1000
NOTES
mercial production of aluminum, a cryolite melting point of 1004 f 1" was obtained using quenching techniques, essentially as described by Roedder.6 Briefly, the method consists of suspending a small amount of test material in the hot zone of a constant temperature furnace for a period of time sufficient to obtain equilibrium. The charge is then dropped from the hot zone into cold mercury. Microscopic examination of the quenched charge will determine the state of the system a t temperature. Phase changes and their relation to temperature are readily observed by quenching a series of samples over a range of temperatures. Melting points of pure substances are fairly simple to obtain. Their small melting ranges permit one to make full use of precise furnace temperature controllers in establishing solid-liquid transformation temperatures. The accuracy of any given experimental result is limited only by the accuracy of calibration, if the precision of the thermostat is within these limits. Extraneous thermal effects that could influence the experimental results are minimized or eliminated by performing the thermocouple calibration in exactly the same manner as the sample runs. The material to be quenched is enclosed in a platinum foil envelope. The envelope is threaded a t the top with platinum fuse wire, suspended by means of the fuse wire from two platinum wire hooked conductors, and located about 1 millimeter from the thermocouple. Calibration of the Pt, Pt-lO% Rh Thermocouple.The e.m.f. of the thermocouple was measured by a Type K-2, Leeds and Northrup potentiometer. The reference junction was at 0'. The melting points of 99.99% NaCl (SOO.S'), recrystallized NaF (992'), and 99.999% old (1063") were used as calibration oints. The NaC!, wfich was obtained from the Delta 8hemical Corporation, exhibited no significant melting range. The gold was obtained from the Sigmund Cohn Corporation in the form of 0.01 inch diameter wire. To determine the gold point, the wire was contained in a small ceramic capsule to prevent access to the surrounding platinum envelope. The gold had no observable melting range. Sodium fluoride was J. T. Baker C.P. grade and required recrystallization from water solution in order to reduce the melting range to approximately 1". In this case the temperature where the last crystals melted was considered the melting point. The deviation of the observed thermocouple readings from standard reference tables was plotted as a function of the observed readings. This curve was linear over the 263 degree calibration range. ?he accuracy of the calibration was considered to be f l . The thermoregulator maintained the temperature i o within f0.2' of the mean both during calibration and during the cryolite melting point determinations. The precision of the regulator was well within the accuracy of calibration. Cryolite Melting Point Determination.-The cryolite employed in these determinations was of the highest purity known to the aluminum industry. A semiquantitative spectroscopic analysis of representative Sam les of the material and a quantitative determination o f the more prevalent impurities are given in Table I. X-Ray powder diffraction could not detect other phases, such as chiolite. Three or four fragments taken from particularly clean clumps of Greenland cryolite were sealed in each of several platinum envelopes. Each specimen was quenched from a successively higher temperature until one had been uenched from the liquid state. This procedure establiLed the "bracketting temperature." The magnitude of the bracket was then reduced until approximately 0.6" separated the completely solid from the totally liquid state. The cryolite quenched from 1003.9 f. 0.2' (max. deviation), and main( G ) E. Roedder, A m . J . Sci., 249, 81 (1951).
Vol. 61 TABLE I Elements detected
Fluorine Aluminum Sodium Silicon ( SiOz) Potassium (KF) Calcium (CaF2) Lithium (LiF) Magnesium Strontium Iron Copper
Qualitative estimate of concn., %
10-100 10-100 10-100 0.01-0.1 0.01-0.1 0.01-0.1 0.01-0.1
