a Hewlett-Packard Model 2801A quartz thermometer (Hewlett Packard Co., Palo Alto, Calif.) which was calibrated against an ice bath. One probe from the quartz thermometer was placed in the main bath with the thermistor probe and another probe was placed in the exit stream in the cell holder. The thermistor probes were calibrated and frequently checked against the quartz thermometer. All temperature readings are given as absolute values (reliable to *0.02 "C). TEMPERATURE EVALUATIONS. The temperatures in the cells were monitored for 15 minutes continuously at each position. The cells were monitored at the top, the middle (light path), and the bottom with the cell holder placed in each of its three positions (a total of 27 readings, each for 15 minutes) for 15,25, and 35 "C. Frequent checks of the temperatures were made of previous readings to be sure the system provided a constant basis for comparison. The average readings for the three cells at the top, middle, and bottom for the 27 readings, each at 15,25, and 35 "C were as follows: (a) 15.288 0.025 "C (temperature of primary bath, 15.221 0.003 "C; and temperature of exit water in cell holder, 15.270 10.003 "C); (b) 25.427 It 0.005" C (temperature of primary bath 25.375 & 0.003 "C; temperature of exit water in cell holder, 25.403 f 0.003 "C); (c) 35.177 f 0.022 "C (temperature of primary bath, 35.206 f 0.003 "C; temperature of exit water in cell holder, 35.199 f 0.002 "C). We wish to emphasize that the temperature variations reported for the cells represent the maximum variations measured between any two points throughout the three cells. The temperature at any measurement point in the cells was always 10.005 "C or better for the 15-minute recording. We also measured the time required for methanol added to an empty quartz cell to reach the same temperature as methanol equilibrated in an adjacent cell. Methanol (at 21 "C) was added to an empty cell for which the adjacent cell was at 25.153 "C. For six trials the added methanol was within
*
0.015 "C of the temperature of adjacent cell in 1 minute and within 0.01 "C in less than 1.5 minutes. CELLHOLDER MODIFICATIONS. The system described above is specifically for the D U spectrophotometer. Slight modification would have to be made to adapt this holder to other single beam instruments. The primary changes would probably be in the attachment of the cell holder to the spectrophotometer and perhaps in the size of the light ports if the collimated beam is significantly larger than for the D U spectrophotometer. For double beam instruments, the fundamental design change is from three cells to two and to space the cells to the geometry of the specific double beam instrument. Other minor changes include the attachment of a holder bracket to the cell holder so that it may be properly positioned in the cell compartment, and placing the inlet and exit water ports on the bottom of the cell holder so that circulated water from the primary bath enters and leaves from the bottom of the spectrophotometer. A cell holder has been designed and constructed for the Perkin-Elmer 202 spectrophotometer but the temperature regulation has not been evaluated as yet for this system. Based upon our experience with the single beam device and because the cell holder for the double beam instrument is constructed of plastic and can be further insulated if necessary, we believe that the temperature will be significantly better than for the single beam instrument and may even approach the limit of regulation of our primary bath (10.003 "C). RECEIVED for review March 24,1969. Accepted July 29, 1969. This work was taken in part from the M.S. Thesis of P. D. Feil (1967) and the M.S. Thesis of D. J. Wells, Jr. (1968), Furman University. Grateful acknowledgement is made to The National Institutes of Health for a grant (No. 1 ROI-AM1124) in support of this work.
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A DiffusionCell for the Production of a Constant Vapor Concentration G . A. Lugg Defence Standard Laboratories, Maribyrnong, Victoria, Australia
A DYNAMIC SYSTEM for establishing a known constant vapor concentration is necessary for many industrial hygiene studies such as the evaluation of detection devices and the development and testing of analytical methods. A diffusion cell can be used for this purpose provided that a constant diffusion rate can be achieved and the diffusion coefficient of the compound diffusing into air or other carrier gas is known or calculable. To maintain a steady diffusion rate, Fortuin ( I ) and McKelvey and Hoelscher ( 2 ) used a reservoir saturated with vapor fitted beneath the diffusion cell. However, Altschuller and Cohen (3) using this system found it impossible to obtain rates of diffusion in agreement with those calculated or to obtain good reproducibility. The work described here on reservoir-type cells led to the design of a cell which is kept at a constant level by an optical sensing device. (1) J. M. H. Fortuin, Anal. Chim. Acta, 15, 521 (1956). (2) J. M. McKelvey and H. E. Hoelscher, ANAL. CHEM.,29, 123 (1957). (3) A. P. Altschuller and I. R. Cohen, ibid.,32, 802 (1960).
