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An improved version of a H.F. mercury drop fall detector
J. Electroanal. Chem., 77 (1977) 391--392
391
©Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Short communication
AN IMPROVED VERSION OF A H.F. MERCURY DROP FALL DETECTOR
(~.
YARNITZKY Department of Chemistry, Technion, Israel Institute of Technology, Haifa (Israel) C.A. WIJNHORST, B. VAN DE LAAR, H. REYN and J.H. SLUYTERS *
Laboratory of Analytical Chemistry, State University, Utrecht (The Netherlands) (Received 17th May 1976)
In a recent publication [1] one of us presented a novel mercury drop fall detector. Its advantages with respect to other methods and its possible applications have been described in that publication and need not be repeated here. In this communication we propose an improved version of the apparatus. The electronic circuit is given in Fig. 1. As will be clear it consists of a Colpitt oscillator the frequency of which depends on the impedance on the antenna ANT. This impedance will change with drop growth if the antenna is coupled to the dropping mercury electrode, for instance by a metal lead coiled around the tube containing the mercury (1--10 windings). In the instrument in Fig. 1 also the amplitude of the oscillator will change with dropping mercury electrode impedance. The difference with the instrument in ref. 1 essentially is that not the sudden frequency change, but the sudden amplitude change at the instant of .ANT
15 K Io.~ ZN2222A~.
1200 p ~ ~1814
M
+15V L "L%:~4I ~ _EI~~0// + -i-15V
~2
,i
J-
?--T'°°°°T Y
+Sv ~ ~ i ] 1OK 1OK 80g
T 1MI - _1~'5v -- 2K2 'lOOK 470~
. TTL
T TL ~'~÷ 5V
Fig. 1. The drop fall detector. (ICla and IClb) #A 747 amplifiers, (IC2) pA 734 (Fairchild), (IC3) SN 74121 (Texas Instruments), (L) self inductance, 6 windings 8 mm diameter from 1 mm copper wire, (M) scope monitor.
* To whom correspondence should be addressed.
392
drop dislodgement is detected. The latter can be accomplished simply by demodulating the oscillator signal by means of the diode 1N 914. The d.c. level is separated from the demodulated signal by means of the 0.33 p F capacitor and 1 M ~2 resistor resulting into a sudden voltage change at the instant of drop fall. This voltage change can be either positive or negative, d e p e n d e n t on the particular properties of the galvanic cell and the setting of the variable condensor. Therefore it has been made possible either to invert (IClb) or n o t to invert (ICla) the voltage peak by means of switch S, in order always to obtain a positive pulse. This pulse is compared with a reference voltage Vre~ in the comparator IC 2. In this way only pulses with a height exceeding Vre~ are transmitted and all pulses smaller than V~ef arising from noise, etc. are rejected. V~f should be chosen as high as possible in order to obtain a good rejection of unwanted signals. A 10× amplifier (not shown) immediately after the d.c. filter under bad circumstances improves noise rejection. The one-shot IC 3 produces a pulse of long duration with which a Light Emitting Diode can be driven. Tuning by the variable condenser and proper functioning of the instrument thus can be surveyed easily. At the same time the instrument is TTL compatible, enabling direct coupling to a timer. The timer after a predetermined time interval can start some measurement procedure, e.g. sampling a current (in d.c. polarography) or a voltage, start a chronocoulometer or a network analyzer. This instrument has succesfully been in use for some time in several applications, e.g. in the measurement of capillary curves, in the potentiostatic bridge [2] and in an automatic network analysis system. Interference by d.c. voltages arising from rectification of the H.F. in additional electronic equipment most elegantly can be avoided by means of a relais automatically connecting a 0.47 p F polycarbonate capacitor with a 1000 pF ceramic disc capacitor in parallel across collector and emittor of the transistor in the oscillator (Z N 2222 A). This connection should be made immediately after dropfall and be broken shortly after a measurement has been made. In this way the oscillations are stopped at all times where the H.F. is not needed. A wiring diagram is available on request. REFERENCES 1 Ch. Yarnitzky, J. Electroanal. Chem., 51 (1974) 207. 2 J.H. Sluyters, M. Sluyters-Rehbach and J.S.M.C: Breukel, Z. Phys. Chem., N.F., 98 (1975) 435.
