AIDS FOR ANALYTICAL CHEMISTS I
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Improved Diffusion Dilution Cell for Introducing Known Small Quantities of Liquids into Gases Anthony C. Savitsky and Sidney Siggia Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002 STANDARD GAS MIXTURES containing trace levels of normally liquid impurities can be made by employing a diffusion dilution cell (1-3). The liquid impurity to be added to the gas is contained in a precision bore capillary tube and maintained at a known temperature by means of a circulating constant temperature bath. The liquid is allowed to evaporate slowly into the gas stream which passes over the mouth of the capillary at a known flow rate. From a knowledge of the capillary radius, liquid level in the capillary, and rate of fall of the liquid level at known temperature, the rate of diffusion of vapor into the gas stream at any given time can be calculated from Equation 1,
s = -XPA 21
where S X
rate of diffusion, gram/sec a constant for the liquid at temperature T, cm2fsec density of liquid at temperature T , gram/cma A = cross sectional area of the capillary, cm2 I = depth of the liquid meniscus below the capillary mouth, cm =
= p =
The value of X must be experimentally determined for each liquid at each temperature at which the cell is to be used. The capillary is partially filled with the desired liquid and thermostated at the chosen temperature, T . The initial value of I is observed with a cathetometer and the time noted. Over a period of days, [ is measured periodically. A plot of I z us. time gives a straight line of slope X (2). The time required to determine X varies greatly from liquid to liquid, averaging about a week. For example, calibration for water at 40 “C requires about ten days. Previous diffusion dilution cells have suffered from several drawbacks. They have required calibration of the capillary at several temperatures which is prohibitively time-consuming, have not been capable of very rapid changes in concentration, and have not enabled the operator to produce the desired concentration exactly without extensive, manipulation. Goldup and Westaway (3),for example, have utilized a diffusion dilution cell to produce standard mixtures of water or methanol in nitrogen at the ppm level. Their cell produces changes in the rate of diffusion by changing the temperature (1) W. Jost, “Diffusion in Solids, Liquids, and Gases,” Academic Press, New York, N.Y., 1960, p 411. (2) D. H. Desty, C. J. Geach, and A. Goldup, “Gas Chrornatography 1960,” R. P. N. Scott, Ed., Butterworths, London, 1960, p 46. (3) A. Goldup and M. T. Westaway, ANAL,CHEM.,38, 1657
(1966). 1712
Figure 1. Modified diffusion dilution cell (A) Precision bore capillary tubing, (B) Stopcock, (C) Threaded adaptors, (D) 10/30 Ground glass thermometer, (E) Gas inlet, (F) Gas outlet, ( C ) Sinter, (H) Constant temperature water inlet, (I) Water outlet, (J) 10-mm Glass tubing, (K) Rubber bulb, (L)60/50 Ground glass joint
of the capillary. This approach requires calibration at each temperature used. At each temperature, more precise changes in diffusion rate can be affected by manually changing the liquid level in the capillary. This presents special problems with water, since the system must be opened to the atmosphere. The diffusion dilution cell of Goldup and Westaway has been modified to overcome these problems and thus produce a device more suitable for routine applications. Figure 1 shows the modified diffusion dilution cell. The precision bore capillary (A) is one arm of a U-tube. The liquid level can be changed by opening the stopcock (B) and forcing liquid into the capillary against the internal gas pressure with a rubber bulb, When the proper liquid level is reached, the stopcock is closed. This method of changing the rate of diffusion requires calibration at only one temperature. Since the value of X depends upon temperature and not capillary dimensions, U-tubes containing capillary tubing of differing radii can be substituted without affecting X. This makes
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
I 1
I 2
I 3 TIME
I
I
4
5
(hours)
Figure 2. Reattainment of equilibrium upon changing water concentration from 14.2 to 18.1 ppm in a helium stream flowing at 40 mlimin through cell ( A ) Westaway and Goldup diffusion dilution cell
( B ) Modified diffusion dilution cell ( C ) Equilibrium peak height (18.1 ppm water)
possible wide concentration changes with only a single determination of X. Using water as an example, concentrations between 0.1-1000 ppm are easily attainable. A major advantage of this design is the ease with which any desired rate of diffusion may be produced by simply adjusting the liquid evel.
The modified diffusion dilution cell has been utilized to produce rapid, reproducible changes in water concentration in helium streams at low gas flow rates. Of major concern in this study was the ability to produce any desired water concentration change and the time required to return to equilibrium. Figure 2 demonstrates the time required to return to equilibrium when the water concentration in the helium was changed from 14.2 to 18.1 ppm. Curve A was obtained using a diffusion dilution cell identical to that employed by Goldup and Westaway in which such small changes in concentration must be made by opening the system to the atmosphere and manually adjusting the water level in the capillary. Curve B was obtained with the modified cell. The return to equilibrium was monitored gas chromatographically by observing the peak height of acetylene when the water in helium was quantitatively converted over calcium carbide (3). Precise water concentrations were more easily obtained with the modified cell and re-equilibration time was shortened from about five hours to about one hour with a helium flow rate of 40 ml per minute.
RECEIVED for review January 28, 1972. Accepted April 11, 1972. This work was supported by the National Science Foundation Grant G.P. 12171.
Versatile Polarographic Function Generator C . J. Neilsen System Design Department, Comaltest, Inc., Commerce Drive, Danbury, Conn. 06810
J. D. Stuart Department of Chemistry, The University of Connecticut, Storrs, Conn. 06268
WE REPORT A VERSATILE polarographic function generator constructed of digital integrated circuits that can be readily interfaced to a small digital computer. The following waveforms are available : triangle, ramp, trapezoid, and pulse (square wave), all of independently variable periods-1 millisecond to 100 seconds, all of either repetitive or single-shot cycling. This function generator provides as much versatility in the adjustment of the rise, hold, and fall times of the trapezoidal, triangular, and ramp waveforms as other function generators, either reported in the literature or commercially available (1, 2). In addition, the digital components used in this function generator are economically competitive with those needed in its analog counterpart. To generate some of the same waveforms, recent articles have described the use of digital electronics to provide the switching to an analog operational RC integrator (3-5). In ( 1 ) W. D. Weir and C. G. Enke, Rec. Sci. Insfrum.,35, 833 (1964). (2) J. R. Tacussel, “Function Generators and Auxiliary Instruments for Programmed Voltammetry,” available through Ryaby Associates, Passaic, N.J., 1970. ( 3 ) J. S. Springer, ANAL.CHEM., 42 (8), 22A (1970). (4) R. Bezman and P. S. McKinney, ibid., 41, 1560 (1969). (5) S. P. Perone, D. 0. Jones, and W. F. Gutknecht, ibid., p 1154.
this function generator, all of the shaping of the output waveform and the switching is done in the digital domain. Inherent in the design is TTL digital logic that allows for higher accuracy, speed, and immunity to noise. Care in grouhding and shielding, so necessary in analog circuits, is far less critical in digital circuits. The advantage of a trapezoidal waveform with a variable extended hold of up to 100 seconds before the reverse scan is initiated, aids in the interpretation of the true reverse peak current in cyclic voltammetry (6). Extended ranges of hold times in the trapezoidal and pulse waveforms are possible because of the greater inherent stability of the digital logic as compared to analog circuitry. Digital generators have excellent amplitude stability in long hold times because their stability is due to only the reference power supply voltage going to the D/A converter. Analog generators, however, are limited in the shape of their hold times by either the eventual discharge of a capacitor or to the clipping of the waveform at the start and at the end of the hold by a diode or other nonideal active component that produce rounded edges. (6) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker, Naw York, N.Y., 1969, p 156.
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