486
ANALYTICAL CHEMISTRY
T a b l e 111. Uranium Added. Y
13.09 12.43 12.43 11.32 11.32 11.10 10.43 5.214 5.098 4.947 4.878 4.952 4.988 4.988
C o u l o m e t r i c M i c r o d e t e r m i n a t i o n of U r a n i u m i n R a n g e of 5 to 10 M i c r o g r a m s Current, pa. 62.41 62.41 62.41 62.47 62.47 62.47 62.47 28.18 29.41 30.28 30.08 29.35 29.35 29.35
Time, Corrected for Blank, )fin.
Uranium Found,
2.734 2.652 2.675 2,362 2.517 2.516 2.215
12,63 12.25 12.36 10.92 11.64 11.62 10.24
2.632 2.565 2.076 2.154 2.108 2.224 2.259
Y
5,494 5 . 508 4.653 4.796 4,579 4.831 4.906
Error
70
Y
- 0 16 -0.18 -0.07 -0.40 $0.32 +0.52 -0.19
-3.51 -1.45 - 0 56 -3.54
+2.84 +4.69 -1.89
S0.2iO $0.410 -0.294 -0.082 -0.373
-0.157 -0.082
+5,36 + 8 04 -5.94 -1.68 -7.33 -3.15 -1.64
its magnitude in coniparison to the size of the sample taken, it did not appear feasible to extend this procedure down to smaller samples of uranium, and to expect any sort of validity in the results with a 50 t o 100% correction factor applied to each determination. Iron, tungstate, and molybdate interfere. I n general, any material that is reduced by the cadmium amalgam reductor, quantitatively or not, and can be subsequently oxidized by ceric ions will give positive errors. Thus a prior purification of the uranium samples is arlvisahle. .iCKNOWLEDG\IEST
This research was supported by Contract AT (30-1)-937, Scope I of the T-.3. .Itomic Energy Commission. LITERATURE CITED
microburet, deaerated for 5 minutes, and then allorred to flow into the cell. Two milliliters of wash solution were pipetted into the funnel, deaerated for 5 minutes, and then drained into the cell. The generation of ceric ions was then begun and the time required to bring the galvanometer back to its original reading was measured. The blank was subtracted from this to give the true generation time for the uranium sample. Results using this procedure on 5- and 10-microgram ianiples of uranium are presented in Table 111. DISCUSSION AVD CONCLUSIONS
The data in Table I11 indicate that quantities of uranium as small as 5 micrograms can be determined by a coulometric titration with electrolytically generated ceric ion using a cadmium amalgam reductor for the prior reduction of uranium(\-I) t,o uranium(1V). The accuracy of this method is 8% for 5-microgram samples and 5% for 10-microgram samples. This error is partially at’tributedto the 1.5% uncertainty in t,he concentration of the standard uranium solut,ion, the 0.5% error in microburet readings on the addit.ion of the sample, and the errors due to adsorption and/or other factors in the reductor column. The blank obtained in these determinations corresponded to about 1 microgram, which can be accounted for on the basis of slight pI3 effects, dilution effect,s, and the addition of extraneous ions-e.g., cadmium, mercury, and any reducible ions in the solutions that are oxidized by ceric ion, It was found to be essentially constant for all det,erminations. However, because of
(1) Arthur, P.. and Donahue, J. F., AXAL.CHEM.,24, 1612 (1952). (2) Carson, IT. S . ,Ibid., 25, 466 (1953). (3) Chen, G., J . Lab. Clin. M e d . , 21, 1195 (1936). (4) Cooke, F.D., Hazel, J. F., and lIcXabb, W, AI., .ISAL. CHEM.,22, 654 (1950). (5) Cooke, W. D., Reilley, C. N., and Furman, N. H.. Ibid., 23, 1662 (1951). (6) Ibid., 24, 205 (1952). (7) Farrington, P. S.,and Swift. E. H., Ibid.. 22, 889 (1950). (8) Furman, S . H., Cooke, W. D., and Reilley. C. S., Ibid., 23, 945 (1951). (9) Kano, K.,J . Chou. SOC.Japan, 43, 333 (1922). (10) Kano, N.. Sci. Repta., TBhobu I m p . Cnio., 16, 701 (1927). (11) Kikuchi, S., J . Chein. SOC.Japan, 43, 544 (1922). (12) Kikuchi, S.,Sci. Rcpts., TBhoku I m p . C*r?ic., 16, 707 (1927). (13) Kolthoff, I. >I., and Furman, N. H.. “Potentiometric Titrations,” 2nd ed., p. 362, Sew York, John Wiley & Sons. 1931. (14) Kolthoff, I. Jf,, and TomicBk, O., Rec. t r m . chirn., 43, 798 (1924). (15) McNabb, IT-. If., Hazel, J . F.. and Dantro, H. F., .%SAL. CHEM..23. 1325 11951). (16) Reilley, C. X,, Cooke, W. D., and Furman, K. H., I b ( d , 23, 1030 (1961). (17) Ibid., p. 1223. (18) Rodden, C. J , editor, “Analyt~cal Chemistry of the Manhattan Project.” DP. 3-159, New Tork, McGraw-Hi11 Book Co., 1950. (191 Ibid.. pp. 54-65. (20) Ibid.; p. 70. (21) Willard, H. H., and Furman, S H , “Elementary Quantitative Analysis.” 3rd ed., p. 254, S e vr I-ork, D. Van S o s t i a n d Co., 1940. RECEIVED for review Sfpiemher 20, 1932.
