Determination of Nitrogen Content of Noble Gases Absorption in Titanium Metal H. S. DOMBROWSKI Pigments Department, Chemical Division,
E. 1. du Pont d e Nemours & Co., lnc., Newport, Del.
In critical studies on titanium and zirconium, it was necessary to determine the impurities in argon and helium. The cuprous chloride method served well for determining oxygen, but there existed no precise method for measuring nitrogen. A method, comprising absorption of nitrogen in titanium metal at elevated temperature, was therefore developed. The method is precise for a few parts per million and is also applicable for gross contamination. It may be adapted for measurement of total impurities.
A
NALYTICAL methods heretofore available for determination of the nitrogen content of argon and helium are not sufficiently sensitive for critical applications of these gases. A new, precise method is described for the determination of as little as several parts per million by weight of nitrogen and the procedure is also applicable to high concentrations of this impurity. The method comprises absorption of nitrogen from a relatively large volume of noble gas by passing it over a definite weight of titanium metal sponge a t 900' to 1OOO' C. to form titanium nitride. Nitrogen absorbed by the sponge is determined quantitatively by a modified macro-Kjeldahl method (20). Adaptations are suggested for the determination of total impurities in noble gases.
Table I.
VARIABLE TRANSFORMER
SILICA TUBE 36"I %"I D $0 D f'I.O.FUSTIC
W N G
7
7 /
Gas
-75
Data Showing Complete Absorption of Impurities Furnare 1 Total Sitrogen wt. gain, pirkup, grain
graiii
n.o m 0 .o m
n,o m
0.0168
0.0'71.? o.nn4a
Furnace 2 Total Sitrogen pickup, gram grain
at. gain,
n,n o m no,ooo? .om
n.nono n.onni ooni
10 GRAMS
For these reasons, titanium was selected for absorption of nitrogen from helium and argon. An absorption temperature of 900' to 1000" C. was chosen to ensure a high rate of reaction, minimize sintering effects, and avoid possible reaction of the sponge with the silica tube, such as might occur a t higher temperatures. The determination of nitrogen by absorption in titanium metal sponge with analysis by a modified macro-Kjeldahl technique (20) has been demonstrated to be precise and applicable to gases of both high and low impurity content.
I"x12"SILICA TUBE
EXHAUST
Sample
Argon A-I Argon h-1 Helium EIe-t
HEATER LEADS
INERT GAS S b h U E
THERMOMETER
Specific contaminants such as oxygen, moisture, and cai bon dioxide have previously been investigated and may be determined with precision by standard analytical techniques (2, 4, 6, 9, IO, 14-16). However, thecie impurities often constitute a relatively minor portion of the total impurity content of noble gases. Studies in this laboratory required high-purity argon in which the major contaminant was found to be nitrogen. It s a s thus necessary to develop a new method for the determination of nitrogen. A number of elements are known to react rapidly and completely with oddizing gases. The alkali and alkaline earth metals have long been wed to purify noble gaseq ( 7 , 11-13, 19, 21). More recently titanium, uranium, and zirconium have been employed (3, 1 7 ) to "getter" noble gaqes when a high degree of purity is required. Titanium metal sponge has the advantages of high surface area, high reactivity at elevated temperatures, excellent stability in air at room tempel ature, and the lack of need for anv surface activation treatment prior to use. Systematic studies ( 5 , 8 ) of the equilibria and rates of oxide and nitride formation indicate that temperatures above 800" C. favor rapid and complete reaction.
1
RESISTAWE RIBBON
FIRE BRICK INSULATION
WET TEST METER
Figure 1. Apparatus for Inert Gas Analysis High-temperature furnace. Fused silica tube (36 x a/8 inch inside diameter X I/* inch outside diameter). Potentiometer and thermocouple. Wet-test gas meter. Equipment for macro-Kjeldahl nitrogen determination. Figure 1 is a diagrammatic sketch of the equipment used. The furnace consisted of a 13 X 1 inch inside diameter silica tube wound with Nichrome ribbon of approximately 7 ohms total resistance. The ribbon was cemented to the silica tube with sillimanite cement and the assembly mounted in an insulating firebrick setting. A 15-ampere variable transformer provided the required electrical input t o maintain the inside furnace temperature a t 1050° C. Previous measurements had shown this temperature necessary to maintain the sponge sample in the range 900" to 1000" C. a t the gas flow rate used. The absorption tube consisted of a 36 X 3/8 inch inside diameter X '/z inch outside diameter fused silica tube. 4 10-gram sample of titanium sponge particles occupied roughly a 4-inch
The purity of commercially available noble gases, argon and helium, has been improved to such extent that conventional methods of determining nitrogen are no longer sufficiently sensitive or reliable. These methods are inadequate for critical applications where it is necessary to ensure extremely low contaminant levels. The mass spectrometer method, for example, is not sensitive to contaminants below about 100 p.p.m. Another commonly employed method (1, 1 8 ) utilizes lithium metal a t elevated temperatures to absorb impurities in a constant temperature-constant volume system. The impurity content is calculated from the change in total gas pressure. With gases of relatively high purity (less than 200 p.p,m. of total impurities) the pressure change is so small that the method lacks precision. Furthermore, the lithium method is not specific to the usually occurring major contaminants.
