APRIL, 1939
TANTALUM as a MATERIAL for STANDARDS of MASS .
WILLIAM M. THORNTON,
.
JR.
Loyola College, Baltimore, Maryland
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N 1911, the International Committee on Weights and Measures (1)entertained the suggestion that a series of standard weights fashioned from tantalum, of 100 g. each, should be established for use in chemical research. It was estimated a t the time that the cost would not be more than one-third that of otherwise similar iridio-platinum pieces. Evidently the plan was actually put into effect, for in 1920 the following brief report (2) made its appearance : "Un essai fait avec du tantale a donnC des rCsultats tr& satisfaisants A tous Pgards, mais il ne nous a pas CtC possible
d'obtenir des pi&cesde ce metal dont la masse fdt supCrieure 1008. De plus, son prix, compare A celui du platine, ne constituait pas un avantage tellement considerable, que l'on fdt encouragC A surmonter de grosses difficultes techniques pour en assurer l'utilisation."
Analytical sets, in which the fractional pieces above 20 mg. are stamped from sheet tantalum, have been used successfully in the United States for a number of years. In this connection, Mellon and Swank (3) state that gold weights (presumably 22-carat) show excessive wear as compared to tantalum. In like manner, the smaller weights in a box, whose occupants had been
tested a t the Bureau of Standards and reported upon (April 27, 1932), were calibrated by the present writer (April 13-16, 1938), and the five tantalum members of the set, ranging from 500 mg. to 50 mg., inclusive, were found to have remained substantially unchanged; notwithstanding the fact that they had been used from time to time in analytical work. In 1928, the Bureau of Standards (59) placed some 50-g. tantalum weights under observation. The metal composing these examples was of low density (16.1 g. per cm.9, yet the weights themselves showed a satisfactory degree of stability; in fact, they appeared t o gain a few hundredths of a milligram only during a period of some fifteen months. Tantalum (No. 73) is truly a rare element, being, in all probability, less abundant in the lithosphere than gold (4). Nevertheless, thanks to the phenomenal progress that bas been made with its metallurgy during the past few years, highly purified elemental tantalum can now be procured in abundance and a t a price that is not excessive (4). Fortunately, also, the scientific literature is already replete with descriptive matter covering such topics as the discovery of tantalum (5), the occurrence of the element in nature, the physical and chemical properties of the metal and its compounds, industrial applications, and the chemical examination of the columbotantalates (6) -minerals that had baffled the analyst for more than a century. Among these papers are to be found writings of the well-known pioneers, Von Bolton (7-9) and Balke (1015), and likewise contributions containing useful information from various other authorities (1642). With the foregoing considerations in mind, it would seem that one is in a fair way to predict the suitability of metallic tantalum as a material from which to manufacture reliable standards of mass. In a previous communication (43), an attempt was made to lay down the requirements that should be met by dependable reference weights, and it may not be inappropriate to give these criteria again. They (the weights) should be: (1) sufficiently resistant to abrasion, (2) nonmagnetic, (3) not readily acted upon chemically by atmospheric agents-whether normal or abnormal, (4) non-porous, (5) not appreciably hygroscopic, (6) not easily electrified, (7) of a density (except in the case of fractional pieces) as near to 8.4 g. per cm.8 as may be, in order that the conventional "weight in air against brass" may be utilized with the greatest possible satisfaction, and (8) characterized by design and workmanship that are, practically speaking, beyond criticism. Though a somewhat formidable list of conditions, it is believed that tantalum standards will not be found wanting save in two possible respects,
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and that even these obstacles are of no serious moment. More specifically, the density of tantalum (7, 44) is about twice that of any ordinary brass or bronze, being 16.6 g. per ~ m a t. 20°C.; ~ furthermore, difficulty has thus far been experienced in producing non-porous specimens of the metal exceeding a certain size. This rather high density, and consequent small volume, need cause no error in the values deduced for other weights, when these latter are compared with tantalum standards, provided the density of the air be determined with sufficient exactness a t the time of making the weighings. The porosity of tantalum will be dealt with in greater detail (see below). In addition, tantalum possesses certain other characteristics that should render it well suited to the present purpose: owing to its low specificheat (45), weights
Designa!ian
Appnrcnr moss vs. brow
Date of calibration: July 26, 1937. Conditions of experiment: 27.3'C..
Truc mars
752.4 mm. (at 29.5-C.), and 72 per 0.00115 g. per ml.
cent. relative humidity; whence density of air =
of this metal may be expected to attain quickly the temperature of the balance case, though this phenomenon may be offset to some extent by the fact that tantalum is not a very good conductor (4547) ; moreover, because of its unusually small thermal expansion (4849), the change in volume of a given weight is wellnigh negligible for the ordinary fluctuations of room temperature. There are yet other considerations. As the outcome of correspondence, which was begun in the autumn of 1936, Dr. Clarence W. Balke, of the Fansteel Metallurgical Corporation, generously donated
Date of calibration: September 8. 1938. Conditions of experiment: 23.8'C.. 764.7 mm. (at 23'C.), and 45 per cent. d a t i v e humidity; whence density of air 0.00119 g. p a ml.
