An Easily Constructed Precision Melting Point Apparatus EDWARD 0.HOLMES, JR. Boston University, Boston, Massachusetts
last two years the author has been engaged F.mORaTHE research problem involving the determination of phase diagrams by the micro-method. This work necessitated a melting point apparatus which would give precise and reproducible results to one-fifth of a degree. Neither the plain Thiele-Dennis apparatus nor any of its modifications with metallic jackets of various forms proved so reliable as the apparatus to be described. The following simple apparatus has given most satisfactory results over a long period of time. It consists of a 250-cc. beaker, three-quarters full of mineral oil, resting on an iron dish and containing a glass stirrer, as illustrated in Figure 1. A calibrated thermometer is suspended in the center of the beaker. The th~rmometerbulb is surrounded by a glass jacket made by drawing down a test tube until its internal diameter is about 1 mm. and then cutting i t off a t the narrowest point. This jacket is hung by two thin copper wires from a clamp on the thermometer stem in such a way as to enclose and surround the bulb of the thermometer. It serves two purposes: first, to hold one or more melting point tubes and, second, to keep the melting point tube and the thermometer bulb close to one another in a bath of oil partially insulated from the main body of the liquid. Thus they are not so likely to be affected by thermal diierences occurring in the main body of the surrounding liquid. The jacket is wide open a t the top and contains a small opening in the bottom allowing a small flow of oil from stirring, which prevents excessive thermal lag. Under the iron dish, which serves to diffuse the heat, is a microburner. No ordinary stopcock or screw clamp was found sensitive enough to regulate the burner flame so that the rate of heating could be made as small as desired-usually one degree in two or three minutes. In order to provide for this careful flame regulation, a Hoffman screw clamp was soldered to a brass plate through which had been drilled two holes near the opposite ends. The plate and attached clamp were fastened to a small wooden block by screws passing through the two holes. Then a s t 8 l-mm. diameter brass rod was soldered to the milled thumbscrew of the clamp (see Figure 2). The gas hose leading to the burner was run t h r o u ~ hthe clamp in the usual manner
and now by turning the brass rod, which acted as a long handle, the rate of heating of the bath could be made as small as desired. The microburner was surrounded by a metal shield made by cutting the top and bottom out of a tin can. A focusing light was clamped in such a position that its beam illuminated the melting point tubeq brilliantly. The author used a microscope lamp from a dark field illuminator.
FIGURE ~.-CONST~~C O FTVALVE ~ O N FOR MICROBURNER The main feature of the apparatus is the glass jacket that surrounds the thermometer bulb, which serves to keep the bulb and the melting point tube at the same temperature and also to hold one or more melting point tubes. The other feature is the sensitive
valve made from a Hoffman clamp which enables one to obtain extremely slow rates of heating. Both of these improvements are easily and inexpensively constructed. The rest of. the apparatus is conventional.
