GURE 1. Coulometer for the determination of the atomic weight of zinc.

1-zinc anode; 2- mercury pool cathode; 3-ground glass flat joint.

0.1 molar NH.Cl solution and then in hot distilled ater and reweighed. Appropriate corrections for the fects of buoyancy of air are applied. Thus the charge mass ratio can be readily computed.

4. Results

The results of experiments with metallic zinc amples of five different origins are summarized the table. The first column in the table gives the riginal number of the experiment. Material code nd the origin of material are given in columns two nd three respectively. The last column, column our, gives the determined values of the atomic eight of zinc in each experiment.

The material designated as 1 is high purity zinc sued by the National Bureau of Standards as Standrd Reference Material (SRM) 682. The material was roduced by Cominco American, Inc. from a special ot of high-grade electrolytic zinc. It was purified by acuum distillation, zone-refining and degasification. 'he overall assessment of impurities indicates that he material is 99.9999 percent Zn [15]. The material esignated as 2 is a special high-grade metal (99.995% 'n) produced by the Electrolytic Zine Company of Australasia Ltd. [16]. Materials 3, 4 and 5 were proluced in Peru, Yugoslavia, and Italy respectively. The calculated values of the atomic weight of zinc column four are based on the coulometrically deermined electrochemical equivalent of zinc and dopted value of the faraday. For the determination.

of charge consumed in the electrochemical reaction, the value of electrical current is based on the NBS ohm and U.S. legal volt of 1969.

Time measurement is based on 100 kHz frequency maintained by NBS. Mass measurements were performed by substitution using a set of weights calibrated at NBS. The value of the faraday employed in these calculations is 96486.70 C mol-1 [17].

5. Discussion

The mean value of the atomic weight of zinc based on coulometric data reported here is 65.37736. A two-sided 95 percent confidence interval for the mean, 0.00086, reflects adequately the uncertainty in this value due to the random sources of error. The accuracy of the result, however, depends not only on the random error of measurement, but also on the biases which could exist in the various stages of the measurement process and in the constants which are employed in calculation of the results.

The assessment of the uncertainties due to all known sources of possible systematic error (uncertainties in electrical standards, time, mass and the faraday) indicates that these sources can contribute as much as 0.002 percent (0.00131 amu) error. Thus, the two components of error yields an uncertainty figure of 0.0022 in the atomic weight of zinc.

It is thus felt that on the basis of this work the atomic weight of zinc can be assigned the value

65.377±0.003.

All the experimental values reported here lie well within the assigned uncertainty bounds. Further reduction in the uncertainty of this constant is anticipated when processing of all data is completed. Nevertheless, even the present uncertainty is an order of magnitude lower than that of the presently accepted value. (See table 1.)

On the basis of data presented here, there appears to be no significant difference between the values of atomic weight for materials of different origins even at the 10 percent level of significance. The published results of mass spectrometric investigations [7] also indicate no significant differences in the isotopic composition of zinc produced by electrolytic and chemical reduction processes. To insure that the value obtained in this research is truly representative of the terrestrial zinc, an experimental survey of primary zinc bearing minerals from world-wide sources is now in progress. This study is conducted on virgin minerals which have not been subjected to metallurgical processing (other than flotation), since the latter could produce isotope separation (isotope separation can conceivably occur in such processes as distillation which is commonly used in purification of zinc).

The authors express their gratitude to D. W. Kohis of The New Jersey Zinc Company, A. S. Gill of Elec

Standard deviation of a single determination s = 0.00134.
Standard deviation of the mean 's/ Vn= 0.00039.

Two sided 95 percent confidences interval for the mean based on 11 degrees of freedom ts/ Vn=+0.00086.

trolytic Zinc Company of Australasia Limited, W. Tunney, Jr. of St. Joseph Lead Company, and J. F. Harris of Cominco Limited for their donation of zinc samples from different origins. We also express our thanks to J. Mandel of the National Bureau of Standards for his help in statistical evaluation of our results and to A. R. Cook of International Lead Zinc Research Organization for his interest in this work.

6. References

[1] International Commission on Atomic Weights, Final Version of the Report 28-9-1967, Comptes Rendus XXIV IUPAC Conference, Prague, Sept. 4-10, 1967, pp. 130-141. [2] IUPAC adopts new look in atomic weights, Chem. Eng. News 48, No. 4, 38 (1970).

[3] Hönigschmid, O., and von Mack, M., Des Atomgewicht des Zinks. Analyse des Zinkchlorids, Z. Anorg. Allgem. Chem. 246, 363 (1941).

[4] Baxter, G. P., and Grose, M. R., A revision of the atomic weight of zinc. The electrolytic determination of zinc in zinc bromide, J. Am. Chem. Soc. 38, 868 (1916).

[5] Baxter, G. P., and Hodges, J. H., A revision of the atomic weight of zinc. II. The electrolytic determination of zinc in zinc chloride, J. Am. Chem. Soc. 43, 1242 (1921). [6] Leland, W. T., and Nier, A. O., The relative abundances of the zinc and cadmium isotopes. Phys. Rev. 73, 1206 (1948).