391
©Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Short communication
AN IMPROVED VERSION OF A H.F. MERCURY DROP FALL DETECTOR
(~.
YARNITZKY Department of Chemistry, Technion, Israel Institute of Technology, Haifa (Israel) C.A. WIJNHORST, B. VAN DE LAAR, H. REYN and J.H. SLUYTERS *
Laboratory of Analytical Chemistry, State University, Utrecht (The Netherlands) (Received 17th May 1976)
In a recent publication [1] one of us presented a novel mercury drop fall detector. Its advantages with respect to other methods and its possible applications have been described in that publication and need not be repeated here. In this communication we propose an improved version of the apparatus. The electronic circuit is given in Fig. 1. As will be clear it consists of a Colpitt oscillator the frequency of which depends on the impedance on the antenna ANT. This impedance will change with drop growth if the antenna is coupled to the dropping mercury electrode, for instance by a metal lead coiled around the tube containing the mercury (1--10 windings). In the instrument in Fig. 1 also the amplitude of the oscillator will change with dropping mercury electrode impedance. The difference with the instrument in ref. 1 essentially is that not the sudden frequency change, but the sudden amplitude change at the instant of .ANT
15 K Io.~ ZN2222A~.
1200 p ~ ~1814
M
+15V L "L%:~4I ~ _EI~~0// + -i-15V
~2
,i
J-
?--T'°°°°T Y
+Sv ~ ~ i ] 1OK 1OK 80g
T 1MI - _1~'5v -- 2K2 'lOOK 470~
. TTL
T TL ~'~÷ 5V
Fig. 1. The drop fall detector. (ICla and IClb) #A 747 amplifiers, (IC2) pA 734 (Fairchild), (IC3) SN 74121 (Texas Instruments), (L) self inductance, 6 windings 8 mm diameter from 1 mm copper wire, (M) scope monitor.
* To whom correspondence should be addressed.
392
drop dislodgement is detected. The latter can be accomplished simply by demodulating the oscillator signal by means of the diode 1N 914. The d.c. level is separated from the demodulated signal by means of the 0.33 p F capacitor and 1 M ~2 resistor resulting into a sudden voltage change at the instant of drop fall. This voltage change can be either positive or negative, d e p e n d e n t on the particular properties of the galvanic cell and the setting of the variable condensor. Therefore it has been made possible either to invert (IClb) or n o t to invert (ICla) the voltage peak by means of switch S, in order always to obtain a positive pulse. This pulse is compared with a reference voltage Vre~ in the comparator IC 2. In this way only pulses with a height exceeding Vre~ are transmitted and all pulses smaller than V~ef arising from noise, etc. are rejected. V~f should be chosen as high as possible in order to obtain a good rejection of unwanted signals. A 10× amplifier (not shown) immediately after the d.c. filter under bad circumstances improves noise rejection. The one-shot IC 3 produces a pulse of long duration with which a Light Emitting Diode can be driven. Tuning by the variable condenser and proper functioning of the instrument thus can be surveyed easily. At the same time the instrument is TTL compatible, enabling direct coupling to a timer. The timer after a predetermined time interval can start some measurement procedure, e.g. sampling a current (in d.c. polarography) or a voltage, start a chronocoulometer or a network analyzer. This instrument has succesfully been in use for some time in several applications, e.g. in the measurement of capillary curves, in the potentiostatic bridge [2] and in an automatic network analysis system. Interference by d.c. voltages arising from rectification of the H.F. in additional electronic equipment most elegantly can be avoided by means of a relais automatically connecting a 0.47 p F polycarbonate capacitor with a 1000 pF ceramic disc capacitor in parallel across collector and emittor of the transistor in the oscillator (Z N 2222 A). This connection should be made immediately after dropfall and be broken shortly after a measurement has been made. In this way the oscillations are stopped at all times where the H.F. is not needed. A wiring diagram is available on request. REFERENCES 1 Ch. Yarnitzky, J. Electroanal. Chem., 51 (1974) 207. 2 J.H. Sluyters, M. Sluyters-Rehbach and J.S.M.C: Breukel, Z. Phys. Chem., N.F., 98 (1975) 435.