Determination of Nitrogen and Hydrogen at Parts-per-Million Levels in Milligram Steel Samples G. L. Hargrove, R. C. Shepard, and Harry Farrar IV Atomics International, A Division of North American Rockwell Corporation, Canoga Park, Calq. 91304 RECENTEXPERIMENTS indicate that nitrogen, through its relatively high ( n p ) cross section, is a major contributor of helium in stainless steel irradiated in fast neutron spectra ( I ) . The helium in turn severely limits the ductility of austenitic stainless steels at elevated temperatures ( 2 , 3), and in the presence of a high fast neutron flux, helium atoms can serve as nuclei for void formation. There is also evidence that hydrogen, which is formed in fast neutron fluxes by (n,p) reactions with many elements, also contributes t o void formation ( 4 ) . Because of the resulting dimensional instabilities, void formation has become a critical problem i n the development of liquid-metal fast-breeder reactors ( 5 ) . High specific radioactivity of irradiated steels makes it desirable in the study of these effects, therefore, to measure small amounts of hydrogen, helium, and nitrogen in specimens n o larger than 10 milligrams. A high sensitivity gas mass spectrometer system (6) is being used to measure helium
(1) W. N. McElroy, H. Farrar IV, and C. H. Knox, Tram. Amer. Nucl. Soc., 13, 314 (1970). ( 2 ) D. Kramer, H. R. Brager, C. G. Rhodes, and A. G. Pard,J. Nucl. Muter., 25, 121 (1968). (3) D. R. Harries, J . Brit. Nucl. Eiiergy SOC.,5, 74 (1966). (4) D. Kramer, “Void Formation in Stainless Steel by Proton Irradiation,” submitted for the 1970 AEC Report on Fundamental Nuclear Energy Research. ( 5 ) C . Cawthorne and E. J. Fulton, Nature, 216, 575 (1967). (6) H. Farrar IV and C. H . Knox, Trans. Amer. Nucl. SOC.,11, 503 (1968).
concentrations down to 1 0 P wt ppm, and the technique reported here is designed t o complement this capability by measuring very low levels of nitrogen and hydrogen. Inert gas fusion methods have been used for years in the determination of hydrogen, nitrogen, and oxygen in metals (7). Holt and Goodspeed (8) adapted a n inert gas fusion method for the determination of oxygen developed by Smiley ( 9 ) t o the analysis of hydrogen and nitrogen in milligram size samples. However, their technique, which uses capillary manometers and a helium carrier gas, has detection limits for a 1-mg sample of -200 wt ppm and -10 wt ppm for nitrogen and hydrogen, respectively, set primarily by the operational blank. These detection limits are a n order of magnitude too high for the application described above. If care is given t o the preliminary outgassing of the sampling system involved, stable isotope dilution techniques ( I O ) and spark source mass spectrometry ( I I ) can yield the required sensitivities. These latter methods were considered, but a simpler steady-state approach seemed quicker and more convenient. Therefore, the inert gas fusion methods men(7) W. E. Dallmann and V. A. Fassel, ANAL. CHEM.,39, 133R ( I 967). (8) B. D. Holt and H. T. Goodspeed, ibid.,35, 1510 (1963). (9) W. G. Smiley, ibid., 27, 1098 (1955). (10) C. R. Masson, Mefallurg. Rec., 117, 147 (1967). (11) G. Vidal, P. Galmard, and P. Lanusse, ANAL. CHEM.,42, 98 (1970).
A INDUCTION COIL
Figure 1. Micro-inert gas fusion system
STRIP CHART RECORDER
GAS CHROMATOGRAPH
MULTI-PORT GAS SAMPLE VALVE
M g C0 4 ) ~ DRYING TUBE
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Figure 2. Fusion furnace detail tioned earlier (8, 9 ) were optimized and coupled with a high sensitivity gas chromatograph to yield a micro-inert gas fusion technique with more than the required sensitivity. Using this technique, an analysis for both hydrogen and nitrogen can be completed in 90 minutes.
from the carrier gas and from system degassing do not contribute to the operational blank as in the earlier trap-out methods (8, 9), but instead they produce a steady-state detector current on top of which the desired elements are determined.