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a sizable rod of selected tantalum. The density of this specimen was taken, and 16.63 g. per ~ m a t. 20°C. ~ was obtained. Accordingly, the "blank" was sent to Mr. C. A. Becker (Christian Becker, Inc.), who, during the summer of 1937, gave his time and attention to the construction of three 10-g. weights of solid tantalum. These weights were finished most handsomely and adjusted with extraordinary nicety. They are in the form of a cylinder with rounded edges and bear a groove not far below the top surface (see illustration). In other words, they resemble somewhat, though in miniature, the Imperial Standard Pound of Great Britain (50). To evaluate these tantalum pieces, they were first compared by substitutional weighing with a "doubleplated" standard (43) which had been certified by the
of the experiences just recorded and partly from a general knowledge of tantalum, it seems reasonable to suppose that 10-g. weights of this metal, if handled with ordinary care, will remain constant for a protracted period of time. Admittedly, if the mass of the standard were greater, any uncertainty as to its value would be relatively less. But practical difficulties arise when one contemplates the construction of large weights of tantalum. Because of its very high melting point, 2850°C., tantalum is not fused in the Fansteel works. On the contrary, it is compacted by means of the so-called fioder metallurgy, wherein the finely divided metal is strongly pressed and the resulting bars are heated to a high temperature in a vacuum furnace (cf. 58). In fact, Doctor Balke, in his letter of October 30, 1936, advised against any attempt to prepare a 100-g. weight of tantalum on the ground that so large an ingot might not be composed of strictly non-fiorous material. And the idea of compromise-use of a 20-g. weight, for example--was abandoned, as in most of the systems for the calibration of metric weights the standards employed are decimally related to one another (43, 57). In view of the present-day tendency to perform analyses on a small scale, as implied in the terms microJJ. It is both interesting and gratifying to notice that, emlysis and semimicroanulysis, there is no longer after the rounding off, all three of the weights are equal, much need for a large mass standard, so far as chemical estimations are concerned; On the other hand, i t has become important to know the values of the smaller weights in a working set with increased accuracy. (The larger pieces are being used, for the most part, simply to counterpoise the containers.) One-piece reference weights of bronze, whether gilded or platinized, have been observed to undergo slight changes with time and circumstance. Consequently, it is hereby proposed Dnte of certificate: September 7, 1938. ~ a t i r m a~ l ureau of Standards Test No. 80971. that one or more weights of solid tantalum, of not too Conditions of experiment: 20°C.. 751 mm., and 65 per cent. relative humidify. large a denomination, be preserved within the quarters of any particular scientific organization for occasional and that the "weight in air" exceeds the nominal value use in verifying other standards. It remains for the writer to express his sincere thanks by only 0.01 mg. Besides, the results obtained by the author do not differ from those of the Bureau of Stand- to those who have cooperated so magnanimously in the ards by more than 0.01 mg. (plus or minus) in any case; above-described research. Among these, Dr. Clarence and, as is more important, two successive values for W. Balke and Mr. C. A. Becker have already been any one weight-approximately thirteen and one-half mentioned; but, as i t hardly seems possible to emphamonths intervening-depart from each other by the size unduly their valuable assistance, it is formally same amount (namely, 0.01 mg.). In all likelihood, acknowledged here. Moreover, the Burton-Levin these differences are to he attributed to observational Foundation, Inc., of this city, on the basis of an inerrors and not to any actual changes in the weights troduction by Dr. Reuben Roseman (chemist for the themselves, seeing that the balance used was not quite Foundation), gave sufficient financial aid to ensure the sensitive enough to take account of the 0.01 mg. with completion of the task; and, needless to say, their gift certainty. Speaking more broadly, partly on the basis is most heartily appreciated. Bureau of Standards under Class M on March 8, 1933, and then with one another; that is, six comparisons were made in all. The work was performed with a fine, magnetically damped, analytic balance, whose semibility recifirocal (51) a t the load, though subject to slight variation with changing conditions, did not depart greatly from 0.175 mg. per scale division on the index plate throughout the entire series of determinations. The atmospheric temperature, pressure, and humidity were observed, whereupon the density of the air was calculated with the aid of Felgentraeger's table (52), in order to apply the buoyancy corrections (cf. 53-55). From the data thus established the most probable values for the weights under test were computed by the method of least squares (52, 56). The results of the first calibration (July 26, 1937) are given in Table 1 ; and, similarly, those of the second (September 9, 1938) in Table 2. Meanwhile, by way of removing, as far as possible, all uncertainty as to the correct values of the tantalum weights, they were submitted to the National Bureau of Standards for test. That organization saw fit to certify the whole group under Class M (bigh-precision laboratory standards) on September 7, 1938 (see Table -\
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