temperature of the air stream was increased gradually by means of the Variac control until sublimation began. Further slight increase in temperature caused movement of the sublimate further up the tube. When sublimation was complete, the heater was turned off and the air supply interrupted. The heater tube and vacuum jacket were removed and nitrogen was introduced into the sublimation tube to within 5 mm of atmospheric
pressure. The tube was sealed off with a torch a t convenient places to include separate samples for transfer to a dry-box. Received for review November 30, 1972. Accepted March 14, 1973. Presented in part as paper No. 152, Inorganic Chemistry Division, 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972.
Simple, Efficient Micro Mixing Device Robert E. Gugger and Samuel M. Mozersky Eastern Regional Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, Pa. 797 18
Thorough mixing of two streams of solution, A and B (Figure l), is important in many kinetic measurements and in continuous assay procedures. Such mixing can be easily accomplished with high velocity Get) streams, i.e., those having velocities of the order of 100 cm/sec or more, where turbulence is readily achieved, as shown by Hartridge and Roughton ( 1 ) . For low velocity streams, i.e., those moving ca. 10 cm/sec or less, the combined solution C is frequently passed through a mixer M consisting of a coil of tubing. However, it is necessary that the time interval for flow of successive portions of solution from the point of mixing, Y, to the point of observation, 0, be constant. To ensure that this constancy is not destroyed during the mixing process, the stream is frequently segmented by bubbles of air prior to entering M. The mixing device described here required neither a mixing coil nor air, nor the peristaltic pump frequently used in such work. The mixing chamber consists simply of a Teflon (DuPont)-coated bar magnet about 2 mm in diameter x 7 mm long enclosed in a glass tube ca. 4-mm i.d. (Figure 2). The bar is restricted to a section of tubing about 10 mm long. This can be accomplished by reducing the diameter of the glass tubing a t one end, and inserting the bar magnet through the other end. An 8-mm length of tightly fitting plastic tubing is then inserted into the wider end, to keep the magnet in place. The wider end of the glass tubing cannot be heated to reduce its diameter since this might melt the Teflon coating on the stirring bar. When the mixing chamber is placed on a magnetic stirrer, the primary motion obtained is rotation of the bar about its cylindrical axis. In addition, the bar wobbles, and there is some movement to and fro in the direction of the axis of the tubing, both of which enhance the stirring action. End-over-end rotation about a vertical axis, the usual motion of a stirring bar, is, of course, impossible, since the i.d. of the glass tubing is less than the length of the bar. The mixing action is easily controlled by adjusting the rate of rotation of the driving magnet in the magnetic stirrer. The volume of liquid in the central 10-mm section of the mixer is ca. 0.1 ml. There is thus little opportunity for unwanted mixing of portions of liquid of different age (age 0 being the instant of combination at point Y, Figure 1). The efficiency of the mixing device was tested as follows. Two Y tubes made from 2.5 mm (internal diameter) (1) H . Hartridge and 376 ( 1923).
E. J.
W. Roughton,
Roc. Roy. SOC., Ser.
A , 104,
Y
M
0
B/
Figure 1. Arrangement for continuous assay procedures Y is the point of confluence of the two streams, A and B, which contain the reactants. The resultant stream C flows through a mixer M before passing the point of observation 0
I 4mm. I P l a sI t i c Tubing
'
r 7 m m , 1
I
I
Stir'ring B a r
Figure 2. Micro mixing device. The direction of liquid flow is from right to left
Magnetic Stirrer
Figure 3. Experimental arrangement for testing the efficiency of mixing
Pyrex tubing were jointed either directly, by a piece of Tygon tubing, or via the mixing device, as shown (Figure 3). The arms of the Y tubes were ca. 1.5 cm in length. The Y tubes were positioned in a common vertical plane, with the connecting tube horizontal. When a mixer was used, a magnetic stirrer was placed below it. A solution of 0.1M NaOH containing sufficient phenolphthalein to give ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
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Table I. Efficiency of Mixing with and without the Mixer Efficiency, % Flow rate, cm/sec
0.02 0.41 1.31
Without mixer 35 6.3 4.0
Complete mixing corresponds to equality of the concentrations of dye in the two outlet arms. The efficiency of mixing, E , is given by the equation
With mixer
E = U z / ( A i + Az)
99.6 100
99.9
a n absorbance of 1-2 a t 550 nm was introduced into the upper inlet arm a t a volume flow rate of 0.001 to 0.065 cm3/sec, yielding a linear flow rate of 0.02 to 1.3 cm/sec. Into the lower inlet arm, a solution of 20% sucrose in 0.1M NaOH was introduced a t the same flow rate. When not mixed, a clear line of demarcation could be seen, in the tube joining the two Y’s, between the upper, light, colored solution and the lower, dense, colorless solution. The outlet arms were connected to two flow cells in a Gilford Model 2000 Multiple Sample Absorbance Recorder. Absorbance measurements were made a t a wavelength of 550 nm .
