Determination of the Triple-Point Temperature of Gallium

Abstract

We have determined the triple-point temperature of high-purity gallium to be 29.77406OC using three standard platinum resistance thermometers (SPRTs) calibrated on the IPTS-68 and using samples of gallium from three commercial sources. All data obtained on the highest purity sample have a standard deviation of +0.0001 1 OC. The overall systematic uncertainty is estimated to be ! y: f m°C. The day-to-day irreproducibility of a single melt of each gallium cell was found to be < 215 p0C, while over a period of 7 months the irreproducibility among melts of each gallium cell was < ~O.OO01loC. These limits on the reproducibility appear to be largely instrumental since every time an SPRT was left in a gallium cell for several days there was no change in the indicated temperature of the triple-point to within the instrumental resolution of + 1.5 p°C. The pressure dependence was measured and determined to be dTldP = -2.01 1 + 0.001 m°C/atm. I. IntroductionApart from the boiling-point of water at 100°C and the melting-point of phenoxybenzene at 26.86OC, both of which are extremely difficult to realize at .the millidegree level, there are no internationally-accepted fixed points on the International Practical Temperature Scale of 1968 (IPTS-68) [ l , 21 between the triple-point of water at 0.01OC and the melting-point of benzoic acid at 122.37OC. Phenoxybenzene is an organic compound rather than a metallic element and, as such, is relatively difficult to purify and has a low thermal conductivity. Consequently, its usefulness as a temperature fixed point is severely limited. Practically speaking, then, there are no accurately-realizable and convenient Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose

Realization of the Gallium Triple Point at NMIJ/AIST

“…The triple point of gallium (302.9166 K) has been found to be a highly reproducible fixed point near room temperature [1][2][3][4][5][6][7][8][9][10][11]. Thus, the triple point of gallium is a useful fixed point for realizing the International Temperature Scale of 1990 (ITS-90) [12] and for calibrating standard platinum resistance thermometers (CSPRTs) near room T. Nakano (B) · O. Tamura · H. Sakurai National Metrology Institute of Japan, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan e-mail: tnt@ni.aist.go.jp temperature [1][2][3][4][5][6][7][8][9][10][11].…”

“…Thus, the triple point of gallium is a useful fixed point for realizing the International Temperature Scale of 1990 (ITS-90) [12] and for calibrating standard platinum resistance thermometers (CSPRTs) near room T. Nakano (B) · O. Tamura · H. Sakurai National Metrology Institute of Japan, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan e-mail: tnt@ni.aist.go.jp temperature [1][2][3][4][5][6][7][8][9][10][11]. The triple point of gallium is usually realized by large cells containing about 1 kg of gallium immersed in a liquid bath maintained near 30 • C [1][2][3][4][5][6][7][8][9][10][11]. Although such large cells are convenient for calibrating standard platinum thermometers, strictly speaking, the traditional large cell is far from the definition of the triple point, i.e., the thermal equilibrium state of the solid, liquid, and gas of a pure sample.…”

Guidelines for realizing the International Temperature Scale of 1990 (ITS-90)

“…[Note: for a fixed point, the temperature values r 68 and T 90 are different, but the "hotness" is the same and so the resistance of a given SPRT remains unchanged.…”

Standard Reference Materials

“…The observed freezing curves of the metals and the results of repeated measurements of SPRT's during a single freeze of the metal (except aluminum) are shown. From reference [ 23 ].…”

“…Figure 23 shows freezing curves that were obtained with five different cells containing the aluminum samples described in Sections 4.1 and 4.4. Although results of consecutive insertion and reading of an SPRT have not been obtained as of this writing, the results of Figure 23 indicate that consecutive readings obtained over several hours should not differ by more than about 0.1 mK.…”