signed and constructed the temperature control system and power supplies for the detectors used in the analysis, and assisted in the design of the flow system. LITERATURE CITED
(1) Cvejanovich, G. J., ANAL. CHEM.34, 654 (1962). ( 2 ) Dal Kogare, S., Juvet, R. S.,Jr.,
“Gas Liquid Chromatography,” p. 126, Interscience, New York, 1962. ( 3 ) Madison, J. J., ANAL.CHEM.30, 1859 (1958). ( 4 ) Merritt, C., Jr., Walsh, J. T., Ibid., 34. 108 (1962). (5) $fitzner, B.‘ M., Gitoneas, P., Ibid., 34, 589 (1962). (6) Mitzner, B. M., Jones, W. V., Ibid., 37, 447 (1965). (7) Ottenstein, D. M., “Analysis of Fixed
Gases, Hydrocarbons and Related Com-
pounds by Gas Chromatography,” presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1962. (8) Simmons, M. C., Snyder, L. R., ANAL.CHEM.30, 32 (1958). J. 0. TERRY J. H. FVTRELL Chemistry Research Laboratory Aerospace Research Laboratories Wright-Patterson Air Force Base, Ohio
Purification of Hexamethylbenzene by Zone Refining and Determination of Its Melting Point SIR: For heat capacity and vapor pressure measurements, it has been necessary to prepare as pure a sample of hexamethylbenzene as possible ( I ) . We have prepared a sample of hexamethylbenzene with 0.1 mole % impurity by zone melting and have determined its melting point by modern procedures. An apparatus has been developed which allows purity determinations to be made by the melting point method of Mair, Glasgow, and Rossini (6) at temperatures above 150’ C. with a precision roughly comparable to that obtained a t room temperature. With this apparatus the purity of the starting material and the purified sample was determined. We were thus able to estimate the melting point of the ideally pure compound. Melting points cited elsewhere, along with the method of purification, are : 164’ C., fractional crystallization from ethanol (4); 165’ C., recrystallization from benzene ( 8 ) ; 164.8 + 0.1’ C., recrystallization from benzene after recrystallization from chloroform and ethanol three times ( 5 ) ; and 165.5’ C., fractional crystallization from ethanol (g), as compared with our lowest initial freezing point of 165.644’ C. for the purified compound. The discrepancies in the values, along with the difficulties in purifying this compound and in determining its melting point, make it desirable to report our results a t this time. The apparatus used for the purification was similar to the one described by Herrington, Handley, and Cook (2). Because of an increase in specific volume on melting, as well as a t the second-order transition a t 110’ C., the specific volume of liquid hexamethylbenzene a t the melting point is about 23 % greater (8) than that of the solid a t room temperature. For this reason the usual procedure of zone refining resulted in breakage of the glass tube. The purification was therefore begun with hexamethylbenzene packed tightly in a borosilicate glass tube, rather than in
the form of a continuous ingot. The zone purification process was carried out four times in a nitrogen atmosphere with a zone travel rate of 60 mm. per hour. At this rate of travel the contents of the tube separated into well defined regions during the first pass with a gap of about 20 mm. between each, thus allowing room for expansion in the second pass. After the fourth pass these sections had united so that a fifth pass was not attempted. Because hexamethylbenzene has a vapor pressure of about 50 mm. H g at the melting point (5), the freezing point determinations must be done in a closed system. The upper part of the inner wall of the double-walled borosilicate glass vessel which contained the sample was ground, to provide a means for closing the apparatus with a mating ground joint which formed part of the cap. The cap itself was provided with a ground joint, to provide entry for the thermometer, and a pumping exit. The stirrer shaft entered the apparatus through a stuffing box, containing two Teflon washers, which was soldered on to a Kovar seal whose glass end formed part of the cap. This gastight seal permitted free reciprocal motion of the shaft, with the aid of a constant speed motor and suitable cams. Teflon sleeves over the male half were used instead of grease to seal both ground joints.
Table 1.
