Jan., 1962
CHEMISTRY OF TJRANIUM
IN
SURFACE IONIZATION SOIJRCES
133
THE CHEMISTRY OF URAXIUM I N SURFACE IONIZATIOX SOURCES’ Argonne National Laboratory,Argonne, Illinois Received August 14, 1061
Ions emitted from the surface of electrically heated filaments on which solutions of uranium had been evaporated were studied with a modified Bendix time-of-flight mass spectrometer. It wag demonstrated that by control of the oxidizing and redwing agents on a heated filnrnent and in the surrounding atmosphere the species of emitted ions can be controlled Oxidizing agents (e.g., oxygen)
uc2+, u +,uo +,UOa +,u03 4
.
+
Reducing agents (e.s., carbon)
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The gascous species UCI+ has been observed.
Introduction Surface ionization sources are used extensively in mass spectrometry for the isotopic analysis of elements in solid samples.2 Variation among individual mass determinations, which depend on sample treatment and technique of analysis, have emphasized the need for a better understanding of pertinent chemical effects. I n the isotopic analyses of small samples of uranium, especially with single filament sources,a bot$hthe species and intensity of ions have shown a high degree of variability. I n general it is preferable to do a uranium isotopic analysis on U + ions rather than on oxide ions to avoid the complications from isotopes of oxygen. Often only oxide ions have been obtained. Experimental Results and Discussion A Bendix time-of-flight mass spectrometer4was modified to permit the measurcmcnt of ions emittrd from the surface of elcctrically heated This instrument permits continuous monitoring of the entire mass spectrum. As modified, it has proved to be a useful tool for the study of high temperature reactions affecting the species and intensity of ions emittcd from heated surfaces. Xeutral molecules evaporating from the surfaces can be determined by using the electron gun of the spectrometer to ionize them. Single filament assemblies were made from 0.001 inch thick tungsten and rhenium ribbons. In the past tungsten was used most rxtcnsively in ion sources for production of U + ions. Since pure rhenium has become available, it has to a large extent replaced tungstcn because of its higher work function and its more desirable physical properties. After it was establishcd that the chemical effects of the two elements were similar, most of the experiments were done with rhenium. Small samples of uranium (10-9 to 10-6 g.) in dilute acid solutions were cvaporated to dryness on the filaments in air. After insertion of the filament into the ion source, the spectrornetcr was evacuated to less than 10” mm. As the temperature of the filament was gradually raised, the species of ioiis emitted from the surface were observed and thcir intensitics were measured. An equimolar standard of Uzs6-U2a was used in all experiments as an aid in identifying uranium bearing speries. It was demonstrated that by control of the oxidizing and rcduring agents on the heated filament and in the surrounding atmosphere the specics of emitted ions can be controlled as (1) Based on work performed under the auspices of the U.8. Atomic Energy Comniission. Presented in part a t the Ninth Annual hfeeting (June. 1981) of the A.S.T.M. Con~mitteeE14 011 I M R Spectrometry. ~ (2) M. G . Inghram and R. .J. Iirrydcn, “A ITandbook on hlnsa Spectroscopy,” National Rcsearch Council. Washington, D.C.,I’ublicntion 311, 1054. (3) L. A. Dietz. Rev. Sci. Insir., 80, 235 (1959). (4) D. 13. IIarrington. “Encyclopedia of Spectroscopy,” Reinhold Publ. Corp., New York, N. Y.. 1960, pp. 628-1347. ( 5 ) M 11. Etudier and E. G. Rnuh. to be publiahed.