PRINCIPLE OF OPERATION
EXPERIMENTAL
The manometers used earlier (8, 9 ) to measure the gases evolved during the fusion of the sample are replaced by a gas chromatograph with a helium ionization detector which has a limiting sensitivity for hydrogen and nitrogen of between 10-11 and 10-12 gram. The operational blank has been eliminated by development of a new fusion furnace design and mode of operation. The fact that the nitrides and hydrides involved decompose quickly during sample fusion allows the fusion furnace, at temperature, to be made an integral part of the chromatographic system. With ultra pure helium as the carrier gas, the standing current of the detector is sufficiently low to allow measurement of very l o w levels of chromatographically separated hydrogen and nitrogen. In this way, impurities
Apparatus. The gas chromatograph, a Model 1532-2B (Varian Aerograph, Walnut Creek, Calif.), was equipped with a helium ionization detector (HID) whose output was fed to a Sargent Model DSR Strip Chart Recorder (E. H. Sargent Corp., Anaheim, Calif.) operated at 5 cm/min chart speed. The chromatographic column consisted of a 2.0 meter long, 6.35-mm 0.d. Type 316 stainless steel tube packed with 50/60 mesh molecular sieve 5A (Perco Supplies, San Gabriel, Calif.). The induction unit used with the fusion furnace was a LECO Model 537, 4.5 kW, 1-mHz generator (Laboratory Equipment Corp., St. Joseph, Mich.) with a 60-mm 0.d. induction coil made from 4.75-mm 0.d. copper tubing. A schematic diagram of the micro-inert gas fusion system is shown in Figure 1. More details of the micro-fusion furnace, which was constructed of quartz, are shown in Figure 2. The furnace
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Table I. HID Calibration Data Calibration Factor, Applicable Element gm/cm2(Attenuation. X 1) range, grams Hydrogen 1.15 ( 3 ~ Q . 1 1X) ~10-@ IC9 -les Nitrogen 1.35 (10.06) X 1W8 10-11-10-8 375 VDC Hydrogen 6.71 (10.47) X Nitrogen 10-10-10-7 8.76 (10.44) X a Standard deviation, u , of calibration factor determined at five points in applicable range. Detector voltage 200 VDC
and crucible designs represent compromises which provide positive sample delivery to the crucible and minimum dilution of the gases evolved during sample fusion. The furnace must be periodically cleaned to remove residues which condense on the walls of the furnace. The buildup of these residues over a period of time results in an increased noise level. The furnace is cleaned by rapidly dipping it in concentrated HF, rinsing in distilled water, and drying at 150 "C for 4 hours. Procedure. Samples to be analyzed are cut to a size (0.310 mg) which will negotiate the quartz sample drop-tube (2 mm id) and are degreased in analytical reagent grade carbon tetrachloride. After drying in air for 20 minutes (in a place appropriate for radioactive specimens, if necessary), they are weighed and stored in a screw-top glass vial. Before an analysis, the gas fusion system is put in the purge mode shown in Figure 1. The chromatographic column is activated for 30 minutes at 300 "C with the 99.9999% pure helium carrier gas (Dye OxygenCo., Gardena, Calif.). Meanwhile the sample introduction port cover is removed and the sample is placed with tweezers into the cup of the sample holder which is positioned as shown in Figure 2. After the port cover is replaced and held with two clips, the stopcock in the port cover and the 3-way stopcock at the base of the furnace are opened and the system is purged for 15 minutes at a rate of -1000 ml/min. Following this, the port cover stopcock is closed, the 3-way stopcock at the furnace base is turned to allow helium to flow as far as the multi-port gas sample valve (Varian Aerograph). After 5 minutes, the fusion crucible is brought to temperature (1800 "C measured by optical pyrometer) by adjusting the induction coil current t o 120 mA. The chromatograph is cooled to ambient temperature (-10 minutes) and the multi-port gas sample valve is switched to the run mode shown in Figure 1. The chromatograph is operated with inlet and outlet pressures of 16 psig and 0 psig, respectively, and with a helium carrier-gas flowrate of 250 ml/minute. When the HID base line stabilizes (5-7 minutes), the sample holder is moved along the tube and rotated with the aid of magnets, dropping the sample from the cup into the hot crucible (ATJ Graphite). All gases evolved during sample fusion pass to the chromatograph and the hydrogen and nitrogen peaks elute from the chromatographic column in 0.5 and 3.0 minutes, respectively. The peak areas of the HID response are measured with the aid of a planimeter (Lietz, Inc., Los Angeles, Calif.). The system is calibrated by the use of an exponential dilution flask (12) (Varian Aerograph) and a calibrated sample (12) J. E. Lovelock, ANAL.CHEM., 33, 162 (1961).