Sample (detn. no.) Unpurified (1) Purified (1) Purified ( 2 ) Purified ( 3 )
Initial freezing point,
tl (“C.) 165 576 165 644 165 693 165 659
The space containing the sample could be evacuated to a dynamic vacuum of about 1 mm. Hg. A positive pressure of about 6 cm. H g of purified nitrogen was maintained over the sample during all runs. The whole apparatus was immersed in a stirred thermostat whose temperature could be controlled to +0.5’ C. at any desired temperature below that of the melting range. Temperatures were measured with a platinum resistance thermometer previously calibrated in this laboratory at the ice, steam, and sulfur points as discussed by Moessen ( 7 ) . The equation of Hoge and Brickwedde (3) was used to compensate for deviations of 0.01” and 0.02’ C., respectively, in the values of the ice and steam points from those of the original calibration. The sample for the purity determinations of the purified material, weighed 42.4 grams (about 25% of the total charge) and was a mixture of two sections of the column of “pure” material symmetrically located with respect to the middle, so that it would give a value representative of the entire pure sample. The results (Table I) show that the unpurified hexamethylbenzene contained 0.26 mole % impurity, and that the impurity present in the product was 0.1 mole yo. The mole fraction of impurity was calculated as outlined b y
Purity Data for Hexamethylbenzene
Mole 70impurity‘
t o C (“(2.) 165 781 f 0 016 165 757 f 0 007 165 755 f 0 005 165 731 f 0 023 Av. 165 75 f 0 02 0 Average of 6 values calculated from fraction frozen. R.m.s. error given for each. * Lowering of initial freezing point due to calculated amount of impurity. c Freezing point of absolutely pure compound, given by t l At. R.m.s. error given for each, and for average.
0 0 0 0
264 f 0 120 + 0 080 f 0 093 f. 0
020 009 007 030
AP
0 0 0 0
205 093 062 072
+
VOL. 37, NO. 9, AUGUST 1965
1167
Mair, Glasgow, and Rossini (6) using 4930 cal./mole for the heat of fusion. These results, along &.ith the initial freezing points, give the melting Point of pure hexamethylbenzene as 165.75 f 0.02O c. The uncertainty arises mainly from discrepancies in the initial freezing points (column 2, Table I) and to some extent from the uncertainty in the total time required for complete crystallization, from which the fraction frozen at any time was calculated.
(7) Moessen, G. W., Ph.D. thesis, The
LITERATURE CITED
( 1 ) Frankosky, M.9 Aston, J. G.9 J. PhYs. Chem. 69, in press. (2) Herrington, E. F. G., Handley, R., Cook, A. J., Chem. Ind. (London) 1956, p. 292. (3) How, H. J., Brickwedde, F. G., J. Res. Natl. Bur.+Std. 28, 217 (1942). (4) ~ ~ fH, H,, f parks, ~ ~G , s,, ~ ~ , ~ ~ A. C., J.Am. Chem. Sac. 52,1547 (1930). ( 5 ) MacDougall, F. H., Smith, L. 1.) Ibid., p. 1998. (6) Mair, B. 'J., Glasgow, A. R., Jr., Rossini, F. D., J. Natl. Bur. Standards 26,591 (1941).
Pennsylvania State University, University Park, Pa., 1955. (8) Seki, S., Chihara, H., Coll. Papers Faculty Sci., Osaka Univ., Ser. C, Chem. 11, NO. 1, 1-8 (1943-9). (9) Spaght, hl. E., Thomas, S.B., Parks, G. S., J. Phys. Chem. 36, 882 (1932). J. E. OVERBERGER i ~ l ~ , J. G. ASTON Department of Chemistry The Pennsylvania State University University Park, Pa. WORKaided by a grant from the National Science Foundation.
