Inert Gas Fusion Determination of Oxygen in Vanadium, Niobium, and Tantalum GEORGE J. KAMIN, JEROME W. Q'LAUGHLIN, and CHARLES V. BANKS Institute for Atomic Research and Department of Chemisfry, Iowa State University, Ames, Iowa --r
b The inert gas fusion technique has been applied to the determination of oxygen in the Group V-A metalsvanadium, niobium, and tantalum. A platinum flux technique was satisfactory for the determination of oxygen in vanadium and a mixed nickelplatinum bath used with a platinum flux technique was satisfactory for niobium and tantalum. Oxygen contents from 100 to lCl00 p.p.m. were determined on standcrd samples with relative standard deviation of 3% for vanadium and for niobium and tantalum.
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gas fusim technique has been applied to the determination of oxygen in a number of materials. Since the availability of commercial instrumentation, it has become increasingly popular. hluch of the original work dealt with the determination of oxygen in steels, but in the recent literature the technique has been applied t o beryllium (4), zirconiim and zircaloy (S), yttrium and yttrium fluoride ( I ) , and mixed fluorides ( 5 ) . Beck and Clark (2)have used the graphite capsule method for determining oxygen in various oxides and in organic compounds. This paper deals with the extension of the inert gas fusion method to the determination of oxygen in vanadium, niobium, and tantalum.
Apparatus and Reagents. The basic instrument used was the Leco Oxygen Analyzer, No. 534-300, consisting of an induction furnace and conductometric bridge. The modifications made on this instrument, a s
Table l.
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Figure 1. Studies of bath to metal ratios necessary for determination of oxygen in the Group V-A metals
well a s outgassing, blanking, and calibration procedures, were discussed previously (1). The temperature was adjusted to a t least 2000' C. as measured by an optical pyrometer. Bath Conditions. Proper bath conditions play a n important part in the evolution of the oxygen from a sample (1, 3, 4). The commonly used platinum flux technique involving a 1 : l ratio of platinum to sample was adequate for the determination of oxygen in vanadium, but much larger quantities of platinum were needed to achieve any meaningful results on niobium or tantalum. I n the case of niobium, the platinum to niobium ratio of a t least 8: 1 was required, while for tantalum, the required ratio was 10: 1. These large amounts of platinum present a major problem, in that they greatly restrict the number of uninterrupted consecutive analyses which can be performed because the volume requirements of the apparatus are rather
restricted. This space problem .is especially acute when samples contaming less than 100 p.p.m. of oxygen are being analyzed. I n an effort to overcome this problem, the use of other bath materials was investigated. Kickel has been previously used as a bath material (4) but it did not prove adequate for any of the Group V-A metals. Attempts a t using a mixed platinumnickel bath showed greater promise. Because nickel contains more impurities than platinum, all t.he nickel and the initial charge of platinum were added to the crucible and allowed to outgas for approximately 1 hour at 1950' to 2000' C. The samples were also wrapped in a t least an equal weight of platinum before analysis. I n the case of vanadium, it was found that a platinum-nickel bath containing as much as 75% nickel gave the same results as a pure platinum bath. Studies on the effect of composition of the platinum-nickel bath on the de-
Determination of Oxygen in Group V-A Pentoxides and Metals
V 02,av. p.p.m. V.F. I.G.F. Taken Found" 2200 2160 f 24 147 155 490 491 f 15 113 118 210 214 i 9 68 66 180 181 i 10 127 118 i 5 107 102 f 6 Determined by inert gas fusion method. 0 2 ,
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BATH TO METAL WEIGHT RATIO
HE INERT
EXPERIMENTAL
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NbzOs 02,
Taken 301 178 105 81 44
.a. Foundo 297 176 101 80 41
Nb 02, av. p.p.m. V.F. I.G.F. 1450 1460 f 20 450 460 f 12 295 289 f 10 185 186 f 5 100 95 f 6
Ta206 02,
Taken 162 105 PS 65
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02,av. p.p.m. V.F. 480 280 160 80
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I.G.F. 415 f 12 270 f 10 155 f 8 $0 f 7
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VOL. 35, NO. 8, JULY 1963
1053
termination of oxygen in niobium showed that baths containing from 25 t o 75y0 nickel yielded good results. A similar study on tantalum showed that baths containing from 25 to 5070 nickel gave good results, the bath to sample ratio in each case being approximately 1O:l. In both cases, the range of the results was far more satisfactory in the case of the mixed bath than with platinum alone. Though nickel alone tends to volatilize a t temperatures above 1800' C., the platinum-nickel bath shows no signs of volatilization until 2200O c. For analysis of vanadium, niobium, and tantalum, a 40% nickel bath was prepared and used with the platinum flux method. Studies on the bath to metal ratios necessary for the use of this bath are shown in Figure 1. The minimum bath to metal ratio for vanadium is 1:1, for niobium, 5:1, and for tantalum, 6 : l . The bath to metal ratios were maintained by periodic addition of nickel with subsequent outgassing. The use of the platinum-nickel bath nermitted the analvsis of 25-30 metal samples, before it" was necessary to change the crucible, which was approxi-
mately double the number analyzed with platinum alone. There is, however, little volume decrease between the platinum-nickel bath and the platinum alone, because of the lower density of the nickel. The increased number of analyses would appear to be caused by increased solubilit,y of the samples and graphite in the bath. Procedure. After the bath was outgassed and the system calibrated ( I ) , the crucible was preheated for 1 minute, the platinum-wrapped sample was introduced, heating was continued for 3 minutes, the power was shut off, the system was swept for 4 minutes, and the final oxygen content was calculated from the change in conductance.
