EFFECT OF SODIUM CONCENTRATION ON THE SURFACE TENSION OF MERCURY YUNG LEE'
Engineering Research Branch, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada The surface tension of pure mercury and mercury containing up to 0.2 weight % sodium has been investigated between about 20' and 350' C., by the method of maximum bubble pressure. The present results for pure mercury are in good agreement with those previously reported, and a n empirical correlation is presented as a function of temperature. Even a trace of sodium in mercury seems to lower the surface tension markedly and a t a sodium concentration of only 0.04 weight YO,the surface tension is reduced to about 60% of that of pure mercury. A further increase in sodium concentration has little effect. The effect of the elevated temperature is of secondary importance in mercury containing dissolved sodium.
interfacial tension between the liquid and vapor phases is an important factor in the evaluation of boiling phenomena, especially for mercury boiling systems (Kutateladze, 1959; Lyon et al., 1955). Additives t o mercury are generally undesirable in heat transport systems because they complicate design. However, if an additive is the only way t o achieve good wetting, the optimum amount of the additive having the least complicating effect must be known. Bircumshaw (1926), employing the method of maximum bubble pressure, reported the results of the surface tension measurement of pure mercury up to the boiling point a t atmospheric pressure, and his results are frequently quoted in reference books (Lyon, 1952). Kemball (1946) has also obtained values up to 480 or 490 dynes per cm. for the surface tension of mercury in vacuo and in some permanent gases a t about 20°C. by the sessile drop method. The inherent difficulties associated with the measurement of the surface tension of mercury are extensively discussed by Bosworth (1938) and Adam (1941). THE
' Present address, Department of Mechanical Engineering, University of Ottawa, Ottawa, Canada.
The surface tensions of very dilute alkali-metal mercury amalgams by the method of maximum bubble pressure are reported by Pugachevich et al. (1959), but only a t 22'C. and relative to vacuum. For our purposes, results were required up to the boiling point and relative to a gas atmosphere. The present study was therefore initiated as a supplemental investigation of a study on pool boiling heat transfer with mercury and mercury containing dissolved sodium a t the Chalk River Nuclear Laboratories (CRNL). This report presents the result of the re-investigation of the surface tension of pure mercury and the extension of the data for mercury-sodium amalgam in argon gas, by the method of maximum bubble pressure. Experimental
The essential features of the apparatus used in the experiment are illustrated in Figure 1. A stainless steel beaker, 2.5 inches in diameter by 3 inches, contains about 200 cc. of test liquid. One of three interchangeable orifice tubes (o.d., 2.014, 1.64, and 1.29 mm.) made of stainless steel hypodermic tubing is immersed in the liquid. These orifices are tapered and MICROMETER
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Figure 1 . Maximum bubble pressure apparatus 66
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change in liquid depth due to the removal of test liquid from the orifice tube, for the volume occupied by the bubble, and for the thermal expansion of the tube. However, sample checks showed that the errors introduced were negligibly small, in the order of 0.05%. As a calibration check of the apparatus, the surface tension of distilled water relative to air was obtained and the agreement of the test results with the values reported in Smithsonian Physical Tables (1954) was within 2%. This may be taken as a measure of the accuracy of the methods and measurements. I n the calculation of pure mercury results, the external diameter of the orifice tube was used for the controlling bubble diameter, since the results of contact angle measurement of Kitts (1952) seem to indicate that the contact angle of pure mercury on steel remains greater than 90" up to about 300°C. at atmospheric pressure. According to Porter's analysis (1930) on the bubble detachment from the tube, if the contact angle drops below 90' at some temperature level, the bubble will form on the internal diameter. For the present test, this would involve a decrease of about 570 in the bubble pressure over a small range of temperature. For the determination with mercury containing sodium, the internal diameter was used as the controlling bubble diameter, because the results of contact angle measurement of mercury-sodium amalgam by Kitts (1952) showed that with about 0.005 weight % sodium concentration, the contact angle dropped below 90". The difference introduced by taking the controlling bubble diameter equal to the external diameter of the tube is less than 4% in the case of mercury containing about 0.006 weight % sodium. The surface tension of the amalgam was almost 35% less than that of pure mercury. Other major sources of error in the determination of surface tension are those involved in the physical constants such as orifice diameter, depth of tube immersion, manometer reading, densities of manometer and test fluids, etc., used in the numerical calculation. The probable maximum over-all error in the present experimental results
