lower than those reported for direct scintillation methods ( 3 , 4).
lOOr
LITERATURE CITED
t
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1
0
1
2
3
4 5 M HN03
6
7
8
Flgure 1. Factors influencing the extraction of americium and curium using nonyl phosphate. (0)0.5 M nonyl batch A Am. (A)1.5 M nonyl batch A Am. (0)1.5 M nonyl batch B Am. (A)1.5 M nonyl batch B
Cm
determination to be made. This can then be used to correct for losses in the sample. The background count rate is dependent upon the sample, as =e the limits of detection, which are approximately 20 times
(1) F. E. H. Crawley and Elizabeth Goddard, Health Phys.,30, 191-197 (1976). (2) A. Lindenbaum and M. A. Smyth, in “Organic Scintillants and Liquid Scintillation Counting”, D. L. Horrocks and L. T. Peng, Ed., Academic Press, New York, 1971, pp 951-958. (3) T. Mo and JaW. Prim, “Inhalation Toxicology Research Institute, Loveiace Foundation, Annual Report 1974-75”, 1975, pp 108-11 1. (4) S.J. Powers, BNWL-1950, Pt. 1, 66-67 (1975). (5) R. F. Keough and S.J. Powers, Anal. Chem., 42, 419-421 (1970). (6) W. J. McDowell, in “Organic Scintillants and Liquid Scintillation Counting”, D. L. Horrocks and L. T. Peng, Ed., Academic Press, New York, 1971, pp 937-950. (7) W. J. McDoweil and C. F. Caleman, in ‘‘Promedings of International Solvent Extraction Conference”, Vol. 3, pp 2123-2133, Lyon, September 1974, SOC.of Chem. Ind., London. (8) E. S. Gureev, V. N. Kosynkov, and S. N. Yokovlev, Radiokhimiya, 6, 655-665 (1964). (9) Aibr!ght and Wilson Ltd., Product Technical Data Sheet, Industrial Chemisby Division, Oldbury, Warley, W. Midlands. (10) F. E. H. Crawley, E. R. Humphreys, and J. W.Stather, Health Phys., 30, 491-493 (1976). (11) P. L. Altman and D. S.Dlttmer, “Blood and Other Body Fluids”, Biological Handbook Series, Federation of American Societies for Experimental Biology, 196 1. (12) D. E. Watt and D. Ramsden, “High Sensitivity Counting Techniques”, Pergamon Press, Oxford, 1964.
RECEIVED for review February 14, 1977. Accepted April 11, 1977.
Digital Device for Precise Determination of Drop Times at Dropping Mercury Electrodes Pamela J. Peerce and Fred C. Anson” Arthur A. Noyes Laboratory, California Institute of Technology, Pasadena, California 9 1 125
Thermodynamic information on the adsorption of ions and molecules a t mercury electrodes is often obtained from electrocapillary curves (1). The necessary values of the interfacial tension can be determined directly by means of a capillary electrometer or evaluated from measurements of the natural drop time of a dropping mercury electrode ( 2 ) . The tedious experimental aspects of the former approach are well known. The latter procedure is more attractive because it is experimentally simpler and readily automated (2-6). Several methods for detecting drop fall have been proposed previously (5). The two most common employ photoelectric or impedance measurements to detect the end of drop life. Corbusier and Gierst (2) were among the first to suggest the photoelectric method, which requires that the capillary be positioned very precisely in the light beam. However, their device and several of those based on impedance measurements ( 3 , 4 ) do not allow the lifetimes of consecutive drops to be measured. In this note, a simple, digital drop timer is described which provides a rapid, convenient means for determining the lifetimes of up to 1024 consecutive drops. The potential of the electrode can be controlled by a standard, commercial polarograph. (A Princeton Applied Research model 174 Polarographic Analyzer was utilized, but any polarograph having a current measuring circuit with an output which can be adjusted for dc level and polarity would be acceptable.) 1270
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
The experimental arrangement for measuring drop times is the same as that for recording a polarogram. The current output of the polarograph is connected directly to the input terminal of the drop timer. Since the device can be set to time up to 1024 consecutive drops at each potential, drop lifetimes can be determined very accurately. Once started, the timer runs continuously without operator intervention and displays the number of clock pulses elapsed during the last run while performing the next one. This permits the operator to record the result of each run while the next run is in progress. The digital output of the timer is well suited for interfacing to a computer if even greater automation is desired. EXPERIMENTAL Description of the Instrument. The drop timer is shown schematically in Figure 1. The abrupt change in current which accompanies the detachment of each drop from the capillary is used to sense the end of drop life. The input signal to the timer is sharpened by a Schmitt trigger, IC1, which is set to fire when the negative edge of the signal at its input crosses the threshold level of +2.6 V. (Since the timer accepts only positive input voltages, it is necessary to reverse the polarity of the current output when the current changes sign.) The clock frequency is 120 Hz. The unrectified, 60-Hz signals from the transformer secondary are differentiated and fed to separate inputs of the Schmitt trigger, IC2. Normally, as one input is high while the other is low, the exclusive OR output is high. However, just before and after the voltage reverses direction (twice
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per cycle), both inputs are low causing the exclusive OR output to change state, thus generating the clock pulses. The inputs of both IC1 and IC2 are protected by Zener diodes (5 V). An experiment is initiated by depressing the START button sometime during the growth of a drop. This clears IC8 and when the drop falls, IC3, IC6, and IC7 are reset; IC8 is incremented by one; and the counting begins. When the preset number of drops has been counted, the output of IC3 undergoes a brief negative transition which triggers the firing of IC5, thus disabling IC6 and IC7 and updating the two digital readouts which display the number of clock pulses counted and the number of times the measurement will have been repeated a t the conclusion of the current run. IC6 and IC7 are then automatically reset and the next run begins. The relative timing of these events is shown in
Figure 2. The execution time of the device is ca. 10-15 ws which represents a negligible correction for electrodes with natural drop times of several seconds. The low pass filter shown in Figure 2 was employed to reduce 60-Hz noise (RC = 0.3 s). The presence of the filter caused the apparent lifetimes of each drop to increase by a constant amount (ca. 0.02 s).
