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Anal. Chem. 1985, 57,389-390
Flowmeter Device for Measuring Slow Gas Flow Rates Avy Ventura and Joost Manassen* Department of Plastics Research, Weizmann Institute of Science, Rehovot 76100, Israel Various slow gas flow (1)and liquid (2) flowmeters have been described in the past, but their lowest rate measurements are about 50 mL/h (3). In our study of photocatalytic reactions to be published elsewhere, the rate of nitrogen evolution was used to measure the reaction efficiency. It was necessary to develop the device described below, which allows differential determination of gas flow rates in the range 1 mL/h to 300 mL/h with an accuracy of 1% (5% a t the lowest flow rates). Computer storage of data, calculation of averages, t)"d plotting of the gas flow rates allow for an automatic control of the gas evolution, as well as real-time monitoring.
EXPERIMENTAL SECTION A U-shaped glass tube (8 cm high, 0.5 cm i.d.), partially filled with mercury is shown in Figure 1. Three Pt electrodes are used for the electrical contacts. A solenoid-activated valve (Skinner Europa, Roosendaal Holland 24V/6W) tightly closes the righthand side of the U-tube. The measured gas enters the U-tube from the right side and causes a pressure buildup. As a result, the mercury rises on the left side from a position below electrode C, to reach the upper electrode B, causing the valve to open and thus allowing a certain volume, dV, of gas-air mixture to escape. The mercury level then falls below electrode C, which signals the valve to close again. A capillary is used at the bottom of the U-tube to minimize oscillations of the mercury (damping effect). The amount of gas freed each time the valve opens is a function of the distance between electrodes B and C and can be regulated by the displaceable electrode B. An electronic device (4011BE) is used to keep the valve closed/open during the ascentldescent of the mercury. A toggle flip-flop (4027AE)is used for the output to the computer, which maintains a constant voltage on the output between consecutive periods of the valve. The overpressure in the U-tube is sensitive to temperature changes, which makes it essential for the entire device to be kept at a constant and well-defined temperature during operation. The computer system included an Apple I1 with an AID interface card from Mountain Computers, Inc.
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Flgure 1. Schematic representation of flowmeter device for measurement of slow gas flow evolutlon: A, 8, C, pt electrodes;D, valve; E, reactor (or pressure equallzer); F, mercury; G, electronic box; H, recorder; I, computer; J, capillary tube.
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RESULTS AND DISCUSSION An output as produced by the computer is shown in Figure 2. The differential variation with time of the gas flow is presented. It is apparent that flow rates as low as 1 mL/h are measured with reasonable accuracy. In actual practice, a direct x-t recorder output is also produced, which only shows the frequency at which the valve is opened, to check the proper functioning of the system. The calculation of the gas flow rate is based on two parameters, namely, the period of the valve and the gas volume, dV, released each time. The time events are stored by the computer and, using calibration data, are converted directly to flow rates. The optimal time difference between two openings of the valve was found to be approximately 20 s. Data points are collected every 0.2 s, to allow 8 h of data collection with regular floppy-disk storage. Thus with a 20 s valve period we achieve accuracies of 1% in the time difference measurements; however, a better accuracy can be obtained simply by increasing the frequency of the data collection (for shorter experiments or if a hard disk is available). In addition, electrode B can be set higher, as a result of which the freed volume d V as well as the valve period are increased. A gain in plotting accuracy is then achieved, but at the cost of resolution loss. Calibration curves for various flow rates are shown in Figure 3. The evacuated gas volume d V was chosen so that for gas flow rates of 5, 20, and 50 mL/h, the valve period was approximately 20 s (for d V = 0.02,0.11, and 0.25 mL, respec0003-2700/85/0357-0389$01.50/0
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Figure 2. Computer output of flow rate varlations wCh time. Flow rates (mL/h) were as follows: 60, 50, 40, 30,20, 10, 5 (A); 5, 2.5, 1 (B).
tively). The curves show the optimal range to be used for a given d V and the corresponding corrections to be taken into account. For d V = 0.02 mL (0.11mL, 0.25 mL), the optimal range is 1 to 10 mL/h (5 to 60 mL/h, 30 to 150 mL/h). The error due to the inertia of the mercury is not substantial in these ranges. The calibration curves were constructed using 0 I984 American Chemical Society
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Anal. Chem. 1985, 57,390-391
Flow Rate lml/h)
Figure 3. Callbratbn curves for different flow rates. The freed volume d V (mL) was 0.02 (U), 0.1 1 (+I, and 0.25 (0). water electrolysis and checked with a soap-bubble flowmeter (using argon). The latter is only reliable for flow rates above 20 mL/h. The curves are corrected to 20 "C and atmospheric pressure.
