An Inexpensive Zone Refining Apparatus Gregory F. Needham, Gerald Boehme, and Roger D. Willeti Chemical Physics Program, Washington State University, Pullman, WA 99164 Duane D. Swank Pacific Lutheran University, Tacoma, WA 98447
The technique for purifying compounds known as zone refining has been in existence for some time.' The intended use for the technique was to uniformly dope an impurity into a lattice and to provide pure materials for the transistor industry. ~ e r m a n was k one of the first elements to be purified using zone refining, to nearly perfect purity-99.99999999 percent pure.2 We desired to is; the techniq"e to purify long chain aliphatic amines which are used in the study of the phase transitions in salts of the type (RNH3)2MX4.These serve as crystalline models of lipid hilayers. Zone refining is a member of a family of techniques known collectively as zone melting. In zone melting, compounds can he purified or given a desired composition. Using zone refining, one or more small molten zones move through the sample. The s a m ~ l is e usuallv contained in a lone tube. As the molten zone moves through the sample, it carries with it a portion of the soluble impurities. The final purity of the sample depends on the size and number of zones, the number of times the zone moves through - the sample, . . the initial concentration of the impurities, and an intrinsic property of the compound known as the distribution coefficient. The distribution coefficient is defined as the ratio of the concentration of the solute in the solid to the concentration in the liquid. A small distribution coefficient means that the sample will purify faster than a samvle with a high - distributioncoefficiknt. Zone refining works because the composition of the solid
1 0
Pure A
25
50
75
I 100 Pure B
Concentrat~onof B ( percent) Figure 1 Phase diagram of an #dealtwo-component system
be summarized in the binary phase-diagram shown in ~ G u r e 1. Consider a solution of composition 1, and lower it to its freezing point, 2. The solid that freezes out, point 3, will have a lower concentration of B than the liquid at the same temperature. This exclusion of the solute B will raise the concentration of B in the liquid, point 4. If the sample is further cooled to the freezing point at this composition, point 5, the solid that forms will be richer in A than the corresponding liquid. This, in turn, raises the concentration of B in the liquid, and so on. The experimental set-up we used is sketched in Figure 2. The heaters were several turns of nichrome wire, and the sample tubes were 8 mm Pyrex" closed at the bottom. The III~:~ICT.w r t - nici.la.\.*r ominy in ,.
