ANALYTICAL CHEMISTRY
1106 fining Co., Argo, 111.: and of the samples of naturally seleniferous waters supplied by 0. A. Beath of the University of Wyoming. BIBLIOGRAPHY
(1) Arthur, Moore, and Lambert, J . Am. Chem. Soc., 71, 3260 (1949). ( 2 ) Baker and Maxon “Inorganir Syntheses,” Vol. I, ed. by H. S. Booth, pp. 119-20, New York, McGraw-Hill Book Co., 1939. (3) Custer and Natelson, ANAL.CHEM.,21, 1005-9 (1949). (4) Furman, “Scott’s Standard Methods of Chemical Analysis,” Vol. I, Kew York, D. Van Nostrand Co., pp. 782-3, 5th ed., 1939. (5) Lambert, Moore, and Arthur, ANAL.CHEM.,23, 1193 (1951).
(6) Lansky, Kooi, and Schoch, J . Am. Chem. Soc., 71, 4066-75 (1949). (7) Pitha, J . Chem. Education, 23, 403 (1946). (8) Schoch, “rldvances in Carbohydrate Chemistry,” Vol. I, pp. 247-77, ed. by Pigman and Wolfrom, S e w York, Academic Press, 1945. (9) Schoch, J . Am. Chem. Soc., 64, 2957-61 (1942). (10) Trelease, Soil Sci., 60, 125-31 (1945). (11) Trelease and Beath, “Selenium,” p. 221, published by the authors, New York, S . Y.,1949. RECEIVEDAugust 21, 1960. Condensed from a thesis submitted by Jack L. Lambert t o the faculty of the Graduate School of the Oklahoma Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of doctor of philosophy, August 19.50.
Determination of Gamma Isomer Content of 1,2,3,4,5,6 -Hexachloro cyclohexane EMORY E. TOOPS, JR., AND JOHN A. RIDDICK Commercial Solvents Corp., Terre Haute, Ind.
A simple, rapid, and direct method for determining the gamma isomer content of lindane was needed for production control and specification analysis. The freezing point of a purified sample of the gamma isomer and its cryoscopic constant were determined with a platinum resistance thermometer. A calculated freezing point of 112.86’ C. was obtained for 100.00 mole % gamma isomer. The application of the freezing point depression method for the routine analysis of the gamma isomer content of lindane was checked on synthetic mixtures. The apparatus consisted of a simple freezing cell and a mercury thermometer specially constructed to the authors’ specifications. The method has an accuracy of about +0.05 mole %.
rontaining two or more components and a crystalline phase containing one of these components is: -InS1
=
-In(l
- Sp)
=
where
R
= gas constant per mole
tlo =
absolute temperature of freezing point of major com-
ponent when 9 2 = 0 A H j o = heat of fusion per mole of major component a t temperature t j o A C ‘ ~ = heat capacity per mole of pure liquid minus pure solid
for major component in pure siate a t temperature tf
t = given temperature of equilibrium DETERMINATION OF THERMODYN4MIC FREEZIYG POlYT
T
HE name “lindane” has been coined and accepted for the in-
secticide containing not less than 99% by weight of 7-1,2,3,45,6-hexachlorocyclohexane (?-benzene hexachloride) ( 4 ) . AIthough there are several methods of analyzing mixtures of the isomeric hexachlorocyclohexanes (1, 5, 8, 9 ) , there are no published methods for lindane. A suitable method should be not only direct and accurate but rapid enough for production control and specification purposes. In the opinion of the authors, a freezing point depression method fulfills the above requirements. Such a method requires only modest equipment and can be used by any intelligent technician. There is a wide variance among the reported values of the melting or freezing point of the gamma isomer. Some work has been done in evaluating the cryoscopic constant and applying i t to the determination of the gamma isomer content in technical benzene hexachloride (3). However, in the absence of a reliable freezing point it was decided that a complete redetermination \vas advisable. The procedure used for determining the thermodynamic freezing point of the gamma isomer was based on the method used a t the National Bureau of Standards by Rossini and coworkers (6,11) for hydrocarbons. The basic thermodynamic relationship (11)between the teniperature of equilibrium and the composition of the liquid phase, or solvent, for an equilibrium mixture consisting of a liquid phase
The freezing cell assembly used for the determination of the freezing point of the gamma isomer was essentially t h a t developed by Mair, Glasgow, and Rossini (11).