RESULTS AND DISCUSSION Characteristics and Properties of the Timer. As the clock frequency employed in this device is 120 Hz, the uncertainty in the lifetime of a single 5-s drop is ca. 0.2%. Such precision in t h e measurement of individual drop times is ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1271
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Figure 2. Timing diagram. The purpose of this figure is to show the relative timing of the indicated events. However, the time scales for the various events are not necessarily the same. For example, the clock pulses (IC2) are separated by 8.33 ms whereas the pulses from the exclusive OR output of IC1 occur only every 3-7 s comparable to that reported by Corbusier and Gierst (2) but not as high as has been achieved with some of the previously described devices (3, 5 , 6). However, when the lifetimes of many drops are averaged, the accuracy and precision of the present timer are very high indeed. A precision time mark generator (Tektronix Model 180 A) was used as the input source to test the inherent accuracy and precision of the drop timer. For marks separated by 5 s and counting 8 marks per run, the interval could be measured to within 0.2%. In 20 replicate experiments, a precision of f0.008% was observed. The time mark generator was also used to test the ability of the timer to measure short time intervals. For marks exactly 0.01 s apart, the timer yielded an interval of 0.010006 s after counting 512 marks. Electrocapillary Data. The timer was used to measure the electrocapillary curve for 0.5 M NaC104(pH 4.2) at 25 O C . At least three runs consisting of the measurement of eight drop times were performed at each potential. Even near the potential of zero charge (pzc) the drop time could be measured with a precision better than fO.l% by using the most sensitive current range on the PAR 174. (Triggering of the drop counter requires that the negative edge of the input signal cross +2.6 V. By careful adjustment of the dc offset in the current output of the polarograph, this requirement can usually be satisfied despite the smaller magnitude of currents flowing near the pzc. However, problems can occur under conditions where
c
1
-E/rnV vs SCE Flgure 3. Comparison of electrocapillary data obtained from differential capacitance and drop time measurements. (0)This work; (0)taken from Parsons and Payne ( 7 ) . The two data sets were superimposed at the electrocapillary maximum
the current-time curves contain depressions which cause the input signal to cross the triggering level prematurely.) The data are plotted in Figure 3 along with the surface tension date of Parsons and Payne for aqueous 0.5426 M HC104 a t 25 "C (7). The agreement between the two sets of data is excellent. Thus, the greater convenience of the drop timer described herein has been achieved without sacrificing the quality of the electrocapillary data obtained. Of course, all methods based on drop time measurements share the limitation that adsorption equilibrium must be reached rapidly for the drop time to be a reliable measure of the equilibrium interfacial tension. ACKNOWLEDGMENT 'Charles Klopfenstein provided much valuable advice in the design of the digital circuit. Helpful discussions with Irving Moskovitz are also acknowledged. LITERATURE CITED (1) D. C. Grahame, Chem. Rev., 41, 441 (1947); D. M. Mohilner in "Electroanalytical Chemistry, A Series of Advances", Vol. 1, A. J. Bard, Ed., Marcel Dekker, Inc., New York, 1966. (2) P. Corbusier and L. Gierst, Anal. Chim. Acta, 15, 254 (1956). (3) L. Meites and J. M. Sturtevant, Anal. Chem., 24, 1183 (1952). (4) A. J. Bard and H. B. Herman, Anal. Chem., 37, 317 (1965). (5) B. Nygard, E. Johansson, and J Ologsson, J . Elecfroanal. Chem.. 12, 564 (1966). ( 6 ) G. Papeschi, M. Costa, and S. Bordi, Electrochim.Acta, 15, 2015 (1970). (7) R. Parsons and R. Payne, Z . Phys. Chem. (Frankfurtam Main), 98, 9 (1975).
RECEIVED for review February 28,1977. Accepted April 18, 1977. This work was supported by the National Science Foundation.
Donnan Dialysis Enrichment of Cations James A. Cox" and James E. DiNunzio Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1
The separation of a sample from a relatively concentrated electrolyte by an ion-exchange membrane results in transfer of ions of the appropriate charge sign from the sample into 1272
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
the electrolyte (I, 2). If the volume of the latter solution is smaller, the concentration of the species which originate from the sample may be increased by their transfer into the