[7] Hess, D. C., Inghram, M. G., and Hayden, R. J., The relative abundance of zinc isotopes, Phys. Rev. 74, 1531 (1948 [8] Cameron, A. E., and Wichers, E., Report of the International Commission on Atomic Weights (1961), J. Am. Chem. Soc 84, 4175 (1962).

[9] Faraday, M., Identity of electricities derived from differer sources, Phil. Trans. Series 2, 3, 23 (1833).

[10] Laird, J. S., and Hulett, G. A., The electrochemical equiv alent of cadmium, Trans. Am. Electrochem. Soc. 22, 385 (1913).

[11] Gladstone, J. H., and Hibbert W., On the atomic weight of zinc, J. Chem. Soc. 55, 443 (1889).

[12] Hamer, W. J., Resumé of values of the Faraday, J. Res. Nat Bur. Stand. (U.S.) 72A, (Phys. and Chem.), No. 4, 435 (1968).

[13] Craig, D. N., Hoffman, J. I., Law, C. A., and Hamer, W. J Determination of the value of the faraday with a silver perchloric acid coulometer, J. Res. Nat. Bur. Stand. (U.S. 64A, (Phys. and Chem.), No. 1, 381 (1960).

[14] Marinenko, G., and Taylor, J. K., Electrochemical equivalents of benzoic and oxalic acid, Anal. Chem. 40, 1645 (1968 [15] This high purity standard reference material, SRM No. 682 is available from the Office of Standard Reference Material National Bureau of Standards, Washington, D.C. 20234 along with the Certificate of Analysis at a unit price of $9 [16] Gill, A. S., Acting General Superintendent of Electrolyt Zinc Company of Australasia Ltd., Private Communication [17] Taylor, B. N., Parker, W. H., and Langenberg, D. N., Deter mination of e/h, using macroscopic quantum phase coher ence in superconductors: implications for quantum electro dynamics and the fundamental phyical constants, Rev. Mod. Phys. 41, 375 (1969).

(Paper 75A6-68

*

1. Introduction

Because of the difficulties involved in performing urate experiments at high temperatures by contional techniques, a high-speed method was eloped to measure heat capacity, electrical resisty, hemispherical total emittance, and normal ctral emittance of electrical conductors. In this er, application of this technique to measurements niobium in the temperature range 1500 to 2700 K escribed.

The method is based on rapid resistive self-heating the specimen from room temperature to near its ting point. During the short experiment, which s less than 1 s, current flowing through the specin, potential across the specimen and specimen perature are measured. The experimental quanes are recorded with a digital recording system, ich has a time resolution of 0.4 ms, and a full-scale al resolution of one part in 8000. Details regarding construction and operation of the measurement tem, the methods of measuring experimental untities, and other pertinent information, such as mulation of relations for properties, etc. are given in lier publications [1, 2].1

2. Measurements

Measurements were performed on two specimens ignated as niobium-1 and niobium-2. Each speci

is work was supported in part by the Directorate of Aeromechanics and Energetics U.S. Air Force Office of Scientific Research under contract ISSA-70-0002. igures in brackets indicate the literature references at the end of this paper.

men was a tube of the following nominal dimensions; length, 4 in (101 mm); outside diameter, 0.25 in (6.3 mm); and wall thickness, 0.02 in (0.5 mm). A small rectangular hole (0.5 × 1 mm) fabricated in the wall at the middle of the specimen approximated blackbody conditions for the high-speed photoelectric pyrometer [3].

The measurements were made in the temperature interval 1500 to 2700 K (approximately 50 K below the melting point). To optimize the operation of the pyrometer, this temperature interval was divided into six ranges: I, 1500 to 1650 K; II, 1600 to 1800 K; III, 1750 to 1950 K; IV, 1900 to 2200 K; V, 2100 to 2550 K; and VI, 2400 to 2700 K.

In each of the first five temperature ranges, two experiments were conducted on niobium-1 and one on niobium-2; in the last range, one experiment was conducted on each specimen. A total of three additional experiments in the ranges II, III, and IV were performed on niobium-1 in which the surface radiance of the specimen was measured.

Before the start of the experiments, each specimen was annealed by subjecting it to 30 heating pulses (up to 2400 K). All the experiments were conducted with the specimens in a vacuum environment of approximately 10-4 torr.

To optimize the operation of the measurement system, the heating rate of the specimens was varied depending on the desired temperature range by adjusting the value of the resistance in series with the specimen. Duration of the current pulses in the experiments ranged from 390 to 450 ms; and the average heating rate of the specimens was approximately

0.5 mm

FIGURE 1. Photomicrographs of the niobium specimen before (upper photograph) and after (lower photograph) the entire set of experiments.

5200 Ks1. Radiative heat losses from the specimens amounted to approximately 1 percent at 1500 K, 3 percent at 2000 K, and 10 percent at 2700 K of the input power.