have their ends ground so that the cross section of each will be in the same plane. The depth of the tube immersion at some depth in the test liquid was measured with a micrometer, the initial contact point being attained by the means of a simple electric circuit. The liquid bath temperature was controlled by a calibrated I-C thermocouple Honeywell Pry-0-Vane and recorded with a Leeds and Northrup potentiometer. The formation and detachment of the bubble from the orifice tube were manipulated with either a microneedle valve or a bellows pump. The maximum pressure attained within the slowly growing bubble before it separated from the orifice tube tip was measured with an inclined manometer. A bubble formation rate of one to two per minute was adequate. The apparatus was contained in an inert dry box and filtered argon gas was used as the blanket gas during determination. All parts coming in direct contact with the test liquid were thoroughly cleaned and dried at the beginning of each test run. Prior to the determination with liquid metal, the apparatus was calibrated by measuring the surface tension of distilled water. Thereafter the dry box was extensively flushed with argon gas to ensure an inert atmosphere in the apparatus and the pressure of the blanket gas in the box was maintained a t a slightly higher level than atmospheric. One master amalgam of about 0.2 weight 70 sodium was made up in an inert atmosphere from triple-distilled mercury and metallic sodium (Fisher S-135), and from this master amalgam, several subsequent dilute amalgams of different sodium concentration were made. The sodium concentration in each test liquid was determined by spectrophotometry before and after the determination. Results and Discussion
The experimental results were calculated by the iterative procedure of Sugden (1922). All calculations were carried out on the G-20 computer a t CRNL. Corrections to the calculation were considered for the
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estimated using the method described by Kline and McClintock (1953) is about 2.3%. The density of mercury containing sodium was calculated from the estimated volume of additives in the amalgam. The results of surface tension measurements of pure mercury and mercury containing 0.006 and 0.037 weight 7% sodium, plotted as a function of temperature in Figure 2, are the means of three to four measurements a t each test condition, and the reproducibility is within the limit of experimental error estimated. By applying the method of least squares, the results for pure mercury obtained from the present study can be expressed by the following equation : u
= 468.7 - 1.61 x lo-’ t - 1.815 X
t 2 ( U dynes/cm., t O C . ) Results of increasing and decreasing temperature cycles or the use of different sizes of orifice tube showed no significant difference. Results obtained in dried air and filtered argon atmosphere are similar. The comparison was made of the results of pure mercury with mercury containing dissolved sodium. The temperature has the primary effect on the surface tension for pure mercury; however, the effect of temperature is dominated by that of sodium concentration in the case of mercury-sodium amalgam. In Figure 2, the results obtained from the present study for pure mercury are compared with the results reported in the literature. The agreement is excellent; all previous results except those of Kemball (1946) reported are within the confidence limit of the present correlation. The results for mercury-sodium amalgams a t 23” C. are presented in Figure 3 as a function of sodium concentration. The present work shows that even a trace of sodium in mercury markedly lowers the surface tension and that when a sodium concentration of only 0.04 weight 70 is reached, the surface tension is reduced t o about 60% of that of pure mercury. A further increase in sodium concentration has little effect. In Figure 3, the results of Pugachevich (1959) for Hg + N a and Hg + Cs are 68
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compared with those of the present study; the present values are much lower. Although the different approach in the experiment may have somewhat contributed to the discrepancy, it would hardly account for all disagreement. Acknowledgment
The author is indebted to R. I. Hodge, Head of the Engineering Research Branch, CRNL, for his encouragement and advice in the work. The author is pleased to acknowledge the service of the staffs in Technical Service Group, Reactor Research Division, CRKL, and also thanks R . W. Ashley and his staff in the Development Chemistry Branch, CRNL, for the analysis of the test liquids. Literature Cited
Adam, N. K., “The Physics and Chemistry of Surface,” 3rd ed., Oxford, London, 1941. Bircumshaw, L. L., Phil. Mag. 2, 341-50 (1926). Bosworth, R . C. L., Trans. Faraday SOC.34, 1501 (1938). Kemball, N. K., Trans. Faraday SOC.42, 526 (1946). Kitts, F . G., thesis, Department of Chemical Engineering, University of Tennessee, 1952. Kline, S.J., McClintock, E . A., Mech. Eng. 75, 3 (1953). Kutateladze, S. S., “Liquid Metal Heat Transfer Media,” Consultants’ Bureau, New York, 1959. Lyon, R . E., Foust, A. S., Katz, D. L., Chem. Eng. Progr. S y m p . Ser. 51, 41-7 (1955). Lyon, R . N., ed., “Liquid Metals Handbook,” Government Printing Office, NAVEXOS P-733 (rev.) (1952). Porter, A. W., Phil. Mag. 9, 1065-73 (1930). Pugachevich, P. P., Timofeevicheva, 0. A., Russ. Phys. Chem. 33,350-2 (1959) (English trans.). Smithsonian Physical Tables, 9th ed., Smithsonian Institution, Washington, D. C., 1954. Sugden, S., J . Chem. SOC.1,858 (1922). Taylor, J. W., Phil. Mag. 46,867 (1955). RECEIVED for review July 24, 1967 ACCEPTED October 23, 1967
