NOTES
690
solution depletion by the solute-mercury reaction, since the current, in contrast with the current function, i/Ch‘/z, actually increases a t each nominally greater concentration (nominal concentration equals the “as-prepared” concentration), ie., while the true solute concentration is less than the nominal one due to depletion, the former, as indicated by the current, is in all cases greater than the last preceding nominal concentration. ;Ilthough the solute-mercury reaction may so interfere with the normal chemical processes a t the reference electrode surface as to produce the potential anomaly, the potential changes observed are more likely part of a more general pattern of behavior for the calomel electrode in sulfur dioxide, first noted 11y Cruse,*mho studied the cell Ag/AgCl/SOz, EtzKHzCl/Hg,Clz/Hg
( 11)
in which the calomel electrode acts as a cathode. For the first few hours, the potential remained fairly constant at 50 mv., which is close to the value (8) K. Cruse, 2. Elektrochem., 46, 571 (1940).
Vol. G5
calculated from thermal data; subsequently, it increased over a period of 12 hr. to a new and fairly constant value of 280 mv. This aging effect was observed in other cells, using either the calomel or Hg-Hg2Br2 electrode, but not in cells in which the Ag-AgC1 electrode was used with, e.g., a hydrogen electrode. Cruse concluded that the aging is characteristic of mercury-mercurous halide electrodes in sulfur dioxide, and that such electrodes are not suitable for measurements of more than a few hours duration. The potential change found in the present study, in which the calomel electrode functions as an anode, is logically in the opposite sense to Cruse’s observation. Experimental The material, apparatus and procedures used have been described.’
Acknowledgment.-The authors wish to thank the U. S. Atomic Energy Commission which helped support the work described.
NOTES DISSOCIATIOK OF MOLYBDENJM(V) CHLORIDE I N CARBON TETRACHLORIDE SOLUTION’ BY IRVING 31.PEAR SON^
AND
CLIFFORDS. GARNER
Department of Chemistry, Cninersify of California, Los Angeles 8.4, California Received A p r i l 11 1960 ~
Xolybdenum(V) chloride is a blue-black dimeric3&solid having normal melting and boiling points3b of 194 and 2G8”, respectively. The dark red-brown vapor apparently consists of MoC16 trigonal b i p y r a m i d ~ ,easily ~ decomposed to lower chlorides and Clz under appropriate conditions. Many studies of MoCI5 require specialized handling techniques because of the extreme sensitivity to moist,ure and oxygen, which form oxychlorides and, respectively, HC1 and C12. We give here observations on the hit,herto unreported solubility and dissociation of MoC15 in CCh solution, as well as on the behavior of MoC15 in vacuum distillation and the removal of oxychloride contaminants from MoC16. Experimental Argon, Hydrogen, Nitrogen.-Tank Ar ( I h d e ) , oilpumped tank Nz (Liquid Carbonic Co.) and electrolytic grade tank H, (Liquid Carbonic Co.) were each passed through Drierite and Ascarite, then through finely-divided Lr metal a t cu. 800” (Ar, AT,) or ca. 600” (H2). (1) Supported by U. S . Atomic Energy Commission under Contract AT(ll-1)-34,Project 12. (2) Electronics Division. The National Cash Register Company. Ilawthorne, California. (3) f a ) D. E . Sands and A. Zalkin, Acta Cryst., 12, 723 (1959); (b) H. Dehray, Compt. rend.. 66, 732 (1868). (4) 11. T’. G . E w e n i and 11.W.Lister, T r a n s . Faraday S o c . , 34, 1358 (1938).
