Isotopic Analysis of Gaseous Boron Hydrides by Neutron Absorption ROBERT
P. HAMLEN and WALTER
Department
S. KOSKl
o f Chemistry, The Johns Hopkinr
University, Baltimore 18,
Pentaborane (B6Hg)was prepared by pyrolysis of diborane a t 180" C. (1). Pentaboranes of various boron-10 content were prepared by pyrolysis of diborane having the appropriate enrichment.
4n isotopic boron analysis of diborane and pentaborane containing varying amounts of boron-10 has been made using a neutron absorption technique. The gas was placed in a cell, the inside of which was coated with a silver-activated zinc sulfide phosphor, and exposed to a neutron flux. The number of events from the B1'J(n,a)Li7reaction was a measure of the boron-10 content. The accuracy and reproducibility of the results agreed to within 1% with mass spectroscopic analysis of the same materials. Preliminary results indicate the method is applicable to other gaseous boron hydrides such as dihydropentaborane and tetraborane.
APPARATUS AND PROCEDURE
In applying the scintillation technique to boron-10 determination, the gas was placed in a shallow aluminum cone with an exit tube a t the apex. The bottom of the cone consisted of a quartz window 2 inches in diameter, the inner surface of which was coated with a silver-activated zinc sulfide phosphor to a thickness of 3.4 mg. per sq. em. The inside of the cone was covered with the same phosphor to approximately the same thickness. The volume of this cell was i ml. This cell was then coupled with a RCA 5819 photomultiplier *be and surrounded with 9 cm. of paraffin for thermalizing the neutrons. The neutron source was a polonium-beryllium source emitting 1 X 10' neutrons per second. The output of the photomultiplier was amplified with an Atomic Instrument Co. Model 204C linear amplifier. The pulses \\.ere then scaled and recorded with a Model 1030A scaler obtained from the same company.
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OR the past several years isotopic exchange reactions involving the boron hydrides have been studied in this laboratory, and neutron absorption has been utilized as a means of determining the total boron-10 content of various gaseous boron compounds. This paper describes the details of experiments with diborane and pentaborane. The method has also been extended to tetraborane and dihydropentaborane. I n handling small amounts of these gases, it was found that satisfactory results could be obtained by putting the gas in a proportional counter and electronically detecting the effects of the alpha particles from the reaction BlO(n,a)Li'. More conveniently, scintillation techniques were utilized by permitting the reaction to occur in a cell, the inner surfaces of which were coated with a silver-activated zinc sulfide phosphor.
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MATERIALS
Diborane of normal isotopic content was prepared by reaction of lithium aluminum hydride (LiAlH,) with boron trifluoride ethyl ether and it was purified by the method outlined by Shapiro, Weiss, Skolnik, and Smith ( 2 ) . Diborane containing 96% boron10 was made by the same procedure. The etherate was made from B1oFaobtained by heating B'OF,. CaF?, which was purchased from the U. S. Atomic Energy Commission. Diborane having an intermediate boron-10 content was prepared by diluting the enriched compound with diborane of normal isotopic content.
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I
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COUNTS PER M I N U T E PER CM. PRESSURE
Figure 2. Variation of counting rate with boron-10 isotopic content in diborane and pentaborane
The counting rate was measured with and without the gas in the cell and the difference Jyas taken as a measure of the boron-10 content. The counting rate did not increase linearly with the pressure of the gas in the cell; consequently, all counting was done a t approximately the same pressures of 4 and 10.9 cm. of pentaborane and diborane, respectively. Corrections were applied for small pressure differences. The calibration curves were obtained by comparing the counting data with the boron-10 content as obtained with a General Electric mass spectrometer. The spectra were scanned magnetically using a 2000-volt ion-accelerating potential and a 50-volt electron-accelerating potential. DISCUSSION-
G R A M A T O M S O F BORON x 1 0 5
Figure 1. Variation of counting rate with gas pressure of diborane and pentaborane
I n attempting to apply the nuclear reaction B1a(n,~)Li7 to an analytical scheme for determining the boron isotopic content in boron hydrides, three methods were considered. The first and most convenient consisted of a cell, the inside of which was coated Tvith a thin layer of silver-activated zinc sulfide phosphor. The number of scintillations, produced in the phosphor by the alpha particles emitted as a result of the nuclear reactions produced by the neutrons, was a measure of the boron-10 content. The second method consisted of putting the boron hydride gas inside a proportional counter and recording the effects produced by the alpha particles resulting from the nuclear reactions. This method could be applied to the analysis, but it was not as trouble-free as the first method. It suffered from the disad-
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ANALYTICAL CHEMISTRY