p.p.m. of oxygen content in the sample, except for the higher oxygen values on tantalum metal. Multiple analyses of the samples showed a relative standard deviation for vanadium of about 3%, and a relative standard deviation of about 5% for niobium and tantalum.
RESULTS
(3) Elbling, P., Goward, G. W., Ibid., 32, 1610 (1960). ( 4 ) Kaliman,'S., Collier, F., Ibid., p. 1616.
The pentoxides of the Group V-A metals were analyzed and the results of these analyses are shown in Table I. A comparison of the results obtained by the vacuum fusion and by the inert gas fusion methods on the Group V-A metals is also given in Table I. As can be seen, the results were in good agreement in the range from 100 to 1000
ACKNOWLEDGMtNT
The authors thank V. A. Fassel and his coworkers for the vacuum fusion results. LITERATURE CITED
(1) Banks, C. V., O'Laughlin, J. W., Kamin, G. J., ANAL. CHEM.32, 1613
(1960).
(2) Beck, E. J., Clark, F. E., Ibid., 33,
1767 (1961).
(5) Potter, J. L., Murphy, J. E., Heady, H. H.,Ibid., 34, 1635 (1962). RECEIVED for review February 11, 1963. Accepted April 4, 1963. Division of An-
alytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962. Contribution No. 1283 from the Ames Laboratory of the U. s. Atomic Energy Commission.
Semimicro Determination of Molecular Weight with a Dew-Point System ANGEL0 DE ROS, OLlVlERO FAGIOLI, and PIER0 SENSI Research laboratories, lepetit S.p.A., Milan, Italy
b A new method for the semimicro determination of molecular weight is described. The method is based on the fact that the observation of the dew point provides a very accurate estimation of the beginning condensation and of the ending evaporation of the solvent on a surface whose temperature varies slowly, in the presence of the vapor of the solution at constant temperature. If a number of cells containing solutions of different molarities are used sirnultaneously with a common condensing surface, the disappearance of the various solvent films follows the order of the molar dilutions, even where the differences among the concentrations are very small. The method described presents the following features: ease of performance, possibility of repeating the measurements many times, good precision and accuracy, use of small quantities of substance (2 ml. of solution from 0.1 to 0.2M), quantitative recovery of the substance whose molecular weight has been determined, and the possibility of using substantially all the solvents having boiling points between 40' and 120" C. 1054
ANALYTICAL CHEMISTRY
M
is usually determined by methods based on the variation of the vapor pressure of a solution relative to the pure solvent (cryoscopy, ebullioscopy, isopiestic, osmotic methods), the determination of vapor density, or functional group analysis (1). The more recent ones, which are microscale methods, utilize improvements in temperature measurements, such as amplified thermocouples and thermistors with low thermal capacities, high speed, and sensitivities not previously attained. These methods are very useful because of the small amount of substance required, but are rather delicate (2-7). The problem of determining molecular weight using small quantities of substance is very important in organic chemistry, and in spite of the considerable amount of work cited, the problem seems not yet definitely solved. In this paper a new method, based on vapor pressure lowering of a solution, is described which can provide, in a semimicro scale, an easy and quick determination of molecular weight with good accuracy and with the possibility of using a very large number of solvents. The method is based on the fact that OLECCLAR WEIGHT
the observation of the dew point provides a very accurate estimation of the beginning condensation and of the ending evaporation of the solvent on a surface whose temperature varies slowly, in the presence of the vapor of the solution a t costant temperature. If a number of cells containing solutions of different molarities are used simultaneously with a common condensing surface, the disappearance of the various solvent films follows the order of the molar dilutions even where the differences among the concentrations are very small. EXPERIMENTAL
Apparatus. The apparatus is shown schematically in Figure 1. The cell block, A , is formed by a cylinder of copper electrolytically plated with platinum, 10 cm. in diameter and 3 cm. high. The six cells on the top surface are 2 cm. in diameter and 1 cm. deep, and are placed a t the corners of a regular hexagon. The center of each cell is 3.2 cm. away from the axis of the block. To the undersurface of the block, a 1-watt heater, B , is screwed. The ground-glass disk, C, is 12 cm. in diameter and 0.23 cm. thick, with plane parallel surfaces. The bottom
