tration was kept constant a t 2.5 X lO-5M while the nickel(I1) perchlorate concentration was varied through 5 values from 1.25 X to 5.0 X 10-3M at each temperature. For both sets the resulting plots of pseudo-first order rate constants (reproducibility 1 2 %) were linear and passed through the origin at all temperatures. However, the slope of the one Arrhenius plot is 13% lower than that of the other, and is consistent with a deviation of the effective valve block temperature (Tb)from the thermostat temperature (T,) given by: 7'' T , - 0.13(Tt - To),where T, is the ambient temperature. It is unlikely that this simple relationship will apply over a much wider temperature range than that studied here. It was verified by removing a valve and inserting a thermocouple into a flow channel that the solution there was closer to ambient temperature than was the thermostat liquid. However, it is difficult to measure meaningful temperatures in this way, because the thermal conductivity of the surroundings under actual experimental conditions is not readily simulated in the absence of the valve and in the presence of the thermocouple; furthermore, a longitudinal temperature gradient exists in each flow channel, with the temperature deviating more and more from that of the thermostat with increasing distance from the drive syringe. Consequently, additional tests were carried out at a thermostat temperature of 45.0 f 0.1 "C and an ambient temperature of 27 i 1 "C. Using a
-
0.20-ml cuvette and dispensing first 0.21, and then 0.36 mlper channel, the apparent rate constants were 7.1 X lo3and 7.8 X l o 3 1. mole-' sec-I, respectively. On changing to a 0.07-ml cuvette and using a second pump with intake from the thermostat well to circulate liquid around the cuvette (as recommended by the manufacturer), dispensing 0.21 and 0.36 ml per channel gave apparent rate constants of 7.6 X lo3 and 8.0 X lo3 1. mole-' sec-', respectively. Varying the level of the liquid in the thermostat well from two-thirds full to completely full, and making measurements a t intervdls from hour to 2 hours after the thermostat liquid had reached a temperature of 45.0 "C, had no discernible effect. Finally, solutions at ambient temperature introduced into the drive syringes reached a temperature of 44.9 "C within 15 minutes. From the above results, the best value of the rate constant at 45 "C is 8.0 X 1031.mole-lsec-l. We conclude that when instruments of the design described here are used at other than ambient temperatures, sufficient volumes of reactant solutions should be dispensed t o ensure that all of the solution being measured has originated directly from the drive syringes.
RECEIVED April 14, 1972. Accepted May 23, 1972. Financial support by the National Science Foundation under grant GP-16342 is gratefully acknowledged.
Simple Inexpensive Flowmeter for Use with Fluorine and Other Corrosive Gases A. B. Waugh Chemistry Department, Unicersity of Melbourne, Parkdle, Victoria, Australia 3052
P. W. Wilson Chemical Technology Dicision, Australian Atomic Energy Commission, P.M.B., Sutherland, N.S. W., Australia 2232 MOSTFLOWMETERS for use with fluorine are both expensive and unreliable, and moreover can often only be used over a narrow range of flow rates. The simple differentialmanometer type flowmeter described here is made from materials resistant to fluorine. In the literature, a number of differential-manometer flowmeters are described, but their restricting capillaries are usually made from glass tubing and therefore are not suitable for use with fluorides (1). Capillaries made of materials other than glass normally have to be purchased and are expensive. The material most suitable for use in a fluorine resistant flowmeter is polytetrafluoroethylene (Teflon, Du Pont), but n o simple way to make a small hole in this material was available previous to this work. A method for making small holes in Teflon has been described but it is complicated and time-consuming (2). EXPERIMENTAL
Figure 1 is a diagram of the flowmeter we have used successfully. The U-tube is made from l/& 0.d. Kel-F tubing and is filled with Kel-F oil. The T-unions are Monel metal and the inlet and outlet tubes are nickel. Presumably, other materials could be used in these components depending on the particular gas being metered. ( I ) G. Brauer, "Handbook of Preparative Inorganic Chemistry," 2nd ed., Academic Press, New York, N. Y . , 1963, p 84. (2) H. P. Raaen, ANAL.CHE44.. 34, 1714 (1962). 2118
TEFLON CAPILLARY RLSTRICTIOII
I
Figure 1. Fluorine-resistant differential manometer The restricting capillary is made as follows (Figure 2). With a small drill (approx. l / d n . diameter) holes are drilled in both ends of a piece of Teflon rod until only a l / d n . thick plug remains in the center of the rod (Figure 2 4 . Metal needles are inserted into these holes. One needle is grounded; the other needle is connected t o a high-frequency discharge coil, in our case, Edwards H. F. Tester, Model T.l. (Figure 2B). The high-frequency coil is turned on and is held in position until a discharge occurs through the Teflon plug. A discharge within the Teflon rod is easily seen since it emits a bright purple glow. If a low flow rate is required, the coil is removed quickly. If the discharge is maintained for a longer period, a larger hole is made and the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
flow rates of 0.5-3.0 cma mine’. A discharge time of 20 seconds gave a hole of 220-ptm diameter and the capillary was used to measure flow rates of 50-200 cma min-1. The flowmeter must be calibrated by normal methods after it is assembled.