Oxidizing agents (e.g., oxygen) ~~~
uc2+,u+,uo+,uo2+,
u03+-
c Reducing agents (e.g., carbon) When filaments were free of carbonaceous materials and the hydrocarbon content of the spectrometer vacuum system was low, only the ions of the higher oxides were observed. On several occasions only UOa+ ions were obtained with loss of the ent.ire sample below 1000”. Emission of U 0 3 +was enhanced by an air leak. When the hydrocarbon content of the vacuum system was high (observed by turning on the electron gun of the spectrometer), varying amounts of U +,UO and UOI+ wcre observed as the oxides were chemically reduced. An air leak increased the relative amount of oxide ions produced. A leak of benzene into the machine eliminatcd the oxide, and a sustained air leak was required before the oxide ions were observed again. Carbon was added to the filaments before placing them into the spectrometer. In some cases sucrose was added with the sample. In others, carbon was deposited by letting benzene vapors strike the heated filaments either before or after deposition of the sample. In general, when samples were treated in this way only metal ions wero observed. On occasion oxide ions could be observed very briefly during reduction. When sufficient carbon had been added it was difficult to produce the oxide ions again with an air leak. To control the amount of reducing material on the filaments each assembly first was cleaned of extraneous organic matter by boiling succcssively in carbon tetrachloride, acetone and alcohol. The fi1,ament then was baked out in a vacuum system by slowly raising the temperature to 2200” while the pressure was kept below 5 X 10- mm. Samples of uranium deposited on such filaments without further treatment yielded only the higher oxides when analyzed in a spectrometer with low hydrocarbon conttmt. Controlled amounts of carbon were deposited on the filaments by the following procedure: The filamrnt assembly was placed in a 300-cc. vessel with a side arm containing 50 microliters of benzene. The benzene was cooled with a liquid nitrogen bath and the vessel was evacuated to a pressure of less than lo-’ mm. The vessel was isolated from the vacuum systcm with a stopcock and the side arm containing the benzene was immersed in a Dry Ice-acetone bath. After allowing five minutes for the benzene vapors to come to equilibrium with the solid benzene in the Dry Ice bath, the filnments were heated for various periods of time a t different temperatures. Analyses were made on 0.1 microgram samples of uranium deposited on rhenium filaments which had been tieated with benzene vapors under different conditions. It was found that a few minutes of “carbonizing” before sample deposition a t temperaturcs between 1200 and 2200” woultl result in almost exclusive emission of U + ions during analysis. However, the degree of carbonizing had a marked effect on the temperatlure of metal ion emission. ( T h e filament temperature during carbonization seemed more critical than the duration.) Metal ions were observed a t temperatures below 1300”. A persistent metal ion beam wm obtained frequently a t temperatures as high as 2500”. As the temperature of one filament was raised slowly a metal ion beam appeared, passed through a maximum, disappeared completely during a range of temperature of 1M0, +
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and then reappeared and ptxrsisted a t much higher temperatures. As a general rule more extensive carbonizing results in metal ion emission a t higher temperatures. When filaments werc carbonized for 20 min. or longer a t 22OO0, no U + ions wire observed until the tcmperature exceeded about ?300°. Then UC2+ ions were observcd also but a t an intensity factor of 100 below that of the T7+ ions. It seems that metal ions may arise from several parent molecules. It is prot)al)le that they come from uranium metal (low temperature emission) and from one or morc refractory carbides (high temperature emission). Positive identification of UC2+ ions was made by mass determination and by resolution of the two uranium isotopes in the sample. I t is likely that UC and possibly U Z Cexist ~ on the filaments also, even though they were not observed as stable gaseous species.6 The tem erature a t which metal ions are emitted is consistent with t i e known physical properties of the metal and carbides.’ The oxide ions seem to have a variety of parents also. The trioxide may be present on the filament after evaporation of the sample and be lost a t relatively low temperatures. It is formed also from U02 by an air leak. As KO2 is reduced, UO+ ions are formed. In addition, UO+ ions are produced when air is leaked into a source which has been emitting metal ions only. Prolific emission of both L O + and UOz+ions may be obtained by such an air leak. In one experiment UOf ions first appeared. Then UOI+ ions appeared and grew to an intensity one hundred times that of the original metal ions. A t the same time the metal ion beam increased by a factor of five before i t decreased and eventiially disappeared. It is probable that the intense oxide ( 8 ) 11. A. Wilhelm, e t d . , J . Chem. SOC.(Suppl. 2), 5318 (1949). (7) 11. Etberington, Ed., “Suclear Engineering Handbook,” 1st Ed., McGraw-Hi11 Book Co., S e w York, N. Y., 1958.
beams produced by an air leak are associated with the burn ing of the highly reactive uranium carbides. Oxide ions produced in this manner can be observed a t temperatures far below that a t which metal ions can be produced from the same source in the absence of the leak. Preliminary experiments with 0.1 microgram samples of the alpha emitting P 3 suggest that well over WO/o of the sample may be lost beforc the temperature for maximum mrtal ion emission is reached. Apparently, the uranium is lost during reduction before the more refractory carbides are formed. Experiments arc under way to devise procedures to minimize these losses. Multiple filament assemblies will be used to investigate the problem. With a multiple filament it ehould be possible to separate some of the surface effects from the chemical effects. For example, the effect of the carbonizing procedure on the work function of the surf:ace is unknon~ibut is susceptible to study by the multiple filament technique.
