pleted and before determining the amount of arsenic collected (Amer. Public Health Assoc., 1967), another 100 ml of cold distilled water was added. The HC1 was thereby diluted and overly violent reaction with the arsenic determination reagents was avoided. Discussion
We conclude that the literature was incomplete in its specification of the distillation method. The acid concentration in particular influences the results quite markedly, based on our results. In applying the results reported here to analysis of drinking water supplies and surface water samples, it must be remembered that all tests involved the same values of V and m. Thus the functionality indicated above has not been completely determined. In particular we did not have the opportunity to vary Vwith m held constant-e.g., at 15 pg. The maximum desirable amount of arsenic in drinking water is 0.01 mg/l. and the maximum safe amount is 0.05 mg/l. (Kopp and Kroner, 1967). Contribution of arsenic by reagents and other normal considerations in both the distillation procedure and in the determination of the amount of arsenic collected (Amer. Public Health Assoc., 1967) dictate that as a practical matter on the order of 10 pg of arsenic be present. Thus in the case of drinking water at or below the desirable arsenic level a sample size of 1 liter or more would be necessary. For 1-liter samples it would not be practical to add concentrated HCl, which is itself over 6 0 z water. A possibility is to bubble HC1 gas into the sample until the acid concentration is on the order of 30 wt or more. It might be best to do this at a very low rate of bubbling, before adding the cuprous
chloride, and with the sample kept cold (-0OC). Once the desired acid concentration was reached, the cuprous chloride could be added and distillation could begin. Literature Cited Amer. Public Health ASSOC.,Inc., “Standard Methods for the Examination of Water and Wastewater,” pp 56-8, New York, N.Y., October 1967. Baumhardt, G. C., Eng. Quim (Rio de Janeiro), 5 ( 5 ) , 10-11 (1953); C A , 48, 3833a (1954). Bonner, W. D., Wallace, R. E., J. Amer. Chem. Soc., 52, 1747-50 (1930). Furman, N. H., Ed., “Standard Methods of Chemical Analysis,’’ 6th ed., Vol 1, p 119, Van Nostrand, New York, N.Y., March 1962. Hillebrand, W. F., Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed., Wiley, New York, N.Y., 1959. Hodgman, C. D., Weast, R. C., Selby, S. M., Eds., “Handbook of Chemistry and Physics,” 39th ed., p 2156, Chemical Rubber Publ. Co., Cleveland, Ohio, 1957. Kopp, J. F., Kroner, R. C., “Trace Metals in Waters of the United States,” p 19, U S . Department of the Interior, Cincinnati, Ohio, 1967. Manufactures de produits chimiques du Nord, French patent, 1.001.712. Feb. 27.1952: CA.51.6102h. (1957). Scherrer, J.’ A., U.S. Dept. of Comme;c‘e, Research Paper R P 871, J . Res. Nat. Bur. Std., 16 (March 1936). Scott, W. W., Ed., “Standard Methods of Chemical Analysis,” 5th ed., Vol 1, pp 91-3, 106, Van Nostrand, New York, N.Y., August 1947. Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,’’ 3rd ed., Vol 2, Van Nostrand, New York, N.Y., August 1967. Received for review April 26, 1972. Accepted July 24, 1972. Financial support for this work was obtained from departmental funds of the McGill Department of Chemical Engineering and from National Research Council of Canada operating grant A-3673.
Particle Counting by Glow Discharge Perturbation Nicholas A. Fedrick, Alexis T. Bell,’ and Donald N. Hanson Department of Chemical Engineering, University of California, Berkeley, Calif. 94720
A constricted glow discharge has been assessed for its use as a means for counting and sizing airborne particulate matter. The device consists of a tube in which two electrodes, separated by a disc containing a small orifice, are mounted. Passage of a particle through the orifice causes a perturbation in the discharge current. The magnitude of these perturbations has been found to be proportional to particle diameter.
M
easurement of the size and concentration of particulate matter in various media has become an increasingly important problem. Considerable effort has been expended on the development of various methods for these measurements. Of the devices proposed and used, one of the simplest is the Coulter counter (Coulter, 1953), which is extensively used to count and size particles in liquids. The use of the Coulter counter, however, is restricted to particulate To whom correspondence should be addressed.
