Analytical Chemistry o f Fused Media
I 11th Annual Summer Ssmposiam, ACS Division of
c,t2E:f;i s~Gt
nectady, N. Y., Jane 1958 COCHAIRMEN: lemonf, 111.
N. H. Nachtrieb, University o f Chicago, Chicago, Ill., and D. M. GRUEN, Argonne National laborafory,
Kinetics and Mechanism of the Bichromate-Bromide Reaction to Yield Bromine in Fused Nitrates. F. R. Duke and M. L. Iverson, Deparfment o f Chemistry, Iowa Stafe College, Ames, Iowa. Techniques for the Study of Metal-Metal Salt Solutions. S. J. Yosim, Atomics International, Canoga Park, Calif. Spectroscopy in Rigid Glassed Solvents. E. W. Abrahamson, Stafe University o f New York, College o f Forestry, Syracuse,
N. Y. Constitution of Molten Salt Mixtures, Mercuric Bromide-Alkali Metal Bromides. G. J. Janz, J. Goodkin, and J. D. Mclntyre, Rensselaer Polyfechnic Institute, Troy, N. Y. Application of Electron Paramagnetic Resonance Methods to Fused Salts. B. R. Sundheim, New York Universify, New York, N. Y. Problems Imposed b y Changes i n Temperature and Solvent Composition on the Spectrophotometric Analysis of Fused Salts. G. P. Smith, Oak Ridge National laborafory, Oak Ridge, Tenn. Electrode Processes in Fused Salts, J. O'M. Brockris and J. Barton, University o f Pennsylvania, Philadelphia 4, Pa. Spectrophotometry Applied to the Analysis of Fused Salts, D. M. Gruen, Argonne National laborafory, lemont, 111. Complexing of Tantalum Pentachloride by Chloride Ion in Fused Media. C. M. Cook, Jr., E. 1. du Pont de Nemours & Co., Wilmington 98, Del. Some Separation Processes from Molten Fluoride Solutions. W. R. Grimes, Oak Ridge National laborafory, Oak Ridge, Tenn. Investigations of the Silver-Silver Chloride Reference Electrode i n the lithium Chloride-Potassium Chloride Eutectic Melt. Ling Yang and R. G. Hudson, Carnegie Institute o f Technology, Pittsburgh 13, Pa. Fused Salt Polarography Using a Dropping Bismuth Electrode. J. J. Egan and R. J. Heus, Brookhaven National Laboratory, Upton, I. I., N. Y. Reference Electrodes for Molten Salts. Seymour Senderoff, G. W. Mellors, and E. R. Van Artsdalen, National Carbon Co., Cleveland 7, Ohio.
**I m
1
~
,
1
A summary of the symposium prepared b y N. H. Nachtrieb i s given in place of publishing the complete papers as in former years. For the record we list the authors and their addresses together with the titles of their papers.
T
11th Annual Summer Syniposium of the Division of Analytical Chemistry was held a t the Research Laboratories, General Electric Co., Schenectady, N. Y., June 19 to 21, 1958. Cohost of the symposium was the Department of Chemistry of Union College. Over 100 chemists attended the 21/2day meeting and actively participated in discussion of the 13 papers presented. F. R. Duke spoke on the kinetics and mechanism of the bichromate-bromide reaction to yield bromine in fused nitrates. I n fused sodium nitrate, bichromate ion appears to behave as a Lewis acid, forming nitronium ion from nitrate ion by the reaction: HE
Cr2&-2
+ NO3-
-$2Cr04+
+ SO*+
Subsequently, nitroniuni ion oxidizes bromide ion to bromide, and is itself reduced to nitrogen dioxide: SOz+
+ Br-
-+
S02Br
-+
SO2
+ Br
The rate of decrease in bichromate ion concentration is first-order in nitro1892
a
ANALYTICAL CHEMISTRY
nium ion. Chromate ion is removed by lead ion to form insoluble lead chromate, and the rate of decrease in bichromate ion concentration is therefore governed by the solubility product constant of lead chromate. The presence of chloride or bromide alters the lead ion concentration by forming PbC1+ or PbBr' complexes, and the altered rate of bichromate ion consumption may be used to calculate their instability constants. Because the over-all kinetics for the oxidation of bromide ion includes the equilibrium constant for the production of nitronium ion, the strength of bichromate ion as a Le& acid is implicit in the reaction rate. The possibility of establishing a relative acidity scale for various Lewis acids-e.g., Zn+2, S207-2, etc.-on the basis of the rate of bromide oxidation was proposed by the speaker. S. J. Yosini discussed various techniques which have been used for the study of metal-metal salt solutions, such as Cd-CdClp, Ag-AgC1, Bi-BiCl3, Ba-BaC12, and Hg-HgClz. These included measurements of the vapor pressure of a volatile component, cell e.m.f. methods, electrical conductivity, transference number determinations, cryoscopic studies, and (in the case of bismuth) static diamagnetic susceptibility. The state of the dissolved metal is of particular interest-Le., the extent of its ionization and the possibility of partially electronic conduction mechanisms.