vantage that the properties of such a chamber can be appreciably altered by small amounts of impurities in the gas filling. For example, the size of the plateau or its position as far as voltage is concerned varied with impurities. At times the plateau was practically nonexistent. Often, trace amounts of impurities cannot be controlled without a considerable amount of effort. For this reason, the use of this approach was very limited. The third method, which was considered and used to an even more limited extent, was essentially one used by Walker ( 3 ) . It consisted of placing a cylindrical cell of boron-containing material in solution, suspension, or gas around a neutron counter and measuring the neutron attenuation produced by the boron. This approach has not been used very much for isotopic analysis, mainly because rather large samples are necessary if reasonable accuracy is desired. Therefore, comments in this discussion are confined to the scintillation method of analysis, Figure 1 gives the counting rate us. the pressure of the boron hydride in an earlier scintillation cell of 21-ml. volume. This variation is not linear, especially a t higher pressures. Spot checks showed that the newer cell of 7-ml. volume described above exhibited the same behavi8 presumably because of the absorption of the alpha particles by the gas in the cell. I n routine analyses, pressures of about 11 cm. for diborane and 4 em. for pentaborane have been used. -4plot of the counting rate us. boron-10 content is given in Figure 2. In this calibration curve the isotopic content was taken from mass spectroscopic measurements. The plot is linear over the range of interest and, in many instances, if the counting rate was measured a t 18.8 and 96% boron-10, the straight line drawn through these points gave very satisfactory results for boron hydrides containing intermediate isotopic concentrations. I t appears from experience to date that the boron-10 content can be determined by this method to within 1%. The total amount
of boron required for the analysis was about 1 mg. In checking the reproducibility of results a number of different fillings and measurements were made on penta- and diborane and all results checked to within 1%. Most of the measurements were routinely made on diborane and pentaborane; however, some have been made on dihydropentaborane and tetraborane. Although the results indicated that this method of analysis was also applicable to the latter gases, experience with these materials has not been sufficiently extensive to quote the data a t this time. ACKNOWLEDGMENT
This research was supported by the United States Air Force through the Office of Scientific Research, Air Research and Development Command. One of the authors (R.P.H.) wishes to acknowledge the support of the H. A. B. Dunning Fellowehip throughout the 4 years of his graduate research. The authors also wish to acknowledge their appreciation to Lewis Friedman, Brookhaven National Laboratory, and Harold C. Mattraw, Knolls Atomic Power Laboratory, for the mass spectroscopic measurements. LITERATURE CITED
(1) Koski, W. S., Maybury, P. C., Kaufman, J. J., ANAL.CREM.26, 1992 (1954). (2) Shapiro, I., Weiss, H. G., Skolnik, S., Smith, G. B., J. Am. Chem. Soc. 74, 901 (1952). (3) Walker, R., Manhattan District, Declassified Document, MDDC 362 (1940). RECEIVED for review March 22, 1956. Accepted M a y 22, 1956. Division of Physical and Inorganic Chemistry, 129th .Meeting, ACS, Dallas, Tex., April 195G.
Potassium Determination with the X-Ray Spectrograph L. B. GULBRANSEN Mechanical Engineering Department, Washington Universsity, St. Louis, Ma.
A procedure has been developed using the x-ray spectrograph, in which potassium K radiation is used for the quantitative analysis of potash concentrates and tailings. Two working curves were established by a series of standard samples over the ranges 0.8 to 18.2Yo and 57.8 to 60.5Yo potassium oxide. X-ray spectrographic results are in agreement with chloroplatinate chemical analyses to within 0.4% potassium oxide. The total time for analysis, after the samples have been ground, is 6 minutes for tailings samples and 3 minutes for concentrates.
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ETERMINATIOS of potassium or potassium oxide in potash concentrates and tailings by chemical methods involves a number of separation steps, and these methods, although reliable, may not be suitable for control of plant processes because of the rather long analysis times involved. The x-ray spectrograph can be used to reduce the time of analysis of potash concentrates to about 3 minutes, and potash tailings to about 6 minutes, once a R-orking curve has been established. APPARATUS
A S o r t h American Philips x-ray spectrograph with a tungsten x-ray tube and sodium chloride analyzing crystal was used
through the analytical work. The following operating conditions were used. Target Analyzing crystal X-ray tube operated at Operation of scaler Scale factor, potash tails Scale factor, potash concentrates Geiger tube operated at Analysis line
Tungsten Sodium chloride 50 kv., 40 ma. Fixed count 16 32 1500 volts
Potassium K , 83.07' (2 0 )
The long wave-length K radiation of potassium is strongly absorbed by air, so that the x-ray path had to be enclosed in a plastic bag and flooded with a gas of low atomic number. In this work helium gas was used. In setting up the spectrograph for work in a helium atmosphere, it was found that the helium pressure, or rate of flow of helium through the system, had a marked effect on the counting rate for potassium K radiation. Higher pressures and more rapid rates of flow of helium in the chamber increased the counting rate appreciably. Because of the irregular shape of the Geiger tube, holder, and analyzing crystal, it was difficult to construct a soft plastic bag that was completely gas tight. However, it was simple to construct a soft plastic bag around the system, using a plastic tape, with a slight
