Excellent results were obtained using this device as is seen in Figure 3, in which polarograms of a 4 X 10-4MCd2+in 4 x 10-2M sodium dodecyl sulfate solution prior to and after degassing with prepurified nitrogen (saturated with water vapor) for 10-15 minutes are superimposed on one another. In this case the flow rate of Nz used to break the foam was greater than that used t o deoxygenate the sample. The complete removal of oxygen is indicated by the absence of a peak in the region of -0.4 to -0.5 volt us. SCE, the region in which we
have consistently observed the oxygen peak in our surfactant solutions. The degassing disk developed can be readily adapted to any type of polarographic cell and can be fabricated out of any easily machined material. While Lucite is satisfactory for aqueous solutions, it must be used with caution, if at all, in other systems. RECEIVED for review April 26,1971.
Accepted July 2,1971.
Exhaust Gas Collector for Mechanical Vacuum Pump J e r r y L. Carter a n d Donald R . Davis Department of Chemistry, University of California, Ircine, Calif. 92664
THEDEVICE DESCRIBED collects oil-free gas from the exhaust of an oil-filled mechanical vacuum pump with a low dead volume. Its low dead volume and transparent construction minimize the mixing and dilution which occur in the large, unobservable dead volume of the unmodified pump. Low dead volume and observability are critical if the gas composition is changing rapidly and an analysis or collection of it is desired. In our case, the collector is used to permit analysis by gas chromatography of gases pumped from a gas flow reactor, similar to the type used by LeRoy and coworkers (1,2). The collector is of simple construction, mounting, and use. Normal pump operation is maintained except for a restriction against unexpected increases in exhaust gas flow rate. Reduction of physical dead volume by factors of 10 to 100 is achieved. The essential features are: An inverted cone (apex angle roughly 110”) with a base diameter sufficient to collect all rising gas bubbles; construction of transparent material which permits easy observation of the bubbles and oil surface and easy adjustment and measurement of dead volume; and, to remove oil mist, a short, transparent exit tube at roughly 45” from the vertical, with a visible filter or absorbant trap. A desirable protection for the pump is a pressure relief device in case the gas exit might become plugged or overwhelmed. The device requires a pump with a removable plate over the exhaust oil reservoir, such as is manufactured by Precision Scientific Co. (Chicago, New York, and Los Angeles) in a variety of sizes. The following example is for the Precision Model 150 two-stage pump which has a rated displacement of 150 l./min and a normal dead volume of about 600 ml. A collector built for this pump is illustrated in Figure 1 ; it is installed in place of the standard exhaust plate. With the pump running at the normal oil temperature of about 70 “C, the oil level is adjusted (by adding oil cia the cone apex or draining oil from the pump) to suit the desired dead volume and exhaust gas flow rate. The results obtained with hydrogen gas are given in Table I. The volume given is (cone base area) X height/3 and does not include the volume of the exit tube or filter, about 3 ml as shown in Figure 1. For flow rates less than 100-200 ml/ min, one might reduce the exit tube-filter volume to about 1 ml. The indicated times 1.6-3.6 sec are a measure of the time (1) W . R . Schulz and D. J. LeRoy, Can. J . Chem., 42, 2480 (1964). ( 2 ) W. R. Schulz and D. J. LeRoy, J. Chem. Phys., 42, 3869 (1965).
Figure 1. Exhaust gas collector Lucite block Pressure relief valve (Circle Seal) c. Mounting bolt holes to fit pump d. Tube to gas sampling valve e. A e r e s o l t r a p from 0.5-in. 0.d. X 0.375 i.d. Lucite and loosely filled with spun borosilicate glass f. T y g o n tube connector g. Perforated disk h. Splash trap from 0.5-in. 0.d. X 0.25-in. i.d. Lucite
n.
b.