For nork a t higher temperature 20 turns of No. 20 Sichrome wire were wound on the outside of the cell and the entire unit was enclosed in a n outer glass jacket (Figure 1). The temperature around the freezing cell, and hence the thermal head, was controlled by a Variac connected to the Sichrome heater. The cooling rate was controlled by varying the pressure in the vacuum jacket. The temperature between the freezing cell and the outer jacket was measured with a n iron-constantan thermocouple. The freezing temperatures were measured with a four-junction platinum resistance thermometer using a Leeds & Northrup No. 8069 Mueller bridge. All temperatures were measured with a resistance thermometer calibrated by Leeds & Northrup against a Sational Bureau of Standards certified resistance thermometer. Preparation of High Purity ~-1,2,3,4,5,6,-Hexachlorocyclohexane. Commercial grade lindane rvas purified by the folloning method.
Recrystallization from acetone, using 1 ml. of solvent for each gram of lindane. Recrystallization from fractional distilled iso-octane (2,2,4trimethylpentane), using 4 ml. of solvent for each gram of lindane. Recrystallization from redistilled Phillips commercial grade n-hexane, using 4 ml. of solvent for each gram of lindane. All recrystallizations were carried out by heating to effect complete solution, then rapidly cooling the hot solution in an ice bath to obtain a slurry of fine crystals. The product oh-
V O L U M E 23, NO. 8, A U G U S T 1 9 5 1
I101
tained from each step in the recrystallization was separated by filtration and air-dried. The linal airdried material was further dried a t 60" C. and 1-mm. pressure for 24 hours. The over-all recovery from the first two recrystallizations vas GO to 65%. A 90% recovery was obtained from the third recrystallization. The purified gamma isomer was found t o hold adsorbed moist h e very tenaciously and complete removal by oven drying under reduced pressure was difficult, This adsorbed moisture and m y residual solvent were removed by dispersing dry nitrogen through the molten material, held a t 115' C. far 2 hours, just prior to determining the freeeing point. The flow of nitrogen was adjusted just to fill the melt with fine bubbles without violently agitating the surface.
the freezing point, the time-resiatanco observations were lotted with 1 om. on the time scale equivalent to 1 minute an$ 1 em.
~
~~
~
~~
ance corresponding to the th&nodynamic freezing point.
If the extent of undercooling is great,, the resistance at zero time is more accurately determined by the geometric method described by Mair, Glasgoiv, and Rossini (11). The mathematical derivation of this relationship and the proof of the geometrical solution are discussed by Taylor and Rossini (18). The resifitance corresponding to the freeeing point for zero impurity (ti-) was also determined by the geometric method of Taylor and Rossini. The precision in determining t p , however, depends to a great extent on the time-temperature observations extending over a large fraction (about the order of one fourth to one half) of the material crystallized. Because of the nature of the gamma crystals, i t %vasimpossible to crystslliae more than about one sixth of the melt before the laboring of the stirrer introduced enough heat to cause a rise in temperature. This is apparent in Figure 2 just hefore the stirrer ums stopped. The precision in determining t,he actual freezing point, t , of the gamma isomer is nearly as good as the precision in determining individual temperatures on the equilihn'um portion of the curve. For calculating the purity of a given compound from freezing point data, i t is more convenient to transform Equation 1to the form: log,o P = 2.00000 -
4 .
(ti"
- 0 I1 + R(tP
- 1)
+ .. . 1
(2) where A = cryoscopic constant (mole fraction per 1' lowering) = 4Htjs/R@ 1 4cp
B=l----\
P = .. .
..,
.
i
.! ;
. -.
.
..
. , ..
.' ,
e.
1
," .
.. 8
...
i
' 1
...
.
.
,.
y!..
,
.
.
.
.' 8 ..
. . . .. . . . *' .. ',
:.I
. -
36810-
Figure 1. Freezing Cell
Procedure. Time-temperature freezing curves were determined using 70 grams of purified gamma. isomer. St,irring ~5-m msjntained constant a t 80 strokes per minute. Time-resistance readings were reoorded for intervals of 0.05 ohm (ea. 0.5" C.) while the melt was cooling to establish the cooling rate. The optimum rate for the purified gamma isomer wm found to be ahout 0.1 ohm every 1 to 1.5 minutes (ea. 1" C. every 1 to 1.5 minutes) when the temperature was still 7" above the freezing point. At the appropriate time the solution ums seeded to prevent severe undercooling. With the onset of ery&allizatian, time-resistance me?surements were recorded for intervals of about 1 minute untd the stirrer began to labor. A preliminary plot was then made of the timeresistance data to establish aero time (the time a t which crystallization m-ould have begun in the absence of undercoolmg). For this plot the time male was taken so t h a t 1 cm. was equivalent to 1 minute and on the resistance scale 1 cm. was equivalent to 0.2 ohm (ca. 0.2" C.). If the extent of undercoofing is small, the equilibrium portion of the curve may he extrapolated back to the liquid cooling line to obtain zero time. This typeof curve is shown in Figure 2. In order to locate accurately the resistance corresponding to
Y)
30
36800-
T
014
.
