fraction, and Superlose HAA-I 1-HV, a completely soluble hydroxyethyl derivative of the same fraction. They are available from Stein Hall and Company, 605 3rd Avenue, New York, N.Y. Both of these materials have been recommended for the preparation of iodirnetric reagents which are easy to prepare, sensitive, and quite stable. The absorbance per equivalent of oxidizing agent has been reported to be 18,700 for the reagent made with Superlose, and 16,900 for the reagent made with Superlose HAA-11-HV. This latter reagent shows considerable temperature dependence on both equivalent response and on “threshold” (the point on a plot of absorbance 6s. oxidizing agent added at which a response curve meets the concentration axis) (6). The convenience of its preparation tends to be outweighed by these factors. TIME, MONTHS
EXPERIMENTAL
In an effort to develop a simple visual method for measurement of iodine used as a biocide in a potable water supply, cadmium iodide-Superlose reagent (5) was chosen as being most suitable. However, the reagent showed a gradual turbidity increase due to retrogradation, with eventual amylose precipitation. Since the planned application requires longterm (more than a year) stability, a study of retrogradation rate as a function of amylose concentration was conducted. The set of curves in Figure 1 shows turbidity increase as a function of time for various reagent concentrations. All concentrations show some increase in turbidity, but only the two highest concentrations show unacceptable turbidity, after more than ten months. In addition, these higher concentrations showed a heavy precipitate after this time. Dilution of one volume of reagent with three parts of water improves stability to the point where retrogradation should be insignificant for considerably more than a year. This dilution decreases the absorbance of the final solution due to dilution of the oxidizing agent being measured with an increased volume of reagent solution, but this decrease is less than 10% (4 ml of reagent, rather than 1 ml, per 50 ml of final volume). In addition to studying the effects of‘ dilution, an attempt (6) C. A. Smith, Ph.D. Thesis, Kansas State University, Manhattan, Kan., 1966.
Figure 1. Turbidity as a function of time for various dilutions of linear starch-cadmium iodide reagent
* indicates heavy precipitate formation was made to improve reagent stability by addition of’sucrose, in order to hopefully hinder attachment of amylose molecules to each other through the presence of a chemically similar but soluble material. Although the reagent with sucrose added was not satisfactory, it was significantly more stable than the unmodified reagent, indicating that this approach is deserving of further exploration. CONCLUSIONS
Dilution (1 : 3 with water) of the linear starch-cadmium iodide reagent produced by a previously reported procedure, which uses a commercially available amylose fraction, gives a reagent solution which is stable for more than a year, with little effect on analytical sensitivity. RECEIVED for review July 7,1972. Accepted August 29,1972. This work was performed as a part of Contract No. NAS96555 of the National Aeronautics and Space Administration.
Application of Differential Flow Controllers to Atmospheric Sampling Arthur F. Wartburg, Herman D. Axelrod, Ronald J. Teck, Miles D. LaHue, and James P. Lodge, Jr. National Center for Atmospheric Research, Boulder, Colo. 80302 ACCURATE MEASUREMENT of atmospheric gases using a bubbler-type system requires knowledge of the amount of air sampled. The typical procedure is to maintain a uniform flow rate of air through the bubbler for a given length of time and thus obtain an integrated air sample. Previous workers ( I ) have used needles as critical orifices to maintain constant flow. The needles are easy to use and are inexpensive, but they require calibration, and they are subject to damage by bending or plugging. Huygen ( 2 ) has recommended the use of glass capillaries in place of needles. His capillaries have to be custom made to a specific design which can be troublesome. Low flow rates would require ( I ) J. P. Lodge, Jr., J. B. Pate, B. E. Ammons, and G. A. Swanson, AirPollirt. Contr. Ass. .J., 16, 197 (1966). ( 2 ) C . Huygen. ibid.,20, 675 (1970).
very small capillaries and would probably be difficult to construct. An alternate method would be to use differential-type flow controllers to maintain constant flow rates. These flow controllers, extensively used in pressure applications, can also be used in vacuum applications such as encountered in atmospheric sampling. Differential flow controllers require either constant upstream or downstream pressure. The model available for the application shown in Figure 1 was a Brooks Instruments Model 8744A differential flow controller containing a 2 psi (13.8 X l o 3 nt/m2) spring. This unit required a constant upstream pressure. The reproducibility of the flow rate relative to upstream and downstream pressure changes was studied. The results shown in Table I indicate, as expected, that changes in the down-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2 , FEBRUARY 1973
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AIR
SAMPLE
40,~
Table I. Effect of Changing Downstream Pressure While Upstream Pressure Remains Constant Downstream pressure, Torr Flow rate, Umin 250 200 180 160 140
VACUUM PUMP
0.150 0.150 0.150 0.150 0.150
Table 11. Effect of Changing Simulated Bubbler Assembly Pressure Drop Pressure drop, With flow Without flow Torr" controller, l./min controller, l./min
DIFFERENTIAL FLOW CONTROLLER
4 10 20 30
40 50
0.158 0.156 0.154 0.152 0.149 0.146
0.158 0.101 0.053 -
e These changes were artificially produced by substituting a needle valve for the bubbler and varying the constriction,
Figure 1. Typical atmospheric sampling system using a differential flow controller
stream pressure have no effect. Table I1 shows that with a flow controller, a 46-Torr change in the upstream side (bubbler) pressure drop produces a change of 8.9 in the flow rate. Without the flow controller, the flow rate is not regulated. In actual practice, the bubbler pressure drop is not truly constant due to pressure differences that can result from evaporation of the bubbler solution and differences in the ceramic frits (3) used for dispersion. (The ceramic frits will always be 1 5 Torr of the average pressure drop.) Consequently, one would not normally expect the upstream pressure to vary more than 20 Torr during sample evaporation or by using different bubblers on the same flow controller. A 20-Torr variation represents a 3.2% error which is quite acceptable. The flow controller can be calibrated using a soap bubble flow meter. The equipment is set up in the configuration to be used, the needle valve is set for the desired flow rate, and the flow rate is measured with a soap bubble flow meter. Once the unit is calibrated, further flow measurements are not required as long as the flow controller is being used in the same application. The silica gel tube in Figure 1 prevents moisture from reaching the flow controller and possibly (3) H. D. Axelrod, A. F. Wartburg, R. J. Teck, and J. P. Lodge, Jr., ANAL.CHEM., 43,1916 (1971).
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corroding the unit. If there is a possibility of particles being lodged in the flow controller, the use of a filter prior to the flow controller is advised. The application described here has been for regulation of sampling flow rates of 0.150 l./min. For higher flow rates, other needle valves have been used in the flow controller and comparable accuracies have been obtained. While the above described system is more costly than the use of needles or glass capillaries, several distinct advantages are evident. Needles, to control properly, require at least 0.5atmosphere differential pressure across them. This flow controller only requires 125-Torr. Therefore, in places where electrical power is a problem, a smaller vacuum pump can be used. Second, if the differential pressure across the needles falls below 0.5 atmosphere, the needles cannot control the flow against changes in the upstream pressures. The flow controller has been operated continuously and intermittently for over two weeks and the flow rate varied by &2z.The data indicate that the flow controllers can be used for longer periods without the need to check the calibration. In contrast, our findings indicate that needles must be regularly recalibrated. A major disadvantage of this flow controller system us. needles is the much higher cost of the former. However, for certain atmospheric sampling applications, the use of differential flow controllers is preferable to other systems.
RECEIVED for review June 26, 1972. Accepted September 25, 1972. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