E. IT. Abrahamson presented a general paper on spectroscopy in rigid glassed solvents. He discussed various processes, according to which photochemically excited molecules may lose their excitation energy. Several kinds of bimolecular reactions with the solvent are possible, as n-ell as unimolecular reactions in n-hich radiation may accompany internal rearrangement. Photodissociation and predissociation mechanisms aere also described. G. J. Janz, in R paper coauthored m-ith J. Goodkin and J. D. McIntyre, summarized their work on the constitution of molten solutions of mercuric bromide with alkali bromides. Cryoscopic measurements, conductance measurements, and determinations of viscosity were reported. From these equilibrium and transport studies mercuric bromide is regarded as ionizing according to the reaction: HgBrl
% HgBr+
+ Br-
Calculations of the shielding efficiency of mercuric ion suggest the unlikelihood that HgBr+ is appreciably dissociated. On the other hand, anions such as HgBr3- appear probable, and may arise either by the reaction: HgBrl
+ Br-
$ HgBrs-
or by disproportionation: 2 HgBrz -$ HgBr+
+ HgBrs-
Benson Sundheim discussed the pos-
sibilities of applying elcctron paramagnetic resonance methods to fused salts. He pointed out the potentially high sensitivity of the method for detecting ions and molecules having electrons with unpaired spins. He discussed various factors which may lead to broadening of the resonance absorption --T-iz., spin-spin interaction, spin-lattice coupling, electron exchange, and anisotropy of the local electric fields surrounding the ion. Several applications \yere mentioned. The resonance absorption which is observed in ferric thiocyanate and manganese thiocyanate in the solid state is destroyed when these salts are melted, and changes in the electronic spin state of the central metal ion were deduced. It was also reported by Sundheim that ferric chloride in molten lithium chloride-potassium chloridc exists in the form of the FeC14- ion. G. P. Smith reported on the effects of temperature and composition on the absorption spectra of ions in molten salts. Using spectrophotometric techniques in which absorbances as high as 5 can be measured, he has found that alkali cations shift the forbidden absorption band of so,- (-313 mp) in fused lithium chloride-potassium chloride. There is a progressive displacement of the nitrate ion band to longer wave lengths in the order: Csf
> Rb+ > Na+
Simultaneously, the band broadens and the molar extinction coefficient a t the maximum decreases. From the area under the peak the integrated optical cross section for was evaluated. Varying the temperature from 273 O to 367" alters the area under the band about 0.1%. Chloride ion appears to enhance the absorption of nitrate ion, the electronic transition being normally a forbidden one. J. Barton described some of the studies by J. O'N. Bockris and his collaborators (Inman and Menzies) on electrode processes occurring in electrolysis cells n ith fused electrolytes. I n the deposition of uranium metal from lithium chloride-potassium chloride with metallic uranium anodes there appears to be a two-stage reduction process. Uranium dissolves a t the anode as U+3. Measurements of the cathode potential ivere observed to rise a t first (to -1.0 volt versus silver-silver chloride reference electrode) and then to decrease as the reduction of dissolved uranium to metallic uranium began, The addition of U+3 to the electrolyte shortened the transition time. It was suggested by the speaker that the reduction of U+3 t o U proceeds in two steps, the first being the. reduction to U+* by dissolved lithium. The subsequent reduction of U t 2 a t the cathode is believed to occur with lo~v over-
voltage. A two-stage process is also believed to operate in the deposition of spongy titanium from titanium chloride dissolved in lithium chloridepotassium chloride. Polarized electrodes revealed a one-electron reduction of Ti+3, but no wave for the reduction of T i f 2 was observed below the decomposition potential of the solvent. Titanium tetrachloride in lithium chloride-potassium chloride showed a tm-0electron reduction to Ti+*. Dieter M. Gruen discussed spectrophotometry applied to the analyis of fused salts, and illustrated his lccture with colored slides shoir ing the characteristic colors of different oxidation states of uranium dissolved in lithium chloride-potassium chloride. U 0 2 + 2is yellow, -C"" is green, and U"' is burgundy red. After describing the experimental techniques for preparing anhydrous lithium chloride-potassium chloride by filtration and storing it under carbon tetrachloride, Gruen outlined the details of a modified Beckman DU spectrophotometer for absorption measurements up to 850". U" in fused pyridinium chloride (melting point 144" C.) shows 16 absorption peaks, and the similarity of the spectrum to that of crystalline uranium tetrachloride suggests similar (sixfold) coordination of the U+4 ion in the