Determination of Hydrogen in Niobium R. J. WALTER and H. G. OFFNER Research Department,, Rocketdyne, A Division of North American Aviation, Inc., Canoga Park, Calif.
b A fast and precise gravimetric method for the determination of hydrogen in metals i!; presented. The method complements the vacuum fusion method because it is useful for measuring higher hydrogen concentrations more accurately when using large samples. This method is also useful down to hydrogen contents of 2 2 / A p.p.m., where A is the sample weight in grams. The analysis is based upon evolving the hydrogen at elevated temperatures, converting to water, and weighing the water.
A
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on the physical properties of niobium as affected by hydrogen content required a method for the analysis of large specimens of the metal for large amounts of hydrogen. The present standard methods of determining hydrogen concentrations are vacuum fusion and hot extraction ( 1 ) . Without extensive modification of commercial equipment or use of very small samples, these manometric methods cannot handle quantities of hydrogen over several hundred parts per million. .in inert carrier gas technique has been developed specifically for the analysis of metals capable of absorbing large quantities of hydrogen. Various inert gas techniques have been reported for hydrogen anal Barr (4) and Coe and Jenkins ( 2 ) extracted hydrogen from steel in flowi1ig nitrogen and argon, respectively, and analyzed the hydrogen by means of a katharometer. Heyn in 1903 [S) attempted to analyze hydrogen in steel by evolving the hydrogen from the heated sample in flowing nitrogen, oxidizing the hydrogen to water, and determining t,he water gravimetrically. However, t,he blank values he obtained were of the same magnitude as the amount of hydrogen in the sample, so the procedure was abandoned. Shanahan and Cooke (6) developed a carrier gas technique in which hydrogen is evolved from steel spcc~iinens in flonmg nitrogen and oxidized to watcr by copper oxide; the water is absorbed in methanol and is titrated with Karl Fischcr reagent,. The proposed method is an adaption of Heyn'c method in which the hydrogen evolved from the heated niobium yiecimen, is transported with argon gas through hot copper oxide n-hich reacts wit'h hydrogen t o -form water vapor. CURRENT STUDY
The water is collected and weighed to determine the hydrogen content in the metal. The main advantage of this method is the ability to analyze large hydrogen concentrations in specimens weighing as much a:, 10 grams. dlso, the total equipment coqt for the proposed method is considerably lo\\ er than for vacuum methods. The analysis can be performed rapidly and, if desired, the specimens can be recharged for further studies. llthouph the analyses were performed on niobium, the method should be applicable to all metals. The minimum amount of hydrogen detectable by this method is 22 A mhere .I is the sample weight in grams. EXPERIMENTAL
Apparatus. The apparatus, shown schematically in Figure 1, consists of a Leco induction furnace (Catalog KO. S-36215) and a gas purification train. T h e high-purity argon carrier gas (99. 998y0 pure) is further purified b y passing over copper and copper oxide wire to remove oxygen and hydrocarbons, respect'ively. Any carbon dioxide and water are removed by passing through a compact of ascarite and anhydrone. Sitrogen removal and final impurity scavenging are performed by passing the argon over hot titanium at 650" C. I t was considered important to remove oxygen and nitrogen from the gas because any oxide or nitride formation on the surface of the specimen could impede diffusion of hydrogen from the metal. Sample Procedure. T h e sample is carefully cleaned with acetone, rinsed with benzene, allowed to air dry, and weighed. I t is then placed in a quartz - enclosed graphite - blanking
crucible susceptor (Leco Cat. No. S-21915) and raised into the induction furnace. T h e furnace assembly is first purged with argon for 5 minutes, then outgassing of niobium is performed a t 1100" to 1150' C., as measured with an optical pyrometer, for about 20 minutes in the presence of flowing argon. The argon flow rate is 80 ml. per minute. The argon plus hydrogen is passed over copper oxide a t 650" C. to quantitatively convert the hydrogen to water vapor. The water vapor is subsequently collected in an absorption tube containing magnesium perchlorate. Calibration. T h e method was calibrated by a series of tests to show t h a t hydrogen is quantitatively converted to water and absorbed in the absorption tube; and all hydrogen is removed from t,he niobium specimens. In the first case, measured amounts of hydrogen gas were used to determine if hydrogen is quantitatively oxidized and subsequently collected as water under the experimental conditions of the sample analysis. -1glass ampoule of known volume was filled with hydrogen a t various pressures, corresponding to the usual weight of hydrogen contained in the niobium samples analyzed, and was introduced into the system aft of the argon-purification train. The hydrogen was then transferred through the furnace and combustion train with the argon gas. The recovery in each of these runs varied between 97.7 and 99.0% of the hydrogen introduced (Table I). It should be pointed out that this test is more severe than in actual practice since the gas contained in the ampoule is transported over twice the total void volume in the apparatus than is hydrogen gas evolved from solid specimens. Second, the temperature and time
TITANIUM CHIPS
ARGON 9 9 . 9 9 8 X PUR COPPER OXIDE 650 C
ANHYDRONE WEIGHING TUBE COPPER OXlD GUARD ANHYDRONE TUBE
Figure 1
I
Schematic representation of test apparatus VOL. 36, NO. 9, A U G U S T 1964
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Table I. Calibration with Known Quantities of Hydrogen
Weight
of hydrogen
Weight in ampoule, of hydrogen mg. collected, mg. 17 71 17 35 9 99 9 90 6 00 5 86
Collected, C7 /C
98 0 99 0 97 7
Table 11. Quantitative Analysis of Niobium Specimens Containing Various Amounts of Hydrogen
Specimen NO.
