682
VOL. 12, NO. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
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FIGURE 1. APPARATTX
diameter of about 0.2 mm., feed velocities of 0.12 t o 4.2 cc. per hour were obtained using sec-propyl and sec-butyl alcohols. I t is, of course, necessary to thermostat vessels H and K . This may be done by immersing them in Dewar flasks with ice. The buret, F , may be estimated to 0.01 cc. Since the coefficient of cubic expansion of most organic liquids is about 0.001, the buret temperature must be maintained to within 0.1" to allow volumes of liquid fed into the preheater to be known to within 0.01 cc. The author was able to achieve this degree of constancy for periods of an hour by passing a steady stream of tap water through the buret jacket. The buret is filled by forcing the mercury to the top by the application of compressed air through the connection a t J . The tip of D is broken off and inserted in a tube containing the feed liquid. Upon releasing the compressed air the mercury sinks, a t a rate controllable by tap G, drawing liquid into the buret. The liquid in the filling tube may be kept boiling during this process to eliminate dissolved air. This is necessary if the feed is to be used a t reduced pressures, in which case gas bubbles would be evolved in the liquid. The tip a t D is then resealed. The small quantity of gas remaining a t D may be dissolved in the liquid higher in the tube by heating with a small flame a little above D. Small bubbles of gas will disengage and will rise and dissolve. In starting a run, the pressure in H must equal the pressure in the apparatus plus the hydrostatic pressure of the mercury column. This may be adjusted by reference t o a manometer connected through J . The catalyst may be evacuated, provided the buret is emptied through D. The last traces of liquid may be removed by passing a reverse stream of a gas which does not affect the catalyst out through D with the mercury almost a t the level of the side tube to D. Warming this section of the tubing hastens this process. The constancy of the feed rate is good. For example, delivering 0.3 cc. in 10 minutes, successive deliveries in each period were constant to +0.01 cc. until a total of 2 cc. had been delivered. However: since the gas volume in H increases as mercury is driven out, the rate of feed for a given current through cell K decreases with the height of mercury. This effect is the smaller, the larger the gas volumes in H and K . It may be compensated by an occasional decrease in the resistance.
Literature Cited (1) Goldwasser, S., and Taylor, H. (1939).
e., J . A m . Chem. Soc., 61, 1260
Aspirating Unit for Collecting Air Samples LESLIE SILVER>I.iN, Harvard School of Public Health, Boston, 3Tass., AND WESLEY B. W.IRDLOW, Texas S t a t e H e a l t h D e p a r t m e n t , Austin, Texas
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N@IAKI?jG air analyses for industrial hygiene and other purposes, there is frequent need for a portable sampling unit which mill operate without a n external power source or where t h e direct use of electric pumps or blowers constitutes a fire or expbsive hazard. Zhitkova (1) describes such a unit', but it is not available in this country and is not of adequate capacity for sintered absorbers. The unit described here is easily handled and can be used for laboratory, field, or routine plant determinations.
Construction The unit is shown in Figure 1. It consists of two standard 19-liter (5-gallon) gasoline containers connected by two 10-cm. (4-inch) sections of 0.625-cm. (0.25-inch) brass pipe and bushings and a 0.625-cm. (0.25-inch) gate valve. The bushings are soldered into holes drilled into the filling caps. Two 3.75-cm. (1.5-inch) sections of 3.125-em. (1.25-inch) thin-walled brass tubing are soldered into the can bottom for the air inlets and waterlevel indicator. Copper tubing, 0.938-em. (0.375-inch), lvith brass draincocks (automobile crankcase draincocks) is used for the air inlet and wat,er-level indicator. These connections are placed in No. 6 rubber stoppers which are easily removed for filling. Small draincocks are soldered to each of the pouring caps for air removal.
The tanks are mounted in a frame constructed of two 73-em. (&foot) pieces of 1.9-em. (0.75-inch) angle iron 0.47-em. (0.1875inch) thick. The carrying handle lugs are bolted to the angle iron after removing the handles, A light metal 0.3 X 2.5 em. (0.125 x 1 inch) strap is fastened around each container and a 10-cm. (4-inch) wooden baseboard is drilled for the filling caps. These baseboards are connected t o the angle irons by means of corner braces. Thirty-centimeter (12-inch) lengths of angle iron are fastened to the ends of the angle iron to act as a base when the unit is removed from the wheeled stand. Thus, the unit can be used with or without this mobile section. The iron pipes are slotted, as indicated, to allow ready removal of the aspiration unit. The turnover catch shown is made from 0.3 X 1.25 cm. (0.125 x 0.5 inch) steel strap bent into a U-shape and pivoted, as shown. An elastic band serves as a spring allowing foot operation for turnover releasing and catching.
Operation The rate of flom varies slightly x i t h falling head, as shown in Figure 2. These curves were obtained using a large wet meter of very low resistance to air flow and with the gate valve completely open. The rate a t any time is found by dividing ordinate by abscissa. For practical purposes when sampling a t low rates (0.5 t o 1 liter per minute), it may be
NOVEMBER 15, 1940
ANALYTICAL EDITION
683
FIGURE1. DIAGRAM OF SAMPLING UNIT
1 1
sintered absorber (G-1 porosity 20-mm. disk) requires a slight opening of the valve each 5 minutes of operation. This was determined by using a vertical capillary flowmeter with 10-cm. (4-inch) pressure loss. Table I gives the maximum sampling period for various units and rates. The values listed are dependent upon the pressure-loss characteristics of the absorber. Reversal of the tanks with necessary connections requires less than a minute. For complete portability, a floor flange and a length of pipe may be fastened to the base to hold the absorber and, if necessary, a flowmeter. This allows the operator to sample anywhere on location or to sample with the unit in motion. For convenience in timing samples, a small egg timer (hourglass type) mag be mounted on the pipe. This timer should be calibrated with a stopwatch. Although the device is somewhat large, it can be readily handled by a single person and is easily transported in an automobile.
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Literature Cited (1) Zhitkova, A. S., “Some Methods for the De-
tection and Estimation of Poisonous Gases and Vapors in the Air”, Service to Industry, West Hartford, Conn., 1936.
TABLE I.
IXTERV.4LS O F T I M E FOR CONSTANT SAUPLIKG WITH V.4RIOUs ABSORBERS
Absorber Sintered glass (G-1 porosity 20-mm. disk) P e t r i bottle (4-mm. tube) hfidget impinger (I-mm. nozzle)
Liquid Volume CC. 50
RATES
Minutes of Sampling Possible at Various Rates 1 l./min. 1.5 I./min.
0.5 l./min.
30
5
..
50
35
17.5
11.5
10
35
17.5
10
kept constant by an occasional slight opening of the gate valve. This adjustment of flow is necessary in a blower system, because of resistance and voltage changes-for example, a sampling rate of 0.5 liter per minute with a FIGCRE 2.
DISPLACEhiENT-TIME CH.+R.+CTERISTICS OF UNIT Petri bottle waa filled with 50 cc of water which gave a liquid height of 5 cm. (2 inches). T h e sintered unit, a G-1 porpsity 20:mm. disk t y p e , for A was filled with 50 cc. a n d t h e liquid height waa 5.625 om. (2.25 inches), and B contained 100 cc. a t 11 25 om. (4.5 inches). T h e midget impinger had 10 cc. a t 1.56 cm. (0.625 inch).