matter suspended in a liquid. If such a counter is to be used for analyzing gas streams, the particulate matter must first be collected and then introduced into the liquid. This procedure necessarily involves uncertainties in collection efficiency and agglomeration. It would be desirable therefore to count the particles directly in the gas phase. This could be accomplished if the gas were made conductive through its partial ionization. The present note describes a technique by which it is possible to develop a gas-phase analog to the Coulter counter through the use of an electric discharge. The device described here makes use of a dc glow discharge maintained between two electrodes separated by a disc containing an orifice of approximately the same size as the particles to be counted. If a particle of solid material is allowed to pass through the orifice, a momentary interruption occurs in both the discharge current and voltage. These perturbations which can be viewed on an oscilloscope can be used to determine the particle size and number. Experimental Apparatus and Discussion The experimental apparatus is shown diagrammatically in Figure 1. The particle counter consists of four glass sections, Volume 6, Number 13, December 1972 1117
Figure 2. (left) Current pulse measurements
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mated to each other by standard taper ground-glass joints. The lowest glass section contains a molybdenum electrode which is connected to the negative output terminal of the power supply. The second glass section functions as a support for a stainless steel disc, 100 p thick, 1 in. in diam containing a 153-p hole drilled through the center. The orifice is affixed to the tube with Silastic cement. The third glass section consists of a support for the powder feeder and a length of 3-mm i d . glass tubing used to direct the particles toward the orifice once they have sifted through the feeder. The lower portion of this section houses a tungsten ring-electrode, which is grounded and serves as the anode. The uppermost glass section connects to the gas supply manifold. The particulate matter used in this work consisted of sized glass beads. Beads having nominal sizes of 50 and 100 p were sieved before use in order to assure a minimum deviation from the mean diameter. Observations of 25 beads through a microscope showed the following spread in bead diameters : Nominal 50 p : high, 56 p ; low, 44 p ; av, 50 p Nominal 100 p : high, 112 p ; low, 88 p ; av, 105 p For the experiments, these beads were loaded into a powder feeder consisting of three fine-mesh screens individually sandwiched between flat plastic washers. By gently tapping the feeder a small number of beads could be caused to fall through the screens and into the glass tube leading to the orifice. Because of the pressure differential existing across the orifice the beads were swept through the aperture and out of the system. The passage of each bead through the orifice produced a perturbation in the discharge current. These perturbations were observed as voltage pulses (with a Tektronix Model 502A oscilloscope) across a 100-ohm load resistor connected in series with the discharge. To isolate the base dc current, the connection to the oscilloscope was made through a capacitor and the oscilloscope was used in its ac mode. Permanent records of the pulses were made by photographing the oscilloscope trace with a Polaroid camera. Results and Conclusions Preliminary experiments were performed without the particles to determine the optimum pressures and current to be used. It was concluded that a stable discharge could be maintained using the following conditions : pressure upstream
1118 Environmental Science & Technology
1
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Figure 3. Correlation between current pulse height and particle size
from the orifice = 20 torr, pressure downstream of the orifice torr, and current = 2.1 mA. =2 X When glass beads were placed in the feeder and then allowed to fall through the orifice a perturbation could be observed in the oscilloscope trace. The decrease in current was noted as a percentage of the base current. Figures 2a-c are typical photographs of the pulses recorded in such a manner that an upward pulse indicates a decrease in current. As may be seen from Figure 2c the pulse heights for a given particle size are quite uniform. A small amount of noise can also be observed in the photographs as very rapid symmetric pulses. Although it was not possible to eliminate these noise pulses with the experimental arrangement used, they were readily distinguishable from the pulses due to the passage of a particle through the orifice. Each of the pulses is characterized by a sudden rise to a maximum value and a more gradual recovery to the base signal. Average pulse times for both the 50 and 10% beads were on the order of 1 . 5 msec. Figure 3 shows a plot of the average pulse height VS. the average bead diameter. For each bead size the spread in the current pulses observed is indicated by the vertical error bars, and the spread in measured bead diameters is indicated by the horizontal error bars. The magnitude of the variations from the mean value of the current ranged from 11 to - 8 for the 50-p beads and from $10 to -20% for the 105-p beads. The spread of particle diameters was i11 for the 50-p beads and +7 to -16% for the 105-p beads. The extent of variation in current pulse height and bead diameter are in very close agreement with each other. From Figure 3 it can be observed that there is a linear relationship between pulse height and particle diameter. Whereas the device with its present orifice of 1 5 3 pis limited to detecting particle diameters on the order of 10 p and larger, the principle of the device could be extended to smaller particles through the use of a smaller orifice. Preliminary results have already indicated that it is possible to sustain a discharge through a 30-p orifice.
+
Literature Cited Coulter, W. H., U. S. Patent No. 2,656,508 (1953). Received for reciew May 12, 1972. Accepted August 21, 1972. Financial assistance qf the National Science Foundation through Grant GK-11929 is gratefully acknowledged.