EXPERIMENTAL
Initially, a diffusion cell with a 50-ml reservoir, similar to that used by McKelvey and Hoelscher, was used but a reservoir of this size was abandoned because of poor agreement of the determined diffusion rates with the theoretical. A smaller reservoir with limited air space above the liquid level was then used. The diffusion rates of benzene and methanol were determined using a cell 3.00 cm in length, 0.61 cm in diameter, and joined to a 4-ml reservoir using a Teflon (DuPont) connector to avoid distortion of the cell. For each experimental run the reservoir was filled to near capacity and weighed. It was then connected to the cell and allowed to come to equilibrium in a water bath maintained at 25 0.1 "Cbefore 1 ml min-' of dry air was passed over the cell. After various time intervals, the reservoir was disconnected and weighed. In the case of benzene the effluent stream was also monitored using a flame ionization detector. In subsequent work, a constant level cell of the type shown in Figure 1 was used. Basically, this cell consists of a water-jacketed, interchangeable, precision bore capillary tube ( A ) the bottom of which is connected to a glass syringe reservoir ( H ) and the top to the carrier gas stream by means
*
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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Figure 1. Constant level cell of a Teflon connector (B). The liquid in the cell is maintained a t a predetermined level by means of coarse (G) and fine ( F ) adjustments which vary the height of a light source ( D ) and photoelectric cell ( E ) . A fall in the liquid level causes a n increase in light intensity when the liquid meniscus falls 0.2 mm below the light path. The amplifier (L) triggers a solenoid operated switch ( M ) which activates an electro-
magnet so that the hammer (4 pulsates on the plunger ( K ) of the syringe reservoir. The liquid is then raised to the required level in the diffusion tube. The travel and oscillation frequency of the plunger weight can be adjusted depending upon the rate of diffusion of the liquid. The electronic circuit for the apparatus is shown in Figure 2. The diffusion rates of benzene and methanol were determined gravimetrically using constant-level cells of 0.61 cm and 0.14 cm diameter and diffusion paths of 3.0 cm and 15.0 cm for each cell; the water jacket was controlled a t 25 i 0.1 "C. The weights of the solvents which diffused during periods shown in Table I were obtained by using a duplicate system. Dry air was passed through each of the two arms a t the rate of 1 ml min-*. Each arm consisted of the two charcoal weighing tubes in series which had been equilibrated by the passage of dry air until a constant weight was obtained. The diffusion cell was placed before the charcoal tubes in one arm of the system. After setting the liquid level in the diffusion tube to the required height, the charcoal tubes were connected, the air flow was commenced, and the time noted. At the desired times the charcoal tubes were disconnected and weighed. The amount of diffused vapor was obtained by subtracting the change in weight for the blank tubes from the increase in weight of the sample tubes, allowing for variations in the total air flow as measured by wet meters. RESULTS AND DISCUSSION The determined diffusion rates for benzene and methanol in air a t 25 "C are shown in Table I and compared with the
4 Figure 2. Electronic circuit for diffusion cell
Table I. Comparison of Measured Diffusion Rates in Air at 25 OC Using Reservoir and Constant Level Cells Observed Observed rate/ Diffusion diffusion rate theoretical Compound Cell type Ratio: time (hours) Pressure torr g sec-l X lo6 rate, Z Benzene Reservoir 0. 0944a 0-5 812 3.784 93 0.0944 5-50 811 3.460 85 0.0944 50-77 810 3.121 78 0.0944 77-120 812 2.609 64 3,688 97 Benzene Constant 0.0944 70 780 0.751 98 level 0.0189 144 782 0.201 99 0.0051 193 783 0.039 97 0.0010 286 777 0-24 768 3,277 93 Methanol Reservoir 0.0944a 3.100 88 0.0944 2450 766 50-77 768 2.808 80 0.0944 77-144 768 2.380 68 0,0944 72 767 3.480 99 Methanol Constant 0,0944 140 770 0.676 97 level 0.0189 215 772 0.180 97 0.0051 310 773 0.036 97 0.0010 a Value used for calculation of theoretical diffusion rate. 1912
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
15.20 X lo-? cm? sec-1 for benzene and methanol, respectively
expected rates calculated from the equation ( 4 )
(5).