(1)
this being the ratio of the concentration (absorbance), Az, of dye in the lower outlet arm to the concentration, yz (A1 + A z ) , which would be obtained if mixing were perfect. The observed absorbance values were used as concentrations, proportionality being assumed. Table I gives the efficiency for each of three flow rates, with and without the mixing device. Over the range in linear flow velocity of 0.02 to 1.3 cm/sec, the efficiency of mixing in the presence of the mixer is better than 99.5%. The mixer simplifies substantially the apparatus required for continuous assay. Several mixers can be placed on one magnetic stirrer and all will perform in the same manner. Received for review October 4, 1972. Accepted March 15, 1973. The mention of commercial items is for convenience and does not constitute an endorsement by the Department of Agriculture.
Continuous Flow Krypton Adsorption for Low Surface Area Measurements Seymour Lowell
C. W . Post College, Long lsland University, Greenvale, N.Y. 7 1548 A previous method for measuring low surface areas by nitrogen adsorption using a continuous flow technique was reported (1). This method required the use of a small volume sample cell, with a simple U tube geometry, in order to overcome the problem of thermal diffusion which obscures the adsorption and desorption signals ( 2 ) .However, small samples of low surface area generate small signals which require a clean flow system with good instrument stability and sensitivity in order to obtain accurate signal integration. In addition, it is difficult to accurately calibrate small signals by injecting low volumes of adsorbate into the flowing gas mixture ( 3 ) . The measurement of low surface areas by the volumetric technique uses krypton gas a t liquid nitrogen temperature to minimize the ratio of unadsorbed to adsorbed gas admitted into the sample cell. For a different reason, it has been found that krypton can be used with the continuous flow method to measure very low surface areas.
THEORY The coefficient of thermal diffusion ( 4 ) for a steady state condition is
The term Nt is the total molecular concentration of adsor(1) (2) (3) (4)
Lowell and S. Karp, Anal. Chem., 44, 1706 (1972). D. Kourilovaand M . Krejcl, J. Chromafogr.,65, 71 (1972).
bate and carrier gas in the flow system. The concentrations N1 and N1’ are those for the adsorbate a t the extremes of a temperature gradient extending from T Z to a low temperature T I . Because of krypton’s low vapor pressure a t liquid nitrogen temperature, uiz, 1.76 Torr ( 5 ) ,its mole fraction in the BET (6) range of relative pressure is in the order of 10-4. This small adsorbate mole fraction causes the difference term in the numerator of Equation 1 to nearly vanish with the consequence that no obscuring thermal diffusion signals are generated within the sample cell when immersed in or removed from the liquid nitrogen bath.
EXPERIMENTAL The continuous flow instrument used was a “Quantasorb” manufactured by Quantachrome Corporation, Greenvale, N.Y. Matheson Gas Products supplied the krypton-helium mixture, which was 0.001124 mole fraction krypton certified to 1%relative accuracy. This mixture was diluted in a continuous flow with pure helium to achieve the desired mole fractions of krypton in the flow stream. Gas flows were passed through a liquid nitrogen cold trap to reduce the concentration of any hydrocarbons or water vapor. Flow rates were measured with a soap film bubble meter. The calibration of desorption signals was accomplished by injecting pure krypton through a septum into the flow stream using a “Precision Sampling Corporation” 25-pl gas syringe. The sample chosen for analysis was granular silica with particle size of roughly 0.1-mm diameter. This sample was used because of its low surface area and high interparticle void volume. The sample was outgassed for one hour a t 200 “C under a helium purge prior to analysis.
S.
S. KarpandS. Lowell,Ana/.Chem., 43, 1910 (1971). S. W. Benson, “The Foundation of Chemical Kinetics,” McGraw-Hill, New York, N . Y . . 1960, p 188.
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(5) S. J. Gregg and K. S. W. Sing, “Adsorption, Surface Area, and Porosity,”Academic Press, New Y o r k , N . Y . , 1967, p 316. ( 6 ) S. Brunauer, P. H . Emmett, and E. Teller, J. Amer. Chem. SOC., 60, 309 ( 1938).