526
V O L U M E 26, NO. 3, M A R C H 1 9 5 4
527
length of the tube thus affording an adequate “packed section” for absorption of impurities. Heat-resistant plastic tubing with low volatility and moistureabsorption properties was used for all flexible connections. A 48-inch length of l/g-inch bore plastic tubing was used to isolate the wet-test meter and sample tube to prevent possible backdiffusion of air and moisture to the sponge sample. Selection and Preparation of Sponge Sample. An ample supply of high quality titanium sponge should be obtained, preferably as approximately -8 mesh, +20 mesh material free from fines and substantially free from needle and platelet shaped particles. The nitrogen content of this “standard” sponge should be determined accurately by analyzing a number of riffled samples. PROCEDURE
Prior to starting a run, the silica absorption tube is conditioned by heating it to 1000” to 1050’ C. by slowly advancing it through the furnace to ensure a clean, dry tube. A 10-gram sample of “standard” sponge is neighed accurately on an analytical balance and charged into the absorption tube so that it remains roughly 12 inches from one end. The sponge should be charged in such manner as to fill the cross section of the tube as well as possible (Figure 1 ) . Sample introduction may be facilitated by using a short, n-ide-stemmed funnel and a fairly close-fitting glass rod to act as a piston inside the silica tube. Extreme care must be exercised to prevent loss of any of the sample. The absorption tube is positioned in the furnace so that the sponge sample portion is n-holly outside the furnace on the gas inlet side. Plastic connections to each end of the tube are wired to ensure gas-tight joints and the flow of gas adjusted to a rate of 3 to 4 liters per minute. 4 t this point it is necessary to cool the outlet end of the absorption tube to prevent softening and scorching of the plastic tubing by the hot gas. This may be accomplished by placing a saturated pad of cheesecloth or gauze over the end of the silica tube. In this position, the noble gas purges air from the absorption tube and sponge while the sponge sample remains cold and unreactive. After about 20 liters of gas have passed through the sponge, the absorption tube is moved along its axis until the sponge section is centered inside the furnace. The wet-test meter is read and the measurement recorded 1 minute later to coincide with the start of absorption of impurities by the sponge. A furnace temperature of 1050” C. and a gas flow rate of 3 to 4 liters per minute should be maintained until the desired volume of gas has passed through the system. The optimum volume of gas depends on its impurity level and its molecular weight-i.e., whether argon, helium, etc. A volume of about 600 liters is recommended for argon with nitrogen in the low range (1 to 10 p.m. ). Smaller volumes are satisfactory for higher Contaminant tvels.
Table 11. Reproducibility of Nitrogen Deterniination Sitrogen. P.P.31. by Weight i n Argon and Helium I n Helium ____ I n Argon -____ Sample .-\-I -4.2 A-3 A-4 €le-1 He-2 1 2 3
4 6 Arernge
48.3 43.2 47.4 44.7 46.1 46.2
318 307 330 303 322
34.4 31.5 3:3.2 33.1
3.8 3 2 4.0 3 o
1390 1380 1380 1390
61 3 65.4 63.3 63.0 60.3
46.4
318
331
zJ.3
1383
6 2 .7
. .
..
. . ..
.. . .
...
Khen the desired volume has passed through the sponge, the absorption tube is returned to its former or purge position and the final reading of the wet-test meter recorded when the sponge sample falls be lo^ red heat. When the sample-bearing portion of the tube is cool to the touch, the outlet plastic connection is removed and the tube is withdrawn from the furnace anti placed on a horizontal surface to cool to room temperature. \Then the entire tube is cool, the gas f l o ~is stopped and the remaining plastic connection removed from the absorption tube. The sponge sample is best removed by inserting the sample end of the tube into the weighing bottle, up-ending the tube, and tapping lightly in the vicinity of the sample to loosen the sponge. The entire sample must be collected in the tared weighing bottle, weighed, and analyzed for nitrogen content. All of the sponge sample must be dissolved and an aliquot taken as recommended in the procedure for nitrogen determination. The weight of gas passed through the heated sponge is calculated from the volume of gas recorded by the wet-test meter
mith appropriate corrections for humidity, gas pressure, and temperature. The nitrogen absorbed by the sponge is calculated from the weight of titanium sponge and the difference in nitrogen analyses before and after absorption. The increase in weight of the sponge sample, the nitrogen pickup, and the weight of noble gas permit calculation of p.p.m. of nitrogen and p.p.m of total impurities in the gas. RESULTS
Preliminary work indicated complete absorption of nitrogen by titanium sponge a t the afore-mentioned temperature.. and flow rates. However, in order t o demonstrate complete absorption, two identical furnaces and absorption tube systems were set up to pass the gas through two heated sponge samples in series. Completeness of abcorption of total impurities and nitrogen could then be determined by comparing the total weight gain and nitrogen pickup of the two Qamples. Rehult.., as shonn in Table I, demonstrate substantially complete abborption of total impurities and nitrogen by the sponge in the first furnace.
