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A precision drop-fall detector for polarographic and electrocapillary work
Talanta, Vol. 35, No. 5, pp. 401402, 1988 Printed in Great Britain. All rights reserved
Copyright
0039-9140/88 $3.00 + 0.00 0 1988 Pergamon Press plc
SHORT COMMUNICATIONS
A PRECISION POLAROGRAPHIC
DROP-FALL DETECTOR FOR AND ELECTROCAPILLARY WORK
J. B. CRAIG*, D. HOPKINSand R. A. HOWIE Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, Scotland (Received 3 November 1987. Accepted 9 December 1987)
Summary-This paper describes the construction of a rugged yet very precise detector for use in polarographic and electrocapillary work. It employs an a.c. bridge technique to utilize the large and abrupt change in capacitance on drop-detachment. Timing and recording of drop-times is achieved with a laboratory microcomputer or a counter/timer.
In polarographic and electrocapillary experiments the lifetime of a natural drop from a dropping mercury electrode needs to be known with millesimal precision. Currently, drop-fall detectors use a superimposed a.c. voltage, or a light pulse,’ or a frequency-modulated transmitter where the DME forms part of the antenna? This paper describes the construction of a rugged yet sensitive drop-fall detector based on the a.c. bridge technique commonly used in differential capacity measurements. Its output is compatible with a laboratory computer to time, record and manipulate data for consecutive drops. *To whom correspondence should be addressed.
EXPERIMENTAL Apparatus The four arms of the Wheatstone bridge, Fig. 1, are formed by resistors RI and R2, the cell, and the variable resistance VRl in series with switched capacitors Cl-CS, this last arrangement allowing a wide range of C/R ratios to be selected to balance the bridge. The out-of-balance signal appearing across the bridge is fed to a preamplifier stage (TRl) through an isolatory screened transformer (TXl). The signal is further amp&d by an operational amplifier stage (ICl) which has its gain manually controlled by VR2. The Y output signal can be viewed on an oscilloscope, and is also applied to a full-wave rectifier stage (IC7, D5, D6, and associated components). The d.c. voltage appearing across C24 is used to drive the meter M2, thus giving visual indication of the out-of-balance signal. The Y output signal is also fed to a squaring circuit (TR2, Dl), the
n COUNT PULSE OUT
--15v Fig. 1. Block diagram of the drop-fall detector.
402
SHORT
COMMUNICATtONS
Table 1