The flowmeter device described herein allows direct measurements of gas evolved in a reactor or (using a pressure equalizer) from any other source (see Figure 1).It is essential that the reactor (or pressure equalizer) be a t the same temperature as the flowmeter, and that the pressurized gas volume (V) always be the same (both when measuring and calibrating). This volume V (about 20 mL in our case) can be reduced when measuring very slow gas flow rates. The flowmeter then allows measurement of flow rates down to 1mL/h (with 5% error due to large valve periods), as was done in obtaining the results presented in Figure 2b. Flow rates higher than 300 mL/h are difficult to measure, due to strong oscillations of the mercury in the U-tube. This can be overcome by using a larger device, which will shift the range of measurable flow rates to higher values.
LITERATURE CITED (1) Kowalik, Zbignlew Pol. PL. I-14,219 (CI. GOlF3/00),1982 (Chern. Abstr. 1983, 98, 36651a). (2) Neth. Appl. N181 05,339 (Cl. G01P5/68), 1982 (Chem. Abstr. 1983, 98. 915419). (3) Coudhary, V. R.; Rajput, A. M. Res. Ind. 1977, 22, 86-88.
RECEIVED for review May 29,1984. Accepted August 8,1984.
Preparatlon of Solutions of Tracer Level Plutonium(V) A. Saito, R. A. Roberts, and G. R. Choppin* Department of Chemistry, Florida State University, Tallahassee, Florida 32306 Studies of the environmental behavior of plutonium have indicated the importance of Pu(V) in many natural systems (1). Laboratory investigations of the parameters determining the stability of Pu(V) relative to other oxidation states are hampered by the difficulty of producing this oxidation state in the absence of the others (Le., the 111, IV, and VI states). Redox agents can be used but their presence introduces uncertainty in the role they may play in interfering with the natural Ehof the systems under study. In this paper we report a method of preparation of Pu(V) solutions a t tracer concentrations initially free from contamination by other oxidation states of plutonium and by redox agents. The method consists of extracting Pu(V1) from an aqueous solution into a solution of thenoyltrifluoroacetone (TTA) in cyclohexane, The separated organic phase is irradiated by fluorescent light, causing reduction of Pu(V1) to Pu(V) and Pu(1V) (2). The Pu(V) is stripped into an aqueous phase at p H 4-5 (3).
EXPERIMENTAL SECTION Reagents and Stock Solutions. Plutonium stock solutions were prepared in 1.0 M "OB (ca. 10-6-10-8 M Pu). Thenoyltrifluoroacetone was purified by vacuum sublimation at 45-50 "C and stored in the dark until use. Immediately prior to use, a 0.5 M solution of "A in cyclohexane was prepared; this solution was protected from exposure to light at aJl times, and new solutions were prepared for each experiment. All solutions were filtered through a 0.45-pm filter before use to remove particulate matter, and all glassware was thoroughly cleaned to remove all redox contaminates. Procedure. (a) Oxidation of Pu. Transfer a 1OO-pLaliquot of the stock plutonium solution to a glass vial with a leakproof cap (e.g., a glass-liquid scintillation vial with a Polyseal cap). To this add 10 p L of 0.01 M KMn04 sohtion and allow the solution to stand for at least 6 h in the dark.
TIME ( h i )
Figure 1. Photoreduction of Pu(V1) to Pu(V) and Pu(IV) In 0.5 M TTA Solution In cyclohexane as a function of time. (b) Extraction of Pu(VZ). Add 2 mL of 0.1 M sodium acetate solution of pH 4.7 and 2 mL of the 0.5 M TTA in cyclohexane to the oxidized plutonium solution, and shake the vial for about 10 min. During this step, it is important to keep light from the solution; this can be accomplished by wrapping the vial with aluminum foil. Separate the phases by centrifugation,and transfer the organic phase to a clean vial. The aqueous phase may be discarded or counted to determine the amount of activity not extracted. (c) Photochemical Reduction of Pu(V0. Irradiate the Pu(VI)-TTA solution. Fluorescent room light can be used (about 2 h for maximum yield), but the irradiation time can be reduced by placing the sample close to a fluorescent light (e.g., use 7 min of irradiation at 15 cm from a G. E. F15T8 Cool White lamp). (d) Back Extraction ofPu(V). To the irradiated solution, add 1mL of an aqueous solution of pH >4 (e.g., 0.10 M NaOAc) and
0003-2700/85/0357-0390$01.50/00 1984 American Chemical Society