Standard 352 lOle
Table 111. Concentration Data on Consecutive Specimens Hydrogen Nitrogen concn, concn, Sample Mass, mg wt PPm wt PPm 1 5.02 1.25 337 2 306 7.21 1.21 270 3 11.10 1.11 4 5.40 1.08 320 Mean 1.16 308 Std dev =kO. 08 f28
loop. C.P. grade calibration gases (Matheson Co., East Rutherford, N. J.) are added to the exponential dilution flask with either 0.25- or 5.0-ml "pressure lok" syringes (Precision Sampling Corp., Baton Rouge, La.) during which time the calibrated sample loop is substituted at the multiport gas sample valve in place of the fusion furnace. Because of the nonlinearity of the HID, calibration factors must be obtained at several detector voltages in order to cover a wide range of hydrogen and nitrogen concentrations. RESULTS
Calibration data obtained at two detector voltages are shown in Table I. The limiting sensitivities, which correspond to twice the base-line noise level for a 1-mg sample, are 0.02 wt pprn for hydrogen and 0.3 wt ppm for nitrogen. A summary of 11 analyses performed on two NBS standards is given in Table 11. T o test the effect of the residue of earlier samples left in the crucible, four specimens of 0.23-mm thick Type 316 stainless steel foil of unknown hydrogen and nitrogen content were analyzed by dropping them successively into the crucible. The data shown in Table I11 indicate no significant bias in the later results due to the presence of earlier specimens and also show the level of reproducibility that can be expected from this technique. This limited amount of data suggests therefore that the number of samples of steel that can be analyzed without changing the crucible is limited only by the capacity of the crucible, which in this case was about 30 mg. Using this micro-inert gas fusion technique, the nitrogen content was determined (13) at four positions along a Type 304 stainless steel safety rod guide thimble irradiated in the Experimental Breeder Reactor-I1 (EBR-11) for four years to a peak total neutron fluence of 8.8 X n/cm2. The results reveal a 35% depletion of nitrogen at the lower axial positions of the guide thimble with respect to the pre-irradiation concentration, and show a steady increase in nitrogen to pre-irradiation levels at the higher positions. The fact that these concentrations followed a temperature gradient along the thimble has important implications (13). If nitrogen moves about in a reactor core, its relatively high (n,a) cross section for fast neutrons gives it the potential to create the same sort of neutron capture and helium production (13) N. D. Dudey, S. D. Harkness, and H. Farrar IV, Nucl. Appl., 9, 700 (1970).
Table 11. Analyses of Hydrogen and Nitrogen in NBS Standards Specimen NBS Micro-inert-gas fusion results, wt. ppm mass value, Mean 1 u std Element wt PPm range, mg Values value dev Hydrogen 0.2-0.7 32 39, 32,29, 29, 34,28 32 4 Nitrogen 0.4-8.0 390 356,370, 388,430,476 404 50
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problems in fast reactors that boron creates in thermal reactors.
be applied to the determination of nitrogen in, for example, Ti, Ta, and W. Work is now in progress, however, to extend the technique to the determination of nitrogen in other materials including those which form refractory nitrides.
CONCLUSIONS Micro-inert gas fusion chromatography has been shown to be a very sensitive, accurate, and relatively precise technique for the determination of hydrogen and‘ nitrogen in small samples. The technique has been used in this laboratory t o determine hydrogen and nitrogen contents ranging from 1 t o 1000 wt ppm. At present the technique is limited t o samples where chemical decomposition is not required for the release of nitrogen or hydrogen, and therefore cannot
ACKNOWLEDGMENT The assistance of C. H . Knox is gratefully acknowledged. RECEIVED for review July 10, 1970. Accepted November 4, 1970. This work was supported in part by the U. S. Atomic Energy Commission.