Table I. Performance of Collector Using Hydrogen Gas Exhaust flow rate 1 atm, 300 OK, ml/min 100 200 500 750 1300
Minimum dead volume of cone, ml
vol._ ~ _Dead _ _
6 8 16 22 35 (cone empty)
Flow rate, sec 3.6 2.4 1.9 1.8 1,6
required to purge the collector cone. In practice, however, the time required to purge the system is longer than is indicated by the physical dead volume, because more gas is dissolved in the pump oil than exists in the dead volume. The “effective dead volume,” Veff, for any particular gas can be measured by following the time dependence of the concentration, C(t), of that species in the pump exhaust while the system is being purged by another gas at a flow rate,f. For HD being purged by HP,we find an approximately exponential decay which implies efficient mixing of the oil and the approximate validity of the following equations: dC(t)/dt = -fC(t)/Veff Veff = (tq
- tl)f/ln[C(td/C(td
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
203
We find V,ff = 150 + 10 ml for HD, while the physical dead volume was 10 j= 1 ml ( f = 40 ml/min). Hence for H D and all more soluble gases, V,ff will depend mainly on the volume of oil, Veil, and on the solubility of the gas in the oil, as long as the gas collector is operated with a relatively small volume. Then Veff = VoilCoi~/Cgss, and since our Veil = 2,600 ml, our results imply COil/Cga3 = 53. = for H D in pump oil a t 70 “C. This may be compared t o X~O = 89.10-3 reported for Hz in transformer oil (3). Other
(3) “International Critical Tables of Numerical Data, Physics, Chemistry, and Technology, Vol 111.” E. W. Washburn, Ed., McGraw-Hill Book Company, New York, N.Y., 1928, pp 255, 262-268,
relevant solubilities are: Ns, 118; 0 2 , 192; CO, 198; CHI, 212, units of 10-3 in transformer oil a t 80 ”C (3). To minimize Veff, one should select the smallest pump consistent with the flow requirements. The Precision Scientific Model 25 should have Veri for H D of not more than 60 ml a t flows up t o 150 ml/min when equipped with a conical exhaust gas collector. The simple conical design works better t o reduce physical dead volume than several more elaborate devices we tried, and the reduction in effective dead volume is well worth the effort. A simpler or more effective modification appears unlikely. RECEIVED for review August 16, 1971. Accepted October 14, 1971.
Evaluation of an Attenuator for Improved Operation of Electrodeless Discharge Lamps R. M. Dagnall and M. D. Silvester Department of Chemistry, Imperial College of Science and Technology, London, S W7 2A Y , UK DEALLY ALL the charge material in a microwave excited electrodeless discharge lamp (EDL) should be in the vapor phase during operation. However, this is rarely true and frequently small amounts of solid are present. This solid material changes position during the operation of the EDL and causes a variation in the coupling efficiency of the microwave cavity, thus changing the level of reflected microwave power. In certain instances, gross changes in reflected power can affect the actual power output of the magnetron. This can cause a further change in the discharge conditions of the EDL resulting in further changes in the position of the solid material within the EDL. This process usually produces a cyclic variation in the intensity of the radiation output of the EDL. An attenuator connected between the magnetron and the microwave cavity will diminish these effects by attenuating the reflected microwave power as well as the forward microwave power. In consequence, an increase in the stability of operation of the EDL will be achieved. Also, certain electrodeless discharge lamps, oiz. Hg, As, and Se,
Table I.
Lamp type Cd Zn PbI2 TIC1
Hg Se
Wavelength of measurement, nm
Power, watts
228.8 213.9 283.3 276.7 253.7 196.1
35 25 25 25 18 20
Effect of Attenuator on Lamp Operating Conditions
No attenuator Short term Drift, %/hr noise, Z 3.1 3.3 4.2 2.8 5.0 6.4
Warm-up time, min
Power, watts
10 12 14 14 10 9
50 50 50 50 36 40
-2.0 -2.2 -1.8 -2.0 -3.2 -3.6
require low operating microwave powers (20-30 watts) and a t such low powers the magnetron output is often unstable. The use of an attenuator with these lamps will increase the required microwave output of the magnetron generator and hence give an increase in the stability of the magnetron output. RESULTS AND DISCUSSION
The short term noise and the drift (long term noise) of the intensity of resonance radiation emitted from several EDL’s were measured with and without the attenuator (Table I) using 204
a Servoscribe potentiometric recorder with a time constant of 1 second. The short term noise was defined as the peak to peak variation of the signal, measured over a two-minute period, and the drift was defined as the percentage change in the signal per hour. In these measurements the time taken before the intensity reached a steady value (warm-up time) was measured also. The attenuator decreased the short term noise, as predicted, but only by some 14%, except for those EDL’s which operate at very low power ratings (i.e.,