59B 58B 57B 56B 62B 73B
Weight gain on HQ hydriding collected H/Sb H/Nb atom atom ratio ratio 0 1135 0 1109 0 1120 0.209 0.2073 0.2022 0.303 0.3110 0,3042 0.400 0.3995 0.3958 0.506 0.5028 0.4961 0.610 0.6035 0.6068
Difference from weight gain,
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-2.0 +1.6 -0.6 -1.2 -0.8
necessary for complete removal of hydrogen from niobium was determined by analysis of niobium specimens charged to 0 . 6 H,."b atom ratio, which corresponds to about the maximum hydrogen concentration attainable in solid niobium specimens without fracturing. For these experiments a graphite crucible was substituted for the quartz-enclosed graphite crucible, so that the specimen placed in tube A (Figure 1) could be pushed magnetically and dropped into the crucible susceptor at various temperatures after the crucible had stabilized a t the desired temperatures, and the water collected z's. time was measured. The results of the tests are shown in Figure 2. Figure 2 shows that 20 minutes at 1100" to 1150" C. is adequate for complete removal of the hydrogen contained in the specimens. I t has been shown by thermal analysis that when niobium specimens are slowly heated, the evolution of hydrogen begins to take place at an appreciable rate at 525' C. and is essentially completed a t 825" C. (Figure 3). Following thermal analysis, the specimen was analyzed and 99.0% of the hydrogen was removed in the thermal anal treatment. Vacuum fusion analysis for residual hydrogen was made on two specimens previously analyzed by the proposed method to determine whether all hydrogen was removed from the niobium specimens. The specimens were hydrogen charged to 0 . 6 0 H,'Kb atom ratio, (6400 1i.p.m.) analyzed by the proposed method, and sent to Sational Research Corporation for vacuum fusion analysis. Vacuum fusion analysis in-
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ANALYTICAL CHEMISTRY
SPECIMEN IN FURNACE, MINUTES
Figure 2.
Hydrogen evolved vs. time at various temperatures
dicated 2 . 5 and 4 . 5 p.p.m. hydrogen remained in the sample, which is equivalent to 0.00023 and 0.00040 H T b atom ratio, respectively. The amount of hydrogen remaining is completely negligible.
routine basis in this laboratory. These samples were first hydrided, then divided, and a hydrogen determination was made on each piece. Column 3 of Table I1 showy the values of the hydrogen concentration on these samples as calculated from the weight of water collected. The differences obtained in the hydrogen concentrations between pieces of the sample are not to be considered the maximum degree of precision of the
RESULTS A N D DISCUSSION
The data in Table I1 are from the analysis of six samples, representative of a large number of samples which have been and are being analyzed on a 70
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0
350
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Figure 3. Differential thermal analysis showing endothermic reaction on evolution of hydrogen from niobium specimen containing 0.625 H/Nb atom ratio hydrogen concentration
method, because the results also reflect real differences in hydrogen concentration within the sample produced during the hydriding process Hydrogen charging was performed so that contamination by impurities either from the hydrogen or elsewhere was kept to an abqolu te minimum, and since the specimens were relatively large, the hydrogen content of the specimens could be estimated fairly accurately from the weight change on charging. Column 2 s h o w the H / S b atom ratios calculated from the weight gain of the specimens after hydriding. Column 4 qhows the relative per cent difference between these values and the average of the values in Column 3. The fact that in all but one case the former values are slightly less may be the result of some contamination of thr. sample during hydriding, as well a? incomplete hydrogen recovery during analysis. The blank mas a function of argon purification and the type of susceptor
used. With adequate argon purification and without the induction furnace heated, there is no measurable blank. The blank using a graphite susceptor enclosed in quartz is 0.20 mg. of water or 0.022 mg. of hydrogen. If the graphite susceptor is not quartz enclosed, the blank is about 0.48 mg. of water. It is suggested, on these results, that impurities such as hydrocarbons could be sloivly evolved from the carbon and thus contribute to the blank obtained. Possible minute cracks or pores in the silica envelope of the blanking crucible would allow hydrocarbons to be evolved from the graphite core, but a t a much slower rate. It is possible that the blank could be reduced by using a refractory metal crucible as the susceptor; this point, however, has not yet been evaluated. A 0.20-mg. HzO blank is equivalent to 22 p.1i.m. hydrogen in a 1-gram specimen or 2.2 p.p.m. in a 10-gram specimen. Thus, the gravimetric
method of hydrogen analysis is limited to a hydrogen content greater than 22/A p.p.m,, where A is the sample weight in grams. LITERATURE CITED
(1) “ASTM Methods of Chemical Analysis of Metals,” Am. Soc. Testing
Materials, Philadelphia, Pa., 1960. F. R., Jenkins, N . , “An Improved Carrier-Gas Technique for Determination of Hydrogen in Steel,” Symposium on the Iletermination of Gases in Metals, Iron & Steel Institute, London, 1960. (3) Heyn, E., Metallagraphist 6, 39 (1903). ( 4 ) Rooney, T. E . , Barr, G . , J . Iron Steel 1 7 1 d . (London) 69, 573 (1929). ( 5 ) Shanahan, C. E. A . , Cooke, F., Ibid., 190,381-5 (1958).
