Anal. Chem. 1963, 55, 1836-1837
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that the magnitude of disagreement between the University of Urbino and OGC is so small as to be inconsequential for comparison of atmospheric measurements, being a t most 5 pptv out of 35 or 40 pptv. The two laboratories agree on the concentrations of F-21 at 35-100 pptv levels, to 1 1 3 % (see Table I). We conclude that both laboratories agree on the identification of F-21 in air samples and agree on its absolute concentration down to about 35 pptv. Whether the concentrations of F-21 in the background troposphere are 1-2 pptv (5) or 10-20 pptv ( 6 , l O ) cannot be reconciled on the basis of difficulties in identification or differences of absolute accuracy.
-20 --II2 6
2 O
N
D
J
F
M
A
M
J
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A
S
O
T I M E (months)
The stability of CHCI,F (F-21) in high-pressure calibration tanks. The initial concentrations (C,)in the tanks were as follows: (0) tank 175, 112pptv;(O)tank 178,94pptv;(O)tank181,48pptv;(o) tank 182, 31 pptv; (A)tank 184, 30 pptv. The percent change is lOO(C(t)- C,)/C,,where C, is the lnitlal concentration as described above and C(t) Is the concentratlon measured at later times. Flgure 1.
higher concentrations (53%) compared to the tank with 35 pptv (516%).
RESULTS AND DISCUSSION The results of the interlaboratory comparison between OGC and the University of Urbino are summarized in Table I. The two laboratories agree on the concentration of sample 1with about 94 pptv of F-21. This agreement is also enhanced by the results from A.E.R.E. a t Hanvell, England. Furthermore, with the precision of measurement, no systematic difference was found in the concentration before the sample was sent out (Ci) and after it was returned (CJ several months later. For sample no. 2 with a smaller concentration of F-21 (35 pptv) there was some disagreement between the two laboratories. It appears, however, that the concentration of F-21 may have drifted upward in this tank during the course of the experiment as shown in Table I, where AS = lOO(Ci - C,)/Ci is found to be significantly greater than zero. The disagreement between the laboratories may be due to this drift since measurements of F-21 at Urbino were done a t times between the initial and final measurements a t OGC, and show concentrations between the initial and final concentrations determined a t OGC. More importantly) the results demonstrate
ACKNOWLEDGMENT We gratefully acknowledge the collaboration and participation of S. A. Penkett of the Environmental & Medical Division of A.E.R.E. Harwell, U.K. Registry No. F-21, 75-43-4. LITERATURE CITED Molina, M. J.; Rowland, F. S. Nature (London) 1974, 249, 810-812. Natlonal Research Council "Stratospheric Ozone Depletion by Halocarbons: Chemlstry and Transport"; Natlonal Academy of Sciences: Washington, DC, 1979. Ausloos, P.; Rebbert, E. R.; Glasgow, L. C. J. Res. Natl. Bur. Stand. ( U . S . )1977. 8 2 . 1-10, herotii, D.; Rasmussen, L. E.; Rasmussen, R. A. Geophys. Res. Lett. 1878, 5 , 1001-1004. Penkett, S. A.; Prosser, N. J. D.; Rasmussen, R. A,; Khalil, M. A. K. Nature (London) 1980, 288, 793-796. Crescentlnl, G.; Bruner, F. Nature (London) 1979, 279, 311-312. Slngh, H. B.; Salas, L.; Shlgelshl, H.; Crawford, A. Atmos. Environ. 1977, 2, 819-825. Slngh, H. B.; Salas, L. J.; Shigeishi, H.; Scribner, E. Sclence 1979, 203,899-903. Crescentini, G.; Bruner, F. Ann. Chlm. (Rome) 1980, 70, 631-636. Bruner, F.;Crescentini, G.; Mangani, F.; Brancaleoni, E.; Cappiello, A.; Ciccioli, P. Anal. Chem. 1981, 53, 798-801. Rasmussen, R. A,; Khalil, M. A. K. I n "Proceedings of the NATO Advanced Study Instltute on Atmospheric Ozone: Its Variatlon and Human Influences"; US. Department of Transportation: Washington, DC, 1980; pp 209-231. Rasmussen, R. A.; Khalil. M. A. K. Atmos. Envkon. 1981, 15, 1559-1568. Bruner, F.; Bertonl, G.; Crescentini, G. J. Chromatogr. 1978, 167, 399-407. Snedecor, G. W.; Cochran, W. G. "Statistical Methods", 7th ed.; Iowa State Universlty Press: Ames, IA, 1980; Chapter 6.
RECEIVED for review Apirl 1, 1983. Accepted June 7, 1983. This work was supported by a contract from the Chemical Manufacturers Association (CMA) (Contract No. FPP-813568, FPP-81-365).
Electrochemical Cell for Low-Temperature Voltammetry Tsutomu Nagaoka*.and Satoshi Okazaki Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto-shi 606, J a p a n Recently, low-temperature electrochemistry has become an important technique in studying the structure, thermodynamics, and kinetics of electrogenerated species (1-5). However, measurement systems employed in these studies were often complicated and inconvenient. In this paper, we present a more simplified cell system for low-temperature electrochemistry (>-lo0 OC) which can be used with a PAR 303 static mercury drop electrode (SMDE). A rather complicated method has been used in low-temperature voltammetry in order to cool the electrolysis cell. Compressed dry nitrogen gas is passed through a copper coil immersed in a liquid N2 (LNJ Dewar. The temperature of
the cryostat is regulated by adjusting the cooling-gas flow rate and the voltage applied to a small heater which is located in the gas inlet line between the cryostat and the LN2 heat exchanger. In this study, we adopted a more simple cooling method as follows: A small heater is immersed in the LN2 Dewar and the cooling gas flow rate is simply regulated by adjusting the switching frequency of the heater. Further, the cryostat was made smaller so it could be used with the SMDE.
EXPERIMENTAL SECTION Figure 1 shows the electrolysis cell for the PAR 303 SMDE. The jacket surrounding the cell was made of Kel-F and a stream
0003-2700/83/0355-1836$01.50/00 1983 Amerlcan Chemlcal Soclety
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Anal. Chem. 1983,55, 1837-1839
1.0 cm
I C
-
t
_7_
--J
D
Flgure 1. Vertical section of the jacketed cell for low-temperature electrochemistry: (A) JIS 1'56 O-ring (Viton), (e) Pyrex glass cup, (C and D) inlet and outlet of tlhe coolant.
small heater (5 Q) was immersed in the LN2 Dewar. The voltage applied to the heater was 10-20 V ac and a solid-state relay was used to switch it. The switching noise generated from the relay was negligibly small with the aid of a zero-crossing detector incorporated in the relay. In order to regulate the cell temperature, the temperature of the cooling gas was monitored at the end of the cooling gas line with a temperature sensor AD590F (Analog Devices, MA). The above lineup provided a very simple cooling system because it required only a voltage comparator and the relay as circuits regulating the cell temperature. A copper-constantan thermocouple was inserted into the cell in order to obtain the exact temperature of the sample solution. The reference electrode used was Ag/Ag+ (10 mM) 0.1 M tetrabutylammonium perchlorate.
+
to Cell
RESULTS AND DISCUSSION When the temperature was