Chlorine, Hydrogen Chloride.-Matheson tank C1, was purified by the method of Downs and Johnson.5 IIatheson tank HC1 was dried by passage through Mg(C104)Z. Carbon Tetrachloride.-J. T. Baker “Analyzed” CC1, was ourified (76.8” normal b.o.i essentiallv “ bv “ the method of W‘allace and Willard.6 Molvbdenum(II1) Chloride.-MoCL (Climax Molvbd e n u k Co.) was boiled with 12 f HCi, then n-ashed i i t h absolute ethanol and dried in a vacuum desiccator. Molybdenum(V) Chloride.-For most of these studies MoC15 was synthesized7,*from purified C12 and %Io metal (obtained by reduction of J. T. Baker “Analyzed” Moo3 with purified Hz at 1000°,followed by heating in anhydrous HC1 to remove possible oxide impurities). Climax Molybdenum Co. MoC15 (special lots selected for large lump size) was used in spectrophotometric studies of the dissociation. MoClj prepared from MooBand CC1, a t 4OOo9 was supplied by Professor S. Y. Tyree, Jr., and used in oxychlorideremoval studies with the same general results as our own MOC15. Removal of Oxychloride Contaminants from Molybdenum(V) Chloride.-MoC16 from the above sources generally had brown and green oxychloride coatings. \+’hen coatings were removed mechanically in the best dry boxes available to us new coatings formed rapidly. Distillation in a C1, atmosphere appeared to distil oxychloride with the MoC~S; attempts to fractionate were unsuccessful. Heating in z’ucuo or in Nz gave essentially no distillation except on approaching the melting point whereupon extensive release of C12 resulted, in accord with findings of Honigschmid and Wittmanns and thermodynamic estimatcls of Brewer and co-workers,1° but in apparent conflict n i t h a clainl” that I
,
(5) J. J. Downs and R . E. Johnson, J . A m . Chem. Soc.. 7 7 , 2098 (1955); J. J. Downs, Ph.D. Thesis, Florida State University, August. 1954. (6) C. H. Wallace and J. E. Willard, J . Am. Chem. SOC.,72, 5275 (1950). (7) A. Voigt and W. Biltz, 2 . anorg. Chem., 133, 277 (1924). ( 8 ) 0. Hdnigschmid and G. Wittmann, ibid.,2119, 65 (1936). (9) K. Knox, S. Y . Tyree, J r . , R. U. Srivastara. V. Norman, J. Y. Bassett. Jr., and J. H. Holloway, J . A m . Chem. Soc.. 79, 3358 (1957). (10) L. Brewer, I,. A. Bromley. P. V’.Cilles and N. L. Lofgren, in L. L. Qiiill (ed.), “The Chemistry and hIetallurgy of Miscellaneouv
April, 1961 ?VIoClScan be vacuum sublimed away from MoCId a t 120" and the claim that MoC15 can be vacuum distilled a t 150°12a or even a t 60-70°.129 The latter paper also stated that solid MoC16 and gaseous MoC16 in equilibrium with it decompose to Cln and solid hfOC14 a t 98-162". Apparently MoC15 cannot be purified by vacuum distillation a t these temperatures unless the distillation is substantially faster than the dissociation. Repeated extraction of solid MoC16 with cc14 in vacuo iii a Pyrex apparatus allon-ing filtration of the solid from the extract and evaporation and re-use of the CCI, gave purified i\.loClSwith a Cl/hIo atom ratio of 5.00. The red-brown final CCI, extracts (including C1, in gas phase) had Cl/Mo atom ratios close to 5, whereas the first cc14 extracts were bright red with CI/?Ao atom ratios of 3.4-4.2. Measurement of Chlorine Produced by Dissociation of Molybdenum(V j Chloride in Carbon Tetrachloride Solution. -In the course of working with R'IoC16 in CCI, LUCUO Clz was found presmt in amounts grossly in excess of that which could be formed from any O? possibly present. Two methods were used (and a third attempted) to measure the Clz formed. Concentrations of Clz in the MoC4-CC1, solutions and in thi. vapor phase above each solution were deduced froni either the C1, in the vapor phase or the total Clz with the aid of the perfect-gas and Henry's laws.13 1. Distillation Method.