Conclusions Single filament ion sources of tungsten or rhenium will not produce uranium metal ions in the absence of reducing agents other than the filament metals. Deposition of carbon before, during, or after deposition of the sample will ensure subsequent production of metal ions to the exclusion of oxide ions uiiless the pressure is high. It is probable that the success of single filament sources in the analysis of uranium as metal ions is due to the forination of refractory carbides which decompose at high temperatures.
POLAROGRAPHY OF SOME AIETAI, COhIPLEXES WITII TltIETHYLENETETR.~iCIISE:lISE BY E. JACOBSEN A X D K. SCHR$DER Ihpartment of Chemistry, Universzty of Oslo, Blindern, .Torway Receiaed August 17, 1861
The complexcs of cadmium, copper, lead, zinc, nickel and cobdt with triethylenetctrarnine have bwn sitmlird by means any of the dropping mercury electrode. The cadmium, copper and lead complex are reversibly redurcbd to the :imalgam :LI concentration of the reagent. In alkaline medium the divalent cobalt complex is emily oxidized by air to a red-colored trivalent cobalt complex. The nickel, zinc and trivalent cobalt complex showed an irreversible rcduction at the I1.M.IC. The half-wave potentials and the diffusion current constants of the various complexes are given.
Introduction Triethyleiietet~ramir~e (abbreviated “trien”) forms stable complexes with certain metals. The composition and stability of the complexes have been determined by poterit,iomet.ric titrat,ion. The cadrnum complex also has been studied by means of t’hc dropping mercury electrode by Douglas, ct n1.3 They clairn t,hat, the complex is irreversibly reduced at t,rien concentrations less than 0.01 ;If. More recently t,he polarography of thc copper complex has been thoroughly investigated by Joriassen and c o - ~ o r k e r s . ~ I-Iit,hert,o,the polarographic behavior of the complexes formed with ot,her metals has not been invest’igated. The present paper is an ext.ensive study of the polarography of the cadmium, l a d , (1) G. Scbwarzenbach, Helo. China. Acto, 33, 974 (1950). (2) C. N. Reilley and R. W. Schmid, J . Elisha Mitchell SOC.,73, 279
(1957). (3) B. E. Douglaa, H. A. Laitinen and J . C. Bailar, SOC., l a , 2484 (1950).
J. A m . Chem.
(4) IT. B. Jonassen, J. A. Bertrand, F. R. Groves and R. I. Stewns, ibid., 79, 4279 (1957).
copper, zinc, nickel mid cobalt complcxes formed with trien. Experimental Materials---The technical grade tric,thSlenctetramin~, obtained from Fluka A. G., Sxitzerland, was purified as described by Reilley and Sehmid.2 The remaining chemicals were reagent grade and used without further purification. Approximately 0.1 M stock solutions of the metal salts were prepared by dissolving 0.1 male of the corresponding salt in redistilled water and diluting to one litrr. The stock solutions werr standardmed by complexomctric titration with EDTA. Standard solutions of trien Rwe prepared and standardized following the procedure given by Reilley and Sheldon.6 Five-trnths molar phosphate buffer was used a8 supporting electrolyte. The buffrr was prepared by adding potassium hydroxide to phosphoric acid, and its pH meaeured m t h a p€I metrr. In order to avoid precipitation of metal phosphates, 1 3.l ammonia buffer was uscd as indiffcrent electrolyte for the lead, zinc and nickel complexes. The pII of the electrolyte was adjusted to the desired value by adding hydrochloric acid to the ammonia solution. Triton X-100, abtained from Rohm and IIaas Co., Philadelphia, was found to be effrctivc as maximum suppressor. Apparatus and Technique.-Polarograms were recorded (3) C . X. Reilloy and &I. 1’. Sheldon, Talanta, 1, 127 (1958)