Table 111. Reproducibility of Total Impurities Determination Argon 8-3
Sample
A-1
.4-2
1 2 3 4 5 G
105 85.5 86.8 74.3 72.2 69.2
334 338 37.5 352 388
52
...
.. .. ..
92.2
361
36
.iverrtge
59
..
-4-4
14 9 11 in
Heliuiii He-1 lIe-9 1920 1940 1880 1870
226 318 183 230
, .
..
..
... ...
11
1900
’38
..
Precision of the method for deterniinatian of the nitrogen content of argon and helium is indicated by the results listed in Table
TI. The calculated weights of total impurities shown in Table I11 are somenhat less precise than the nitrogen results. DISCUSSION
Sfter recovery, noble gape? may be fractionated in their liquid stater: to exceptionally high degrees of purity. Suhequent handling operations such as transfer to tank cars and gas cylinderp, however, frequently lead to contamination with air. Determination of nitrogen in conjunction with the cuprous chloride method for oxygen permits precise determination of the extent of this atmospheric contamination. The determination of total impurities by this method is not a3 reproducible as the determination of nitrogen. The amount of total impurities should nevertheless be useful as a guide to overall gas contamination. Several factors may account for variations in the total impurity results. Tit,anium and hydrogen reach an equilibrium absorption condition in which hydrogen is not completely removed from t,he gas stream. Molecular hydrogen as a contaminant would not be expected to be present to any great extent in commercial noble gases with the possible exception of helium. However, hydrogen in the form of moisture, methane, and other light hydrocarbons could be present in any compressed gas as a result of improper storage and transfer. il second factor to consider is that impurities such a? moisture and carbon dioxide are probably not released from a cylinder or other container as a constant weight percentage of the noble gas. These impurities most probably are adsorbed on the inside surfaces of the cylinder and are released a t a rate dependent on the repidual pressure and temperature of the compressed gas. Total impurity measurements, therefore, would vary depending on the residual volume of gas in the cylinder.
ANALYTICAL CHEMISTRY
528 Zirconium sponge, or turnings of either titanium or zirconium should be equally as effective as titanium sponge in this method. Thin turnings and a longer “packed length” may be necessary t o ensure complete absorption of impurities. ACKNOWLEDGMENT
The author Tvishes to cypress his appreciation to J. M.Thompson of this laboratory for performing the analytical work upon which this method is based. LITERATURE CITED (1)
Bowman, R. E., and Hartley, C. B., Welding J . (-V. Y.),29,
258-625 (1950). (2) Brooks, F. R., etal., ASAL. CHSM.,2 4 , 5 2 0 3 (1952). (3) . , Diecke, G. H., and Crosswhite, A . AI., ,J. Opt. SOC.Amer., 42, 433 (1952). (4) Fellows, C. G., preprint of paper presented a t meeting of Instrument Society of America, Sept. 8-12, 1952. (5) Gulbransen, L. G., and Reavell, F. R., Metallurgta, 39, 63-5 (1948).
(6) Harris, F. E., and Nash, L. K., .ANAL. CHEM.,23, 736-9 (1951). (7) Hendry, R. G., L-. 9. Patent 1,769,025 (July 1, 1930). ( 8 ) Hickman, J. W., and Gulbransen, L. G., A N ~ LCHEY., . 20, 1586.5 (1948). (9) Ilfieid, R. AI., lbzrl., 23, 1086-9 (1951). (10) Johnston. ,J,, and Walber, A . C . , J . Am Chem. Soc., 47, 1507 (1925). (11) Jones, R. G., 8. Patent 1,475,036 (Dec. 18, 1924). (12) Klenia, E. D., U. S. htomic Energy Commission, AEC Rept. AECD 2157 (1948). (13) Krauss, A , C. S. Patent 2,204,501 (June 11, 1940). (14) Lunge, George, and Ambler, H. R., “Technical Gas Analysis,” pp. 221-3, Sew York, D. Van Sostrand Co., 1934. (15) lIacHattie, I.