molten salt. I n lithium chloridepotassium chloride the spectrum is again similar, but the bands are broadened and washed out as they are in water. It was suggested that U+4 in both fused lithium chloride-potassium chloride and water may have a coordination number of 8. U+3 in the lithium chloride-potassium chloride eutectic is colored burgundy red, as it is in concentrated hydrochloric acid. Seventeen absorption bands are possessed by the ion, but not all are resolved. The uranyl ion in lithium chloride-potassium chloride shows about four poorly resolved bands. Turning to other systems, Gruen discussed the influence of the solvent cation radius on the structure of the solute complex. It seems likely that nickel chloride in fused lithium chloride a t 700" has sixfold coordination. I n cesium chloride, hon-ever, the effect of the larger cation is probably to diminish the coordination number of Xicz to 4, and the structure of SiC14-2 is presumed to be tetrahedral. Chloride ion enhances the absorption of both ~ molten nitrates and N i f 2 and C O + in shifts the absorption to longer wave lengths. Octahedral coordination is preserved in both cases. Charles 11. Cook, Jr., discussed the complexing of tantalum pentachloride by chloride ion in molten NaFeClr over the temperature range 300" to 400". The method employed was the determination of the vapor pressure
of tantalum pentachloride as a function of the concentration of both tantalum chloride and escess sodium chloride in the melt. Several possible equilibria were considered and excluded on the basis of the experimental results, and the data were shonn to be consistent with the coniplexing reaction: TaClj
+ C1-
TaCIG-
At 300" the logarithm of the equilibrium constant, loglo
(xTaC1s-) UTaCls XCl-
, has the value
2.4 = 0.3. At 400" the value is 1.7 f 0.3. Warren Grimes discussed some of the problems associated with the use of molten salts containing dissolved uranium tetrafluoride as fuels in nuclear reactors. Several binary and ternary molten fluoride systems were discussed, and the limitations imposed by the requirements of low neutron cross section n-ere pointed out. For example, the cross sections of sodiuni and potassium in the ternary melt, XaF-KF-LiF, are too high, although the NaF-ZrF4 binary s h o m promise as the fuel solvent for an aircraft reactor. Grimes turned his attention to the molten LiF-BeF2 system, and described n-ork v,hich has been carried out a t the Oak Ridge Kational Laboratory on the removal of fission products from spent liquid fuel melts. The fission products range from zinc to barium in the periodic table, and many have uncertain valence states or may undergo secondary reactions with the containing metal system. An extensive study was made of the solubilities of the noble gases in the NaF-ZrF4 melt as a function of temperature, since krypton and senon are among the fission products ivhich must be removed. The rare gas solubilities in molten SaF-ZrF4 decrease in the order He > A > Xe, etc., and increase with increasing temperature. The radioactive rare gases were removed by purging the molten salt with helium. Uranium is recovered b? oxidation of UF4 t o cFs with either fluorine or bromine trifluoride, and the UFs is recovered after its volatilization from the melt. Rare earth fission product fluorides are soluble to the extent of several mole per cent in SaF-ZrF4, and decrease in the order YF3 > SmF3 > CeF3. Advantage is taken of the formation of mixed crystals n hich form nhen inactive cerous fluoride is added to the melt. Thus, LaF3 i q extracted from the melt by precipitation as the solid solution, LaF3-CeF3. Ling Yang and R. G. Hudson presented the results of their studies on the silversilver chloride reference electrode in the lithium chloride-potassium chloride eutectic melt. They revieived the reference electrodes which have been employed by various investigators, noVOL. 30, NO. 12, DECEMBER 1958
* 1893
tably the platinum-platinous electrode (Laitinen and Liu), and various forms of the silver-silver chloride electrode used by such workers as Senderoff, Flengas, and Bockris. Junction potentials amounting to 20 mv. or more map arise n-ith e.m.f. cells which contain porous diaphragms. The measurement of these junction potentials Kas undertaken by the comparison of the e.m.f.'s of cells with and Tvithout transference. Fused salt polarography in lithium chloride-potassium chloride with a dropping bismuth electrode was the subject of the paper by J. J. Egan and R. J. Heus. After describing their method for filtering the eutectic and refining the bismuth, Egan discussed several forms of the dropping bismuth electrode and the silver-silver chloride reference electrode found to be most satisfactory. Because of the rapid drop time (less than 1 second) the currents observed are not diffusion-limited, Yevertheless, well-defined waves were observed for lead chloride and zinc chloride, in lyhich the current is proportional to the reducible ion concentration. Diffusion coefficients for lead and zinc ions in the lithium chloride-