.icce;,ted November 18. 1 ! l 5 2 .
Microthermal Analysis of the System 1,2,3,4-TetrachlorobenzenePentac hlorobenzene CLAUDE J. A R C E N E A U X Ethyl Corp., Baton Rouge, La.
E””-
h though it is widely known that many organic compounds may exist in several polymorphic modifications, the chemist may fail to consider this possibility n-hen engaged in normal laboratory investigations of these compounds. It frequently happens that polymorphic relationships are not considered in the construction of phase diagrams by the conventional niacromethods. Consequently, erroneous conclusions may be drawn owing to the influence of unrecognized polymorphous forms of the compounds being investigated. Fortunately various applications of the microscopic fusion methods as advocated by Kofler ( 4 ) and hIcCrone (6, 7) provide means for accurately determining the polymorphic attributes of organic crystalline materials and the physical properties of their mixtures. Regardless of the specific techniques employed, howeve1 , the niicro-
scopic identification of organic conipounds by a study of their melting poinh, polymorphism, and thermotropic behavior in binary and ternary cystems has been termed microthermal analysis. A melting point diagram of the system 1,2,3,4-tetrachlorobenzene-pentachlorohenzene, obtained by the conventional macromethod of determining points of primary crystallization, exhibited extremely abnormal behavior. h microthermal study of the system was therefore undertaken to obtain fundamental information concerning the polymorphic nature of these compounds, and to explain the behavior of their mixtures. A third objective was the application of these physical properties in ~ I P riving a rapid analytical method for binary mixtures of thew t n o materials.
V O L U M E 25, NO. 3, M A R C H 1 9 5 3
487
9 miemthermal stud) of the two compounds, 1,2,3,4-tetraehlornbsnzene and pentaehlomhenzene, was mads to explore the polymorphio nature of there compounds in explanation of the ahnormal hehavior of their binary mixtures. Both compounds are dimorphous, and melting point diagrams obtained frmn crystalline forms show two set8 of ideal he t w o stable forms of the two materials orms. A rapid analytical method, hased :stimating the composition of binary mixouraoy within approximately +2%.
... .
..
Figure 3: "Photomierngraph . of ?,2,3,f7'~et~etroaohlorobenzene r ~
CHLOROBENZENE AND PENTACHWROBENZENE
l,z,3,4-Tetrachlorobenzene. 1,2,3,4-Tetrsehlorobenz~ne Dan bc prepared as monoclinic needks by crystallization from salutions in organic solvents or by sublimation on the hot stage a t about 38" C. Crystals from either of these sources melt with a strong tendency toward sublimation a t 47.5" C . Hot stage observations show that this melted material supercools t o 42" C. ttnd then crystallises into a crystalline film of medium birefringence (Figure l), which upon reheating melts a t 42"C. This crystalline form is very unstable and will transform into a highly hirefringmt form on standing or on jarring the slide (Figure 2). This transformed material melts a t 47.5' C . Further investigation showed that 1,2,3,4-tetrachlorohensene is dimorphous. The stable farm (melting point 47.5" C . ) will be designated as form I and the unstable form (melting point 42 C . ) as form 11. Figure 1 Shows a thin crystalline film of 1,2,3,4tctrachlorohmzene 11,.and Figure 2 shows the same microscopic view after the transformation from form I1 to form I had taken place. With pure material this transformation occurs instantaneously, and single crystds of form I1 will always yield numerous smaller cry&ls of the stable form I. It is possible to produce form I of 1,2,3,4tetrachlorobensene directly from the melt by seeding below the melting point of the stahle form I and above the melting point of the unstable form 11. In this manner form I can be induced to crystallize in highly birefringent rods (Figure 3). Individual crystals of form I1 can be produced by sublimation a t 42" C . These single crystals of form I1 are almost as highly unstable as the crystalline film of form I1 obtained from the melt. They are imperfectly shaped, and it is rather difficult to recognize definite habits.