Novel Method of Determining Weak Bases in Small Amounts Eli0 Scarano, Marco Mascini,’ and Giovanni Gay Institute of Analytical Chemistry, Faculty of Pharmacy, University of Genoa, Italy
CRITCHFIELD AND JOHNSON (1, 2) have found that concentrated (up to saturation or 8 M ) aqueous solutions of strong acid-strong base salts (NaCl, NaI, LiCI, CaC12) can be conveniently used as media for visual and potentiometric titrations of bases with pKb as high as 11-12. This is due to the enhanced potentiometric break at the equivalence point. The high protonation of weak bases is explained in terms of low activity of water and high hydrogen ion activity. Hisashi Kubota and Costanzo (3) have carried out potentiometric titrations of hydrolyzable cations in 10M LiCl. Rosenthal and Dwyer ( 4 ) have studied acid-base equilibria in concentrated salt solutions, particularly 4 and 8M LiCI. These authors also agree in the enhancement of the hydrogen ion activity in these media. Moreover dilute hydrogen chloride and concentrated lithium chloride solutions have high values of the hydrogen chloride gaseous pressure, which are much higher than that of hydrogen chloride aqueous solutions a t the same hydrogen chloride concentration ( 5 ) . So a n aqueous 0.1M HC1 solution can be boiled for 1 hour without appreciable loss of hydrogen chloride if the evaporated water is continuously replaced (6), while a solution -5 X 10-3M of hydrogen chloride in 13M LiCl loses 1 (or 50%) of hydrogen chloride if a nitrogen volume 6 (or 420) times that of the solution is forced to pass through it (5). In this paper a simple and inexpensive method is described for weak base (pKb up to 10) determinations in the pmole range. The method is based on the high hydrogen ion activity and the high hydrogen chloride pressure of dilute hydrogen chloride-concentrated lithium chloride solutions. A nitrogen flow forced through the hydrogen chloride-lithium
chloride solution strips the hydrogen chloride which is collected and determined. Two hours of stripping are sufficient to collect practically all the hydrogen chloride. The base is dissolved in a known amount of hydrogen chloride and introduced into the lithium chloride saturated solution. Only the excess of hydrogen chloride is stripped, thus allowing the base determination. EXPERIMENTAL C. Erba reagent grade LiCl (with 0.03%, as Li2C03,declared alkalinity) was used as purchased or in the form purified by crystallization. The acid-base substances [with pK, at 25 “C (7)] were: Merck (according to Sorensen) glycine (2.35); Merck reagent grade succinic acid (4.21); Merck reagent grade aniline (4.60); Merck p-toluidine (5.09); Aldrich Chemical picolinic acid (5.32); Merck sodium-5-5 ’-diethylbarbiturate, for buffer (7.98). The apparatus shown in Figure 1 consisted of two 15-ml glass tubes, with ground glass sockets; a glass bridge with caps for the tubes and holes for burets; two microburets (1 ml, 0.01-ml division) with the closure on the tip (8). Experiments were carried out in a thermostatic bath a t 25 “C. Saturated lithium chloride solution 13 grams (about 10 ml) and 1.2 grams of lithium chloride were weighed in the A tube; 2 ml of water and 0.1 ml of lO-*M methyl red solution were poured into tube B. The tubes were tightly joined together with the glass bridge. A buret filled with a standard hydrogen chloride solution was inserted in tube A ; a buret with 0.01 or 0.02M NaOH in tube B. A stream of nitrogen (120 + 5 ml/min) was then passed through the solutions into the tubes. The hydrogen chloride solution was poured into tube A . The stripped hydrogen chloride passed into tube B where it was titrated a t regular intervals of time with the sodium hydroxide solution. Preliminary investigation proved that hydrogen chloride did not escape from tube B.
Present address, Institute of Analytical Chemistry, University, of Rome, Italy. (1) F. E. Critchfield and J. B. Johnson, ANAL.CHEM., 30, 1247 (1958). ( 2 ) Ibid., 31, 570 (1959). (3) H. Kubota and D. A. Costanzo, ibid., 36, 2454 (1964). (4) D. Rosenthal and J. S . Dwyer, ibid., 35, 161 (1963). (5) D. Glietenberg and M. von Stackelberg, Ber. Bunsenges. Phys. Chem., 72, 565 (1968). (6) H. A. Laitinen, “Chemical Analysis,” McGraw-Hill, New York, N. Y., 1960, p 85.
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RESULTS AND DISCUSSION The influence of lithium chloride purity is shown in Figure 2. The presence of a base in the commercial product was ~
~
(7) V. E. Bower and R. G. Bates, “Handbook of Analytical
Chemistry,” L. Meites, Ed., 1st ed., McGraw-Hill, New York, N. Y., 1963, pp 1-20. (8) E. Scarano and M. Forina, J. Chem. Educ., 47,482 (1970).
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