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
With mixer
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 .
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
E = U z / ( A i + Az)
(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.
1576
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
(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).
,
I
0
0.2
0.3
O.'
Figure 1. BET plot of krypton
adsorbed o n 10.238 grams of coarse silica powder
P I P o is krypton relative pressure. X is weight of krypton adsorbed. Specific surface area is 0.0187 m 2 / g including approximately 0.0025 m 2 for the cell wall area. Cross sectional area of Kr taken as 19.5 A2 ( 7 ) . Data points were obtained in duplicate with a maximum deviation from the average of 0.28%
RESULTS AND DISCUSSION Figure 1 shows the BET plot obtained using the krypton desorption signals. These signals were nearly Gaussian in shape and showed no thermal diffusion. Attempts to use nitrogen-helium mixtures with the same sample proved to be impossible because of thermal diffusion which completely obscured the adsorption and desorption signals. As previously discussed ( I ) , increasing the sample cell volume in an attempt to give larger desorption signals, using nitrogen as the adsorbate, results in thermal diffusion signals which are larger in proportion to the adsorption or desorption signals. However, no thermal diffusion signals have been observed using krypton in sample cells ranging from 3.5 to 18 cm3 with krypton re1ati;e pressures from 0.1 to 0.35. These observations were confirmed with several adsorbents including zinc dust and low area organic powders. The adsorption signals using krypton-helium mixtures were small in height and broad in width due to the adsorption rate being limited by the flow rate into the sample cell. At flow rates of 15 cm3/min, the adsorption signals had a base line width of 7 to 12 minutes depending
upon the krypton partial pressure. The desorption signals were sharper, since the rate of desorption is governed by the rate of heat transfer into the powder bed. Immersion of the sample cell into a beaker of warm water produced peaks with a base-line width of 2 to 4 minutes. The ability to use larger samples of low area powders greatly facilitates the measurement of low surface areas because of the larger signals generated in the absence of thermal diffusion signals. In addition, it is worth noting that this method does not require the measurement of the sample cell void volume as is required with the volumetric method, nor is there a need to measure the small equilibrium vapor pressures of krypton with the associated problem of thermal transpiration (8). Also, no corrections for ideality are needed since the volumes of desorbed krypton are measured a t ambient temperature. Received for review January 29, 1973. Accepted March 26, 1973. (7) S.J. Gregg and K. S. W. Sing, ref. 5, p 86.
(8) S. J. Gregg and K. S. W . Sing, ref. 5 , p 319
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1577