ANALYTICAL CHEMISTRY
488
Conoscopic observatioiis 011 thin crystalline films and individual crystals show that form I1 of 1,2,3,4tetrachlaroboneene is uniaxial and the sign of the birefringence is negative. On the other hand, the stable form (form I) of the same compound is biaxid, but a180 with a negativeGign of birefringence. Pentachlorobenzene. Investigation of the thermal and crystallographic properties of pentachlorobenzene reveds a striking similarity between i t and 1,2,3,4tetrachlorobenzene. As in the case of 1,2,3,4tetrachlorobenaene, i t is dimorphous. Form I nislta at 87" 6 . and form I1 melts a t 82.5" C. Single crystals of form I1 that m e almost identical with crystals of the corre sponding form of 1,2,3,4tetrachlorohenaene produced by suhlimation a t 42" C. can be obtained by sublimation at 82' C. Monoclinic needle crystals of pentachloroheneene I are obtained, by sublimation on a micro hot stage a t 60" C., in the ~ a m eway that 1,2,3,4tetrachlorobenzene I sublimes below its melting plint. Form I of this compound can also be produced directly from the melt by seeding a t a temperature above the melting point of form I1 but below that of form I. Crystals produced in this manner are highly birefringent. rods, similar to the tctre. chlorobenzene I shown in Figure 3. Figures 4 and 5 show the same microscopic view of pentachlorobenzene before and after the transformation from form I1 to form I
crystallizes in mixed crystals of tho two unstableforms, and trsnsformation into mixed crystals of the two stable forms follows. However, this transformation proceeds a t a much slower rate than in the pure materials. I n mixtures containing between 50 and 60 mole yo pentachlorobenzene, complete transformation may require zs long as 2 weeks or more. The rate of transformation increases as the concentration of either component approaches 100% and is instantaneous in either of the pure materials.
Fisure 5. Same Microscopic View as Figure 4 after Transformation to Pentaehlorobenzene 1 H a s Occurred Crossed Nicoli
Figure 4.
Pentaehlorobenzene I1 Crom Fusion Cmnsod Niools
Conoecopic ohservat,ions of these two crystalline forms shoa that, as in the case of tetrachlorobenzene, form I1 of pentachloro, benzene is uniaxial and the sign of the birefringence is negative The stable form (form I) of thin compound is himid, with IL negative sign of birefringence. Thus, with the exception of tht difference in melting points, the thermal and cryatdlographic properties of the corresponding forms of 1,2,3,4tetrachlora. benzene and pentachlorobensene are identical. BINARY MIXTURES O F 1.2.3.4-TETRACHLOROBENZENE ANI PENTACHWROBENZENE
Mixed fusions (4-6) provide simple means of determining in a qualitative way, the general classification of the phase die gram. Figure 6 shows a mixed fusion of 1,2,3,4tetrachlorobenzene and pentachlorobenzene in which mixed crystal forma. tion in both the correwondinr unstable forms (11) and come
A melting point diagram of the system 1,2,3,4tetrachlorobenzene 11-pentachlorobeneone I1 was conatructed from micro melting point data obtained 6 t h mixtures of the two unstable modifications. This was made possible by first melting small amounts of the binary mixture of known composition between a slide and cover glass directly on the hot stage and allowing the mixture to cool gradually until crystallization occurred. In this manner the mixture always crystallized into thin films of mixed crystals of the corresponding unstable modifications. Melting point determinations were then made on these sitme mixtures before transformation to mixtures of the corresponding stable forms began. The melting point versus composition curve obtained in this manner (Figure 7) is essentially a straight line. indicating ideal mixed crystal formation a t all points. The melting point diagram of the corresponding stable farms (I) was also constructed (Figure 7). This also proved to he an ideal mixed crystal system. I n this experiment, t h e melting points were made on intimate mixtures of the two stable forms which had not heen previously heated in order t o avoid interference from the unstahle crystalline forms. Although Kofler (S) recommends the use of metal mortars in grinding such mixtures to avoid handling difficulties induced by statio electricity, an agrtte mortar proved satisfactoryin this instance. The above findings indicate a true case of parallel isodimorp h i m . This simply means a mised crystal series formed b y a pair of stable and a pair of unstable crystal modifications. Brandstater (3)reports isopolymorphism in the system I-chlor* and l-bmmo-2,4-dinitrobenzene where aeveral mixed crystal series occur in the same system. ANALYSIS OF BINARY MIXTURES
