Efficient Method for Degassing Surfactant Solutions for Polarographic Analysis Jack Novodoff and Horst W. Hoyer Hunter College of the City Uniuersity of New York, Graduate Center, 33 West 42nd Street, New York, N . Y . 10036
IN THE POLAROGRAPHIC analysis of most ions, it is imperative that there be complete removal of any oxygen dissolved within the solution, since otherwise the polarographic waves for dissolved oxygen at -0.05 and -0.9 volt us. SCE might interfere with the analysis. Removal of dissolved oxygen in aqueous solutions of inorganic salts is a procedure which is well documented. One method is t o bubble an inert gas (He, Ar, N2) through the solution for some length of time to displace the dissolved oxygen (1-3). A second method is to add a substance which will react with the dissolved oxygen such as S032- ( 2 , 3 )or a n enzyme (4). When surface active materials in any significant concentrations are involved in a polarographic analysis, removal of oxygen by the usual degassing techniques results in extensive foaming. In previous work ( 5 , 6 ) removal of dissolved oxygen was accomplished by first flushing the surface of the solution
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1-I 1L
AI
II
E? CONTROL
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Figure 2. Degassing device on polarographic cell
(1) H. Willard, J. Merritt, Jr., and J. Dean, “Instrumental Methods of Analysis,” 4th ed., Van Nostrand Co., Princeton, N. J. 1965, pp 691. (2) G. W. Ewing, “Instrumental Methods of Chemical Analysis,” 2nd ed., McGraw-Hill Book Company, New York, 1960, pp 207-208. (3) I. M. Kolthoff and J. J. Lingane, “Polarography,” 2nd ed., Interscience Publishers, New York, 1952, pp 395-397. (4) R. E. Benesch and R. Benesch, Science, 118, 447 (1953). (5) C. Tanford and J. Epstein, ANAL.CHEM.,23, 802 (1951). (6) H. W. Hoyer and J. Novodoff, J . Colloid Interface Sci., 26,490 (1968).
E
(VOLTS
vs
SCE)
Figure 3. Polarograms of 4 X 10-4M Cd2+in a 4 XlO-*M sodium dodecyl sulfate 1. Prior to degassing 2. After degassing
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3 D
2 53 A L L DIMENSIONS
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IN MILLIMETERS
Figure 1. Cross section and bottom view of degassing disk 202
with a n inert gas, H2 ( 5 ) or N:! (6), and then breaking the surface either by rocking (5) or shaking (6). These procedures, although adequate, involved use of an elaborate apparatus (5), or were rather time-consuming (6). A new simple method for speedier degassing of solutions containing surface active materials will be discussed below. A device which simultaneously degasses surfactant solutions and breaks any foam that is generated was fabricated out of Lucite plastic (Figure 1). In essence it provides for the simultaneous flow of gas through the solution for degassing and above it for breaking of the foam. Figure 2 shows the placement of the degassing disk on the polarographic cell of the Leeds and Northrup Dropping Mercury Electrode Assembly. The flow of inert gas for deoxygenation of the sample is controlled by valve A ; valve B controls the gas flow which breaks the foam. The rate of flow of gas in each line is dependent on the volume of liquid to be deoxygenated, the rate of production of foam, and the height of the disk above the level of the solution.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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 Minimum vol._ ~ 1 atm, 300 OK, dead volume _Dead _ _ ml/min of cone, ml Flow rate, sec 100 6 3.6 200 500
8 22
2.4 1.9 1.8
35 (cone empty)
1,6
16
750 1300
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
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