Characterization of one of the specimens (niobium-1) was made by the following methods: photomicrography, spectrochemical analysis, and residual resistivity ratio. Photomicrographs of the specimen, shown in figure 1, indicate that considerable grain growth took place as the result of pulse heating to high temperatures. Chemical analyses were made of the specimen before and after the entire set of experiments. Comparison of results does not indicate any detectable change in impurity concentration. A list of the nature and composition of impurities in the specimen is given in table 1. The residual resistivity ratio (ratio of electrical resistivity at 273 K to the residual resistivity) of the specimen, measured before the experiments, was 11.

The data on voltage, current, and temperature were used to obtain third degree polynomial functions for each quantity in terms of time, which then provided the input information for the determination of properties.

3. Experimental Results

This section presents the thermophysical properties determined from the measured quantities. All values are based on the International Practical Temperature Scale of 1968 [4]. In all computations, the geometrical quantities are based on their room temperature

(298 K) dimensions. The experimental results are represented by polynomial functions in temperature obtained by least squares approximation of the individual points. The final values on properties at 100 degree temperature intervals computed using the func tions are presented in table 2. Results obtained from individual experiments, by the method described previously [2], are given in the appendix (tables A-1. A-2, and A-3).

3.1. Heat Capacity

Heat capacity was computed from data taken during the heating period. A correction for power loss due to thermal radiation was made using the results on hemispherical total emittance. The standard deviation of the individual data points from a third degree polynomial function in the range 1500 to 2700 K for each specimen is 0.7 percent for niobium-1 and 0.9 percent for niobium-2. The average absolute difference be tween the results on two specimens is 0.7 percent. The function for heat capacity (standard deviation= 0.8%) that represents the combined results of the two specimens in the temperature interval 1500 to 2700 K is:

-3

nere T is in K, and p is in 10-8 m. In the computaons of the specimen's cross-sectional area, which needed for the computations of electrical resistivity, e density of niobium was taken as 8.57 × 103 kg m ]. The measurements, before pulse experiments, of e electrical resistivity of the two niobium specimens 293 K with a Kelvin bridge were in agreement within 1 percent. Electrical resistivity of niobium at 293 K ɔtained by averaging the results of the two specimens 15.9 × 10-8 m.

3.3. Hemispherical Total Emittance Hemispherical total emittance was computed for iobium-1 using data taken during both heating and nitial free cooling periods. The function for hemipherical total emittance (standard deviation = 0.4%) hat represents the results in the temperature range 700 to 2650 K is:

€= -0.144+2.88 × 10-4T-4.46 × 10-8T2

where T is in K.

4. Estimate of Errors

Estimates of errors in measured and computed quantities lead to the following estimates of errors in the properties over the temperature range 1500 to 2700 K: heat capacity, 2 percent; electrical resistivity, 0.5 percent; hemispherical total emittance, 3 percent; normal spectral emittance, 3 percent. Details regarding the estimates of errors and their combination in highspeed experiments using the present measurement system are given in a previous publication [2]. Specific items in the error analysis were recomputed whenever the present conditions differed from those in the earlier publication.

In all the earlier experiments with the present highspeed system, the lowest temperature at which measurements were made was 1900 K. In the present work, the measurements were extended down to 1500 K without creating any significant uncertainties in temperature measurements. The imprecision 2 of temperature measurements was 0.7 K at 1500 K, and 0.5 K at 1900 K. The imprecision of voltage and current measurements was less than 0.02 and 0.03 percent, respectively, over the entire temperature range.

5. Discussion

The heat capacity and electrical resistivity results of this work are compared graphically with those in the literature in figures 2 and 3, respectively. Numerical comparisons are given in tables 3 and 4. Heat capacity results of this work are, in general, lower than those of most other investigators, with the exception of Kraftmakher [8]. Electrical resistivity results are in good agreement with those of others. Estimates of errors in papers cited lead to an estimate of inaccuracies in previously reported heat capacity and electrical resistivity of approximately 5 to 10 and 1 to 3 percent, respectively.

50

3.4. Normal Spectral Emittance Normal spectral emittance was computed for niobium-1 using data from three sets of two experiments, one in which the pyrometer was aimed at the surface of the specimen, and another in which it was aimed at the blackbody hole in the specimen. The measurements were made at the effective wavelength of the pyrometer interference filter (650 nm; bandwidth 10 nm). The function for normal spectral emittance (standard deviation = 0.6%) that represents the results in the temperature range 1700 to 2300 K is:

JAEGER (1934)
LOWENTHAL (1963)

KRAFTMAKHER (1963)

CONWAY (1965)
KIRILLIN (1965)

PRESENT WORK

30

251

1000

FIGURE 2. Heat capacity of niobium reported in the literature.

2 Imprecision refers to the standard deviation of an individual point as computed from the difference between measured value and that from the smooth function obtained by the least squares method.

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