1
ing the 8ame techniques as used in the microscopic analysis of benzene hexachloride (1). This showed that the transformation from unstable to stable forms occurs in mixed crystals of the two unatahle forms as well as io the pure materials. The mixture first
In order to determine the composition of binary mixtures of the two compounds, Kofler's method for refractive indur measurements of melts ( 4 ) was applied. It was necessary to modify this method for this investigation. The red filter supplied with the set of certified glass powders vas discarded, and a, green filter (Wrattm No. 58) was substituted. This was done in order to
V O L U M E 25, N O , 3, M A R C H 1 9 5 3 facilitate the observations hy reducing eye fatigue t o a minimum. Furthermore, i t was shown by laboratory tests that the refractive index measurements made with the green filter gave reproducible results. The volatility of the compounds investigated in this study caused difficulties in measurements of the refractive indices of their melts. Xofler (3)has been able to overcome this difficulty with the benzene hexachloride isomers by special prepam tion of the samples. This was accomplished by the so-called "salt chamber" technique in which the samples are enclosed in B melted, low-melting eutectic mixture such as mdium nitritepotassium nitrate. With low-melting compounds like these materials under investigation, a relatively high-boiling liquid such as glycerol may well be substituted for the melted salt. This "liquid chamber" technique was still further improved as follows:
489
complished bfplacing 1 drop of the l h i d at the edge d the larger glass and allowing it to seep under this glass by capillary
fractive index meashrements are inclided with each set of Kofler's certified glass powders (Arthur H. Thomas & Co.).
A sufficientamount of the sampie is mixed with a small amount of the standard powder and melted between a slide and a 12-mm. round cover glass. The mixture is allowed to solidify, and an 18-mm. circular cover glass is now placed directly over the smnl-
IO
0
I
I
eo
10
I 30
I
I
I
40
50
60
PENIACHLORoBLHIENr.
.I.
.,.
I
I
I
70
80
90
100
Fi gum 8. Refractive Index-Composition Relationship for a n d PentaehlorohenM elts of 1,2,3,4-Tetraehlorob~n~~n~ zene with Glass Powder Nd
= 1.5195 usins W n l t e n
No. 58 filter
To obtain the refractive index relationship with composition (Figure S), only one refractive index powder NZS used. The temperatures recorded are the ones a t which the refractive index of the melt i s equal to the refractive index of the standard powder (1.5795). A single refractive index measurement requires not more than 15 minutes, and the cornpasition of a binary mixture can be determined with an acouracy within approximately iZ% hy weight. CONCLUSIONS
Figure 6 . Mixed Fusion of 1,2,3,4,-Tetraohlorohenzenea n d Pentachlomhenzene
This investigation has provided fundamental information concerning polymorphism of the two compounds 1,2,3,4-tetrachlorobenzene and pentachlorohensene. Application of fusion teehniques for accurately constructing the phase diagram of these two compounds has been demonstratid and a simple adaptation of the liofler method of refractive index of melts has been developed. This adaptation can he used with any relatively volatile material such as the ohlorohenaenes, provided they have a relatively low melting point. ACKNOWLEDGMENT
The author wishes to thank S. N. Hall for supplying the samples of pure 1,2,3,4-tetrachlorobeneene and pentschlorohenaene and for suggesting a microscopic investigation of these compounds, and F. C. Holmes and J. T. Clarke for their helpful suggestions and encouragement. LITERATURE CITED
j
-,---....,
~
L.,and Koflei, A,,"Mikromethoden
sur Kennveiehnung organischer Stoffe und Stoffegernische," Innsbruek, Universititsvedag, Wagner, 1948. (5) Lehmann, O., "Die Krystalhndylyse," Leipeig, Wilhelm Engelmann, 1891. (4) Kofler,
(6) MoCrone, W. C.. ANAL.CnEx., 2 1 , 4 3 6 (1949). (7) McCrone, W-.C.. Mikroehenie vel. Mikrochim. Acto, 38,
4, 476
(1951).
am System: 1,2,3,4-Tetra:aohlorobenzene
~~~~g~~~ lor review July 10, 1952. Accepted Deoember 5. 1952. Preen+& before the Division of Analytical Chemistry &tthe 122nd Meeting of the A ~ Cxewro~r. ~ Soormr, ~ Atlantic ~ City. ~ N. J. ~ ~ N