-Aliquots (10- or 20-ml.) of ? V ~ O (puriiied C~~ by extraction method) in CCI, nere delivered from an evacuated buret equipped with a bellowstype brass vacuum valve with Cu-P) rex seals into an evacuated Pyrex evapor:ttor5 and the CCla, CIz and any HCI distilled in 2 min. ("fast") or in 18 min. ("slow") into traps immersed in liquid IYZ. The trap contents mere distilled into OZ-free:tqueous KI, and the 1,- formed titrated with standard n'aZSzO3,after which any HC1 a-as determined iodometrically . 2. Aqueous KI Method.-Aliquots of the same MoCl6CCI4 solutions n-ere delivered into the evaporator, then 0 2 free aqueous K I nxs rapidly added vith vigorous stirring and the 11-formed titrated with standard Na2SzO3. Clz so determined i: a measure of the "equilibrium" concentration if the assumptions he made that the l\IoCl6 is instantaneously hydrolyzed before further dissociation occurs and that oxidation-reduction is negligible here except between Clz and Ia-. 3 . Spectrophotometric Method .-Spectrophot ometric measurement of Clz in MoCl&CI, solutions was not considered feasiole because of complications. At the suggestion of Professor J. D. McCullough, we attempted to measure the intensity of the Clz absorption band a t 328 mp in known aliquots of the gas phase in apparent equilibrium with ?V1oCI5-CCl4 solutions. Because of considerable a t tack of MoClj by rnoieture during the extensive handling the results w r e useful only in showing independently of the other two methods that C1, is released when MoCI, dissolves in CC1,. Molybdenum and Chlorine Analyses .-lloC15 and its CC1, solutions and residue? were hydrolyzed in sealed apparatus with excess 1f KaOH, the ?\.Iooxidized to hlo(V1) with 30% H202, then m x s s H202and any CCI, pSesent removed by boiling. Aliquots of the colorless solutions mere used for determining M o by the a-henzoinoxime gravimetric methodI4 and C1 by Clarke's methodlo (satisfactory at pH 3.0-3.5 in the p~esenceof Mo(V1)). Dry Box.---Among several drv boxes used, the best was a Lucite box cbquippcd with an air lock, a Sa-arc purification system (No. 106, Caemco Iuc., Florida), and two Seoprene gloves Treated with Fluorolube 11 to reduce diffusion of 0, and moisture into the box. A second pair of ?;eoprene gloves was worn by the operator. A purified Ar flow was used inside the box. Although the box contained Materials: Thermodynamics," McGraw-Hill Book Co., Inc., N. Y . , 1st ed., 1950, Paper 8, p p . 276-311. (11) D. E. Couch and -4.Brenner, NBS Report No. 532G, J u n e 14, 1957, p. 11. (12) (a) S. .A. Schukarev, I. V. Vasil'kova and B. N. Sharupin, Zhur. Obshchei R h i m . , 26, 2093 (1956); (h) Vestnik Leningrad Unir., 14, No. 10, Ser. Fiz.i Khim., No. 2, 72 (1959). (13) Henry's law ooniitants were calculated from the data of IT. J. Jones, J . Chem. Soc., 99, 392 (19111, and N. W. Taylor and J. €I. Hildebrand, J . Am. Chem. Soc., 46, F82 (1923). (14) H. B. Knowles. Bur. Standards J . Research, 9 , 1 (1932). (15) F. E. Clarke, Anal. Chem., 22, 553 (1950).
KOTES
691
fresh PZOLand gave negative tests'6 for Oz with liquid Ka-K eutectic alloy and with a mineral-oil suspension of sodium benzophenone ketyl for 15-30 min. periods, hIoCl5 exposed to the dry-box atmosphere formed coatings within a minute or less.