( 2 ) Coe,
RECEIVEDfor review August 20, 1962. Resubmitted December 16, 1963. Accepted May 19, 1964. Work perforrned under cnntract S o . XAS8-19 under the technical direction of George C. Marshall Space Flight Center, Huntsville, Ala.
Cold Combustion for the Quantitative Determination of Amalgable Metals in the Presence of Their Oxides in Treated Ores M. G. HABASHY Special Research Laboratory, 25 Sultan Hussein Si., Alexandria, Egypt, U.A.R. Amalgable metals, such as copper and tin, are quantitatively determined in the presence of their oxides by mixing the sample with mercury and water, and then shaking the mixture in a known volume of oxygen. The volume of gas consumed by the amalgamated metal-which a is converted to the oxide at room temperature-can b e directly measured a t atmospheric pressure with the “cold combustimeter.” The consumed, measured volume of oxygen is converted to STP, and the metallic content of the sample is obtained. Interferences from lower oxides may b e determined quantitatively. The results show a relative error of
f0.7Yo.
A
of the efficiency of various reductants-including hl-drogen, coke, and petroleum-in obtaining metals from their oxides exemlilifies a study nhich requires a n accurate, quantitative, analytical method for the determination of metallic ratios in the presencr of oxides. The CuS04-Hg method ( I is a suitable one for use wit$h iron in combination with COMPARTSOX
iron oxides. However, the determination of either copper or tin, contaminated with their oxides through the various steps of the reduction, has not been accomplished by n direct analytical method. The CuSOa-Hg method ( 1 ) gives low., unreliable results when applied to the determination of metallic tin, aluminum, and zinc. This suggests that amalgamation catalyzes the partial oxidation of the metal. This probably results from a reduction in particle size of the metal during amalgamation, thus increasing the reactivity of the metal towards oxygen. This occurs even a t room temperature and atmospheric pressure, and resembles, on a quiet scale, the charring of sodium amalgam (4). T o verify this hypdthesis, approximately 0.05 gram of pure aluminum powder was weighed accurately and placed in a 25-ml. test tube. Five milliliters of distilled water and 1.5 ml. of pure mercury metal were added. The stoppered tube was shaken strongly for 20 minutes, with ocrasional removal of the stopper to permit contact with air. When the contents of the tube were tested, no indication of the presence of
metallic aluminum was found upon addition of a concentrated solution of CuS04. The metal was converted quantitatively to the white precipitate, Al(OH),, by mere shaking with mercury, water, and oxygen at room temperature and atmospheric pressure. This experiment led to an analytical method based upon what could be termed quantitative cold combustion. The procedure necessitated the construction of the apparatus shown in Figure 1, the “cold combustimeter.” EXPERIMENTAL
Procedure. Approximately 0.05 to 0.5 gram-determined by the suspected metal content-of the sample is weighed accurately and placed on t h e shelf, C, of the apparatus. Pure 0 2 is passed for two minute? from a suitable aspirator system through the t a p s 1 and G, while the buret, H. is full of the acidulated solution which is composed of 50 ml. of HsS04 and 200 grams of XaC1 per liter of distilled water, and tap J is in the off position. During this procedure, the stoi)pcr, F , .on the reaction vessel, . I , is open, and serves as the only outlet for the sweeping gas. Thus, the atmos;i)hrre in the VOL. 36, NO. 9, AUGUST 1964
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