potassium chloride eutectic are about five times too large, as calculated from the IlkoviE equation, because of the short drop time. The probability of successfully determining aluminum in the eutectic melt was mentioned. In the final paper of the symposium by Seymour Senderoff, G. W. Mellors, and E. R. VanArtsdalen, Senderoff revietved the criteria n hich must be met for reference electrodes in molten salts if the results of cell measurements are to have thermodynamic validity: reversibility evidenced by the ability of a cell to recover its orisinal e.m.f. after a rather large current has been passed, conformity to the Nernst equation, and the reversibility of both electrodes. The latter is necessary if the free energy of the cell reaction is to be obtained from the e.m.f. Various reference electrodes (gas, metal-metal ion, and amalgam electrodes) were discussed from the standpoint of their reversibility, stability, and range of useful performance. The conventional chlorine-graphite electrode was reported to be reproducible up to 660" and to be nonreproducible a t 860". The troublesome polarization may be eliminated by the uqe of a
porous graphite electrode which extends the range of reproducibility of the chlorine-graphite electrode to 950". It is prepared by heating graphite in chlorine to 2300" C. Several designs for the silver-silver chloride electrode were described. Attention was called to the fact that it is very noble, as compared with most couples, and not greatly affected by impurities which may diffuse into it. Senderoff concluded his presentation with a discussion of a cerium-cerous chloride electrode. Cerium metal dissolves to the extent of about 16 mole % in cerous chloride, and is therefore alloyed with tin (CeSn3) to reduce the activity of the cerium. The activity of cerium in the alloy is determined by measurement of the e.m.f. of two cells: CeSnr/hICl, CeC13(dilute)/Clt and Ce/CeC4/CeSna
E". the standard electrode potential of the cerium-cerous couple, is calculated from the e.m.f. of the first cell from knowledge of the activity of cerium in the alloy and the concentration of cerous chloride in the electrolyte.
X-Ray Spectrophotographic Determination of Tantalum, Niobium, Iron, and Titanium Oxide Mixtures Using Simple Arithmetic Corrections for Interelement Effects B. J. MITCHELL Research and Developmenf Analytical laboratory, Elecfro Mefallurgical Co., Division o f Union Carbide Corp., Niagara Falls, N. Y.
,A general method of arithmetic corrections for absorption and enhancement interelement effects has been developed and applied specifically to the quantitative determination of mixtures of tantalum, niobium, iron, and titanium oxides. The combined oxides of these elements are prepared chemically from liquid, powder, or metallic samples; weight percentages are determined b y reference to calibration and intensity correction curves relating fluorescent intensity to concentration and sample matrix. Tantalum, niobium, iron, or titanium oxide from 0.05 to 100% concentration can b e determined with a reproducibility to 1% of the amount present in the 10 to 100% concentration range; 5% of the amount present in the 1 to 10% range; and 10% o f the amount present in the 0.05 to 1% range. X-ray analytical time re-
1894
0
ANALYTICAL CHEMISTRY
quired averages less than 15 minutes per determination. The corrections which have been established are generally applicable to combinations of these four oxides which may b e obtained by chemical treatment from such materials as columbite or tantalite ores, tantalum or niobium metals, and titanium alloys.
T
the accurate and rapid analysis of numerous samples for tantalum and niobium (columbium) in widely varying concentrations necessitated a survey of available methods. Chemical determination of tantalum and niobium is tedious and time-consuming. Optical spectrographic analysis generally is limited to the measurement of trace or small quantities. Various published articles (1-5, 10) as m-ell as exploratory semiquantitative work which has been done in this laboratory on a HE DEMAND FOR
X'orelco x-ray spectrograph indicated the speed and applicability of x-ray techniques in the determination of tantalum and niobium present as either trace or major sample constituents. Although the major analytical problem was the development of quantitative x-ray spectrographic methods for the determination of tantalum and niobium, iron and titanium were also to be determined in samples containing these elements plus many others in wide ranges of concentration. Analyses of liquid samples, columbite and tantalite ores, and tantalum and niobium metals were required. The development of a method of x-ray analysis for materials of such widely varying composition and physical state can be simplified by preparing samples of known matrix. Therefore, it was decided to make a prior chemical separation of the combined oxides of tan-