TIME MUUUTES
36796
7
I IO
I
I IS
25
If We purity of the samplc is high-,.e., \w - t ) is smallthe second cryoscopic const,ant, B, may he neglected without significant err011 and Equation 2 reduced to: lag,, P = 2.0000 -
A T 2.302a9 (""
- t,
Since A = 4HplRt?o>i t was necessary to know L/U= IWAL/ uf fusion. I n the absence of cdorimetric data, the heat of fusion n'a8 approximated from Walden's rule (7) to he 4900 calories per male.
ANALYTICAL CHEMISTRY
1108 Freezing Points of Purified y-1,2,3,4,5,6-Hexachloroc5-clohexane Freezing Point,
Sample No. 1 2
Calcd. F . P . for Zero Impurity,
c.
3 4
5 Mean
0
FIGURE I 4
I
I Table I.
WhlA
36730'
Calculatpd Purity, Mole yo
c.
112.839 112.842 112.838 112.830 112.837
1 12.866 112.862 112.863 112.857 112.669
99.96 99.97 99.96 99.95 99.96
112.837
112 863
99.96
+ ALPHA
WT% CAW:9899 WT % ALPHA: 101 COOLING RATE, Io/MIN
36720
Table 11. Determination of Cryoscopic Constant Freezing Point,
KO.
hI o!e Fraction Gamma Isomer
1
0 9946
112 481
2
0 9897
112 141
3
0.9897
112,149
4
0.9896
112.180
5
0 9896
112.176
6 7 8 9
0.9893 0.9838 0.9772 0 9696
112.111 111.823 I l l , 492 111 023
10
0.9505
109.862
11
0.9335
108.890
12
0.8891
106.228
Sample
A.
c.
Mole Fraction 11 Lovering
Solute Alpha !?onrer Aliiha isoilier Alpha isoilier Beta isouier AlDha isoiner Kaphthalene Phenanthrene Xayhthalene Alpha isomer blpha isonier hlpha isonier Alpha isomer
arc.
0 0141 0 0143
36700-
TIME, MINUTES
0 0145
I
I
15
IO
5
0,0153
20
0 0156 0.0156 0 0157 0 0168 0.0168
0.0169
I
I
I
I FIGURE 5
GAMMA + ALFtL4
36.760-
m%GAMMA
0,0173
:
W o k ALPHA =
0.0177
COOLING WTE
99.48
0.52
,I ~ M I N .
36750.-
The freezing point data for the purified gamma isomer are summarized in Table I. DETERMINATION OF CRYOSCOPIC CONSTANT
The caryoscopic constant was calculated by Equation 3 from the freezing points of a number of synthetic samples ranging from 88.91 to 99.46 mole yogamma isomer.
I
0.IOC.
I FIGURE 3 GAMMA WT% WT%
+
ALPHA
GAMMA * 98.49 ALPHA : 1.51
COOLING RATE, Ie/hN.
\
8 36740,
0,l~C
stainless steel sheet, flaked, and powdered. It showed no surface adsorption of moisture when placed in humidistats ranging from 30 t o 91% relative humidity for 7 days. X-ray diffraction patterns showed that rapid solidification caused no 'apparent change in crystal structure. The freezing points of the synthetic samples, the solutes used, and the calculated values of the ciyoscopic romtnnt are given in Table 11. Only the value of the first cryoscopic constant, A , was calculated. In the absence of accurate calorimetric data no attempt was made to evaluate the second cryoscopic constant, B. The vaiiation in the value of A , shown in Table 11, is to be expected, as the limiting form of the freezing point lowering equation was used.
Table 111. .4nalysis of Known Samples of Gamma Isomer Sample S o .
The synthetic mixtures were prepared hy accurately weighing the desired amount of solute into the fieezing cell. The purified gamma isomer was then weighed into the cell, the mixture melted, and the time-temperature freezing curve determined in the manner previously discussed. T h e purified gamma isomer was dried by paEsing dry nitrogen through the melt for 2 hours. The melt was then poured cn a
?-Isomer Concentration. Mole cC; Calculated F Z x 99 98 98 98 98 96 98
30 99 97 82 74 35 11
99.23 98.93 99 03 98.84 98.75 98,-10 98.23
V O L U M E 2 3 , NO. 8, A U G U S T 1 9 5 1
1109
Lindane is by definition a t lrast 99% by weight of the gamma isomer. Therefore the value of -4used should be the best value determined for the concentration range, 99 to 100%. The value of A used for t,he analytical mcthod was the mean of the first five values in Table 11, or 0.0148 mole fraction per 1O lowering of the freezing point. Should it be drsired to analyze samples of the gamma isonleu containing inore than 1% and up to 11% of t,he other isomerp, the appropriate valur: of d may be obtained from Table 11. hccording to fusion data (2, 10) there is evidence that the gamma isomer can rxipt in three polymorphic forms. I n determining the cryoscopic constant d d i t i o n a l evidence n-as obtained that polymorphism can (xist. I
I
36.760-
I FIGURE 6
Table IV.