Factors Affecting the Selection ,of a Cobalt Analysis Line for Atomic Absorption S pectro met ry SIR: Information available in the literature concerning cobalt investigations by atomic absorption spectrometry has been generally limited to the evaluation of detection limits and analytical methods. Allan (1, b), Gatehouse and Willis (6),and McPherson, et al. (6) indicate the 2407.2 A. line as the most sensitive analysis line. Fuwa and Vallee (4) established very low detection limits for cobalt using the 2424.9 A. line. Robinson (9) listed detection limits for three relatively low absorbing lines, the strongest of which was 3529.0 A. in a n oxycyanogen flame. Menzies (8) found the 3533.4 A. line best suited to his apparatus. A study of several cobalt absorption lines indicates the proper choice of analysis line to be influenced by experimental parameters. The factors controlling such selection are discussed with respect to atomic absorption variables.
slot atomizer-burner (Perkin-Elmer Corp., Norwalk, Conn.) assembly was operated at the following conditions: 2.2 liters/minute acetylene, 7.1 liters/minute .*.atomizer air, and 10.9 liters/minute .auxiliary air. The sample uptake rate was 7.0 ml./minute. A D.C. solenoid shutter system mounted on the adjustable 'slit in front of the tube allowed the hollow cathode radiation to be cut off. An H T V R106 photomultiplier tube was used as the detector in the Model 139 photomultiplier attachment. The readout device was the direct reading meter on the spectrophotometer. A Heath EUW20'4 potentiometric recorder was also used as a readout for some measurements. RESULTS A N D DISCUSSION
Cobalt exhibits a line rich spectrum in both emission and absorption. Table I shows the relative absorption of both ground state and non-ground-state transition lines of cobalt (7) for a 50-
p.p.m. solution. There are eight lines which show a n absorption greater than 25% and could prove reasonably useful in atomic absorption cobalt determinations in the range of 0-100 p.p.m. The 2424.9 A., 2521.4 A., 2411.6 A, and 2407.2 A. lines would be suitable for analyses at the low p.p.m. range. The complexity of the cobalt hollow cathode emission spectrum necessitates the use of narrow slits to avoid the inclusion of radiation other than the analysis line. The absorption values shown were determined under conditions which do not represent maximum sensitivity and could be improved if necessary for specific determinations. Working curves were prepared for several cobalt absorption lines to determine their linearity over a particular concentration range. Figure 1 shows working curves run at a hollow cathode tube current of 25 ma. Most of the lines show a pronounced curvature
EXPERIMENTAL
Reagents. T h e cobalt stock solution was prepared from reagent grade CoC12.6H20 dissolved in distilled water. Working concentrations (0100 p.p.m.) were obtained by appropriate dilutions. Apparatus. The atomic absorption unit was a D.C., single beam system assembled from a Hitachi-PerkinElmer Model 139 spectrophotometer and a n optical bench containing the atomizer-burner, hollow cathode source, and associated equipment. The Westinghouse WL 22814 cobalt hollow cathode tube was powered by a Kepco Model Al3C 42511 0.05% regulated I1.C. poner supply (Kepco, Inc., Flushing, K . Y.), operated in the constant current configuration (Kepco Instruction Manual, &lode1 ABC 425M). X small beam, constricted by adjustable slits on each side of the burner, as collimated prior to passage through the flame and then focused on the monochromator entrance slit with bi-convex quartz lenses. A 10-cm. 1 168
a
ANALYTICAL CHEMISTRY
Table I.
Wavelength
% Absorption
4234.0 4190.7 3909.9 3526.8 3474.0 3465.8 3412.6 3121.4
ND ND
A.
Absorption of Selected Cobalt Lines
% Wavelength Absorption A. A. Ground state transitions ND 2424.9 3082.6 Wavelength A.
3044.0 3013.6 2989.6 2987.2 2928.8 2521.4 2435,8
ND
9.5 4.2 7.0 10.3
ND
Absorption %
2407,2 2384.9 2365.1 2309.0 2295.2 2274,5 2174,6
17.5
ND
5.2 3.4
ND
68.6 27.3
75.2 58.1
ND
ND 4.5 4.0 2.2
ND
B. Non-ground state transitions 3.8
3453.5 3442.9 3.0 3431,6 5.2 3405,l 4.2 2536.0 3.5 2529.0 Cobalt solution = 50 p.p.m.
3575 3533 3529. 3513, 3506 3502
ND
ND
=
not detectable
10.0 3.6 4.2 6.0 16.0 26.4
2439.0 2432.2 2419.1 2415.3 2411.6
Slit width = 0.10 mm. HCT current = 18 ma.
12.6 42.2 1.5 28.8 59.6