1tDRILLED HOLES
CONCLUSIONS
A WIGH*FREQUWCY COIL
Teflon capillaries, prepared as above, should have a number of uses in addition to their use in differential-manometers. Two obvious uses are in dropping mercury electrodes (3), and as controlled leaks in mass-spectrometer inlets.
ACKNOWLEDGMENT B
The authors acknowledge discussions with R. N. Whittem and practical assistance from J. H. Levy.
Figure 2. Diagrams that show how Teflon capillary is prepared capillary so formed can be used to measure higher flow rates. If several capillaries of different hole size are prepared by using different discharge times, flowmeters can be constructed to cover all flow rates encountered in normal laboratory work with fluorine. A discharge time of 1 second produced a hole of 70-gm diameter and this capillary was used to measure
RECEIVED for review March 2,1972. Accepted May 22,1972. (3) A. Bond, T. A. O’Donnell, and A. B. Waugh, ANAL. CHEM., in
press.
Coulometric Calibration of a Thermal Conductivity Detector for Oxygen and Nitrogen W. Gary Williams and Dayton E. Carrittl Nova University Oceanographic Laboratory, 8000 North Ocean Drive, Dania, Fla. 33004 A TECHNIQUE HAS BEEN DEVELOPED which enables the quantitative introduction of either oxygen or nitrogen into the sample stream of a gas chromatograph. The method has facilitated the calibration of a thermal conductivity detector for a continuous range of sample sizes of these gases. Either oxygen and hydrogen, or nitrogen and hydrogen, are generated in a coulometer within the gas chromatograph. The calibration gases are desorbed into the sample stream by bubbling the carrier gas through the electrolyte for a five-minute period commencing with the end of electrolysis. A molecular sieve column separates the gases and allows oxygen and nitrogen retention times sufficient to prevent interference from the desorption pressure peak. In these studies, sample sizes of oxygen and nitrogen between 0.25 and 5.00 micromoles were generated and injected, although it certainly should be possible to extend this range in either direction. The accuracy of calibration curves determined by this technique is confirmed by the correct analysis of two different standards. EXPERIMENTAL
Apparatus. A Varian Aerograph 90 P-3 gas chromatograph modified for the analysis of dissolved gases in aqueous solutions is used (Figure 1). The desorption chamber (Figure 1, insert) is like that described by Swinnerton, Linnenbom, and Cheek ( I , 2) in their papers on injecting dissolved gases by stripping the gas-liquid mixture with the carrier, except for Present address, Institute for the Study of Man and His Environment, A-305 Graduate Research Center, University of Massachusetts, Amherst, Mass. 01002. (1) J. W. Swinnerton, V. J. Linnenbom, and C. H. Cheek, ANAL. CHEM., 34, 483 (1962). (2) Ibid.,p 1509.
the addition of a pair of one-centimeter square bright platinum electrodes one centimeter above the glass frit (3). The constant current source is a Leeds and Northrup Coulometric Analyzer (Satalog No. 7960). The 0.25-in. X 30-ft molecular sieve 5 A column, which is operated a t - 17 OC, was originally designed for the separation of argon and oxygen. The molecular sieve is activated by heating the column to 400 “C while purging with dry nitrogen for 12 hours. With a helium carrier flow rate of 60 ml/min, retention times for hydrogen, oxygen, and nitrogen are 10,24, and 180 minutes, respectively. A Leeds and Northrup Speedomax-W equipped with a Disc Integrator records and integrates the signal from the hot wire thermal conductivity detector. The precision of the gas chromatograph is f1 as measured by the relative standard deviation of five air injections from a gas sample valve. Procedure. The procedure described by Lingane (4) regarding oxygen-hydrogen and nitrogen-hydrogen coulometry is followed. A 0.5M K2S04electrolyte is used for the generation of oxygen and hydrogen and 0.1M hydrazine sulfate for nitrogen and hydrogen. The overall electrolysis reaction for the hydrogen-oxygen coulometer is
If the minimum current density of 50 mA/cm2is observed, the experimental yield of oxygen is 0.2498 gmole/pequivalent. For the nitrogen-hydrogen coulometer, N2H5+ is oxidized at the anode
+ 5H+ + 4e-
N2H5+ + NZ(g)
(2)
(3) W. G . Williams, “A System for the Analysis of Dissolved Oxygen, Nitrogen and Argon in Natural Waters,” M.S. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1968. (4) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed., Interscience Publishers, New York, N.Y., 1968, pp 452-7.
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