s
=
~
DMP A RT ( i ) l n h
where S is the diffusion rate grams sec-’, D is the diffusion coefficient cm2 sec-I, M is the molecular weight of the vapor, P is the total pressure dynes m r 2 , R is the gas constant, erg K-’ molecl, T is the operating temperature K, A is the cross-sectional area of the capillary tube cm2, L is the length (cm) of the diffusion path, X
=
~
where P and p , the
p-P vapor pressure of the diffusion component at temperature T, can be expressed in any convenient units. The diffusion coefficients used were 9.32 X lo-* and
With the fixed reservoir-type cell used, the gravimetric results showed a steady decline in the diffusion rate with time; these results with benzene were confirmed by the flame ionization detector. It is not possible to maintain steady conditions due to the concentration gradient in the space above the liquid level changing as the level falls. With the constant level diffusion cell, the parameter
(3
re-
mains fixed at its chosen initial value and concentration gradient effects are eliminated. The observed diffusion rates are found to be better than 95 of the theoretical values for continuous operating periods which are limited only by the capacity of the reservoir.
RECEIVED for review June 2, 1969. Accepted July 3, 1969. ( 5 ) G. A. Lugg, ANAL.CHEM.,40, 1072 (1968).
(4) J. Stefan, W e n Ber., 63,63 (2) (1871).
A Rapid-Response Variable-Temperature Thermostat Bath Employing Analog-Digital Control Circuitry Theodore E. Weichselbaum and Raymond E. Adams Sherwood Medical Industries, Inc., Subsidiary of Brunswick Corp., S t . Louis, Mo. 63121 Harry B. Mark, Jr. Department of Chemistry, The Unicersity of Michigan, Ann Arbor, Mich. 48104 WITH the advent of highly accurate chemical instrumentation based on both analog and/or digital logic circuitry such as those recently described for kinetic (1, 2) and electrochemical (3) measurement, experiments have shown the temperature control of the systems often become the limiting factor in the accuracy of the data (2, 4 ) . Also, it was found that for experiments, such as kinetic based analyses and differential thermal activation of enzyme reactions ( 4 ) , it was often necessary to change reaction temperatures (2, 4 ) . Thus, a rapidresponse variable-temperature thermostat bath was found t o be necessary and convenient. Previous designs of utility constant temperature fluid baths used for the thermostating of chemical reaction vessels, rate studies, electrolysis cells, etc., generally have employed a temperature-controlled switch, such as a mercury or bimetallic switch, t o the application of a heater (or cooling element) on (or off) when the temperature of the fluid in the bath or cell varied from the preset value desired (5). Although the temperature-sensitive switch can be made to be very sensitive to small temperature[changes, it acts only as a n on/off switch for applying full voltage or current t o the temperature element. Thus, “overshoot” in reaching temperature equilibrium cannot be avoided. This problem can be partially circumvented by employing a thermistor bridge temperature sensor and a (1) T. E. Weichselbaum, W. H. Plumpe, Jr., and H. B. Mark, Jr.’ ANAL.CHEM.,41,(3) 103A (1969). (2) T. E. Weichselbaum, W. H. Plumpe, Jr., R. E. Adams, J. C. Hagerty, and H. B. Mark, Jr., ibid.,p 725. (3) G. Lauer and R. A. Osteryoung, ibid.,40 (lo), 30A (1968). (4) T. E. Weichselbaum and J. C. Hagerty, Sherwood Medical Industries, Inc., unpublished results, 1968. ( 5 ) C. N. Reilley and H. T. Sawyer, “Experiments For Instrumental Analysis,” McGraw-Hill, N. Y . , 1961, Chapter 13.
SAMPLE CELL
-+
TH2
Ts P ERROR AMPLIFIER AND BRIDGE COLD REF CIRCUIT
CONTROL AMPLIFIER
BRIDGE-PD POWER SUPPLY
PHASE DETECTOR AND PULSE TRANS,
Figure 1. Schematic diagram of the constant temperature controller circuit which controls the magnitude of the effective voltage applied t o the heater element so that the thermal control is proportional to the error signal sensed by the bridge (6). However, temperature oscillations (discussed below) are still present and affect measurement results ( 4 ) . The dual thermistor bridges employing the analog-digital heater control circuitry presented here circumvents this problem. Also, temperature sensitive switches are quite difficult and time consuming t o reset to a new temperature value. However, the circuit described can be programmed to a new cali(6) “Silicon Controlled R.ectifier Manual,” 4th Ed., General Electric, Syracuse, N. Y . , 1967, pp 150 and 274, Chapter 9.
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