Effect of Bubbling Nitrogen through a and 7 JIixtures Ilitrogen Treatnlent Time, Hours 0 0
Sample NO.
1 2 3
0 . ,j 1.0 1 3 2 0
?
:/-Isomer Concentration, Mole Calculated Found 98.99 98.95 98.96 98.94 98.92 98,87 98.96 98,97 98.96 98.99
7*
I
GAMMA c ALPHA
11
W T % GWMA; 99.01 W T % ALPHA 099 COOLING RATE ~P/2MIN.
36750
TIME, MINUTES
22
25
1
1W9 SEMI-BALL 7 -
I 30
I 35
40
FIGURE 7
20NICHROME TURNS : 0 WIRE NO. 20 TO VARIAC
Figure 8. A.
B. C. D.
A SBESTOS T &
1 j7;'
STAINLESS STEEL ROD
Routine Freezing Point Assembly
Windshield wiper stirrer, 60 strokes per minute Thermometer Variac, t o control cell heater Potentiometer, direct reading ironconstantan
The first determinations of the cryoscopic constant wrre made using the alpha isomer as the solute and a cooling rate of about 1O per minute. For a gamma isomer concentration of about 97.8 t o 99.8 mole yo,this fast cooling rate gave time-temperature curves with two maxima. Three of these curves are illustrated in Figures 3, 4, and 5. However, T? hen the cooling rate LYas lowered t o 1 every 2 to 3 minutes, normal time-temperature resulted (Figure 6). For the same concentrations, the temperature of the 8econd maximum corresponded n ith the temperature obtained from a normal type curve. As yet no further work has been done on the polymorphic foims of the gamma isomer. O
DETERtIIN4TIOY OF G A h I t l 4 ISOMER CONTEYT IN LINDANE
Apparatus. The dimensions and general construction of the apparatus used for the routine analysis are illustrated in Figures 7 and 8. FREEZING POINT STIRRER 6 C E L L
The freezing points ;ww measured with a thermometer constructed to the authors specifications by Palmer Thermometers, Inc., Cincinnati 12, Ohio. They were 44 cm. long, of RedReading Mercury constructed without lens front and calibrated
1110
ANALYTICAL CHEMISTRY
For 100-mm. immersion. The graduations were in 0.1" intervals from 98" to 120" C. with 25 graduations per inch. When in use the thermometers were calibrated a t least once a day a t the steam point. The precision of measurement with these thermometers, as determined on six identical samples of known freezing point, was 1 0 . 0 1 " C. The thermometers proved completely satisfactory. T h e only change contemplated for future thermometers is a 75-mm. immersion line instead of 100 mm.
Analysis of Known Mixtures. I n order to piove this method, known mixtures of dry gamma and alpha isomers were analyzed by the freezing point depression, using the value of 0.0148 for the cryoscopic constant. The known samples were heated to 118" to 120" C. and mixed thoroughly, and the jacket heater and cell pressure were regulated to give a cooling rate of not more than 0.5' C. per minute. The freezing point was taken as the temperature plateau that occurred immediately after recovery from undercooling. The thermometer was read to zt0.01" C. with the aid of a small magnifying glass. The gamma isomer concentration was calculated from the equation : Log mole %gamma = 2 where At = 112.86
-
X At - 0.0148 2.30259
(4)
freezing point of sampk.
The results obtained on known samples are given in Table 111. The cryoscopic analysis of commercial sample? of lindane is complicated by the possible presence of moisture and/or residual solvents. Complete drying can best be effected by dispersing dry nitrogen through the melted material. The possibility of change in composition was investigated by preparing known samples of
the dry alpha and gamma isomers, passing dry nitrogen through the melted samples held a t 115' C. for varying lengths of time, and then determining the gamma isomer content. These data are summarized in Table IV. ACKNOWLEDGMENT
The authors are indebted to Frederick D. Rossini and iZnton J. Streiff of the Carnegie Institute of Technology for their many helpful suggestions while this work was in progress. The preparation of the high purity isomers by Robert H. Cundiff of this Iaboratory is also gratefully acknowledged. LITERATURE CITED
( 1 ) Aepli, 0. T., Rlunter, P. d.,and Gall, J. F., ANAL. CHEM.,2 0 , 610 (1948). ( 2 ) hrceneaux, C. J., Ibid., 23, 906 (1951). (3) Bowen, C. V.. and Pogorelskin, M. A , , Ibid., 20, 346 (1948). (4) Chent. Eng. A-ews, 27, 2210 (1949). ( 5 ) Daasch, L. K., Ax.4~.CHEY., 19, 779 (1947). (6) Glasgow, A. R., Krouskop, K. C . , Beadle, J., Axilrod, G. D.. and Rossini, F. D., Ibid., 20, 410 (1948). (7) Glasstone, S.,"Textbook of Physical Chemistry," p. 454, Sew York, D. Van Nostrand Co., 1940. ( 8 ) Harris. T. H.. J. Assoc. Oifc. Am. Chemists. 32. 684 (1949).
(9) Kauei. K. B., DuVall. R: S., ind dlquist, F. K., I n d . Eng. Chem., 39, 1335 (1947). (10) SIcCione, IV. C., A N ~ LCHEM., . 21, 862 (1949). (11) Mali, B. J., Glasaoi3, -1.R.. and Rossini, F. D., J . Research .Yutl. B u r . Standards, 26, 591 (1941). (12) Taylor, IT.J., and Rossini, F. P., Ibid., 32, 197 (1944). RECEIVED January 18, 1951. Presented before the Division of hnalytical Chemistry a t the 118th Meeting of the AIIERICANCHEMICALQ O C I E T Y , Chicago. 111.
Separation of Chromium from Vanadium By Extraction of Perchromic Acid with Ethyl Acetate ROBERT K. BROOKSHIER, Northwest Electrodevelopment Laboratory, Bureau of Mines, Albany, Ore., AND HARRY FREUND, Department of Chemistry, Oregon State College, Coroallis, Ore. Research by the U. S. Bureau of Mines on the recovery of vanadium from low grade ferrophosphorus required the determination of small amounts of chromium in vanadic oxide products. The titrimetric method is inaccurate, and Foster's perchromic acid extraction procedure gave erratic results. Study of the variables influencing the extraction led
A
CCURACY in the titrimetric determination of chromium in the presence of vanadium (9) is difficult to attain when vanadium preponderates. The usual colorimetric determination of chromium ( 5 ) as chromate or with s-diphenylcarbazide necessitates a preliminary separation from vanadium. Foster (3) employed ethyl acetate as an immiscible solvent to extract perchromic acid and thereby effected the separation and concentration of small amounts of chromium in vanadium products. Attempts in the laboratory of the U. S. Bureau of Mines to applv the separation to determination of chromium in red-cake vanadic oxide led to erratic results and indicated the need for a systematic study of the factors influencing the separation. Since 1847, when Barreswil (I) first reported the formation of blue perchromic acid by the action of hydrogen peroxide on dichromate, the reaction has been used a s a sensitive qualitative test for chromium. The object of later research ( 2 , 4,7, 8) was the study, in a homogeneous system, of catalytic decomposition of hydrogen peroxide by dichromate. The reaction of oxidized compounds with hydrogen peroside, replacing oxygen atoms with
to the following optimum conditions: pH at equilibrium 1.7 r!c 0.2, concentration of hydrogen peroxide 0.02 mole per liter, temperature 20" C. or less, and number of extractions 3. Adherence to these conditions, with final estimation of chromium with s-diphenylcarbazide, yields a more sensitive and reliable method than an) heretofore described.
peroxide groups, generally leads to unstable products that lose oxygen very easily. Studies by Spitalsky ( 7 , 8)'and Bobtelsk'y (2) confirm the transitory nature of perchromic acid as an intermediate in the catalytic reduction of hydrogen peroside with dichromate. S o attempt was made to stabilize the intermediate perchromic acid, although the free acid was knonn to influence the reduction, and the presence of an organic solvent in the homogeneous system was shown to have a stabilizing effect ( 2 ) . REAGENTS AYD APPAR4TUS
Chemically pure reagents were used throughout this work. Absolute ethyl acetate was used and was recovered and purified by distillation after each esperiment. The hydrogen peroxide solution was prepared and standardized daily from Baker's C . P . 30% hydrogen peroxide solution. The standard chromium solutions were prepared from accurately weighed amounts of Kational Bureau of Standards potassium dichromate No. 136. C.P. ammonium metavanadate was used for preparing the standard vanadium solution. All p H measurements were made with the Beckman Model .1I pH meter.