Results and Discussion Our experiments show clearly that Clz is released when M0C15 dissolves in CCl, a t 2-26", unlike PC15 and in contradiction with the inipression gained from the literature that nIoC15 dissolves in CCl, without reaction. Because of the fantastic sensitivity of hIoC15 to moisture and the low solubility of MoCl5in CC14, our experiments do not establish the nature of the &!to species formed in the dissociation. A!toC13 appears improbable since CC14 equilibrated for days with 1\IoC13powder had a solubility of f at 25" and our hIoC16CC1, solutions gave no solid phase on centrifugation. Residues from the total distillation of Clz and CC1, from MoC16-CC14 solutioiis were brown-black solids with green discolorations and C1/ N o atom ratios of 3.35-3.37, suggesting the presence of oxychlorides and possibly i\IoC14 (attack of hIoC15 by moisture in a t least one distillation was shown by finding 0.106 mmole of HC1 in addition to 0.066 mmole of Clz). Thermodynamic estimates of Brewer, et U Z . , ~ and ~ our own data are compatible with dissociation by the path
l Richmond, Virginia, Kovember 5, 1959 ( 2 ) See, f o r ~ u n n i i > l e . \. N Campbell and E. M. Kartzmark, Can. J C/LPVL57, 1 IO0 (1959). €1 5 Iireinemshers, Z physik. Chem., 1 1 , 80 (1893). 1'1'1 tnrl 1 C Ricri J .4m Chem Sac , 63,4305 (1931). IIf r < ,I C h m Lduc , 35, 510 (195s).
(
Vol. 65
perature and pressure constant) of a ternary system composed of mater and two salts, the components identified, respectively, as 1, 2, and 3. Assume a tie line joining a solid phase D with the solution A in equilibrium with the solid. This tie line is the locus of points of mktures from which the phases A and D will form at equilibrium. Consider two such mixtures on this tie line, B and C. If the points B and C are fixed, the direction of the tie line is known, although this would not fix the location of either A or L). If, however, any one of the coordinates A1, dzor ilgof A were also known16 that point would be fixed. This follows from the linearity of tie lines, which requires that
In like manner knoving any one of the coordiiiat,es 01, Dzor D 3sufices to fix point D, because
Thus in order to apply the method described here, the following must be known: (1) the location of point B, ( 2 ) the 1ocat.ion of point C, (3) oiie coordinate of point A, (4)one coordinate of point D. The first of these is available upon synthesis of niixture B. While in principle C could also be established by synt,hesis, this mould be highly impractical. Instead t,hat point is found indirectly, as described below, after determination of the third quantity. By the methods used here the third quantity is AI. It is determined by evaporation to dryness of a weighed fraction of the solution phase of mixture B. The fourth quant'ity is det,ermined as in the method of algebraic extrapolation, after the location of point C is fixed. A knowledge of -41found for the solution phase of mixture B would fix point C if A1 were known as some systematic function of the compositions of mixtures. Such a relat'ionship can be established by the detjerminat8ionof A1 for a series of mixt,ures having an arbitrary but fixed weight per cent. mater and variable ratios of the two s a h , i.e., variable C2/C2 C3 at const>aiitGI. Because t,he location of point C on the tie line is, in principle, arbitrary, C may be considered a member of the series of mixtures a t consttant C1. Thus data are available for a reference plot of Al vs. Cz/Cfz Ca a t known C1. For a value of Al determined for the solution phase of mixture B it is thus possible t o fix the compositjion of the corresponding point C from this plot; point's B and C must have the same A1 value if they lie on t,he same tie line. ,4 dctermination of A1 for a number of such mixtures us B thus h e s corresponding points C on the same tie lines, and defines both t'he solubility curve and the solid phase. As there are no t,ie lines in an invariant region, an invariant point cannot be established by the above method. This point must be fixed instead by extrapolation of Olie solubility curves t o t'heir
+
+
(6) The codrdinstes (in weight per cent.) of a point such as .Iare given by AI, Az and A3, where the subscripts refer to the components. (7) Equation 2 will be recognized as the basis of solid phase identification b y the method of algebraic extrapolation,* except t h a t t h e co111Dosition of the solution phase has been replaced by t h a t of a second mixture lying on the same tie line.
