are other commending features of this material. Although dibutyl phthalate decomposes and darkens somewhat when kept at elevated temperatures for long periods of time, it does not discolor seriously enough to render the determination of the desired points impractical. Clarification can be effected by distillation under reduced pressure, but it is usually more expedient to simply recharge the bath with fresh fluid. Students have experienced little difficulty in using dibutyl phthalate baths throughout the school year without recharging. Care must be exercised that rubber does not come in contact with hot dibutyl phthalate, as it will melt and discolor the bath. Melting-point tubes are readily held in place by the usual method of capillary attraction. The replacement of sulfuric acid by dibutyl phthalate in melting-point baths removes one of the major sources of acid burns in the organic laboratory and, a t the same time, furnishes a bath suitable for the determination of the highest melting points or boiling points commonly encountered which is liquid a t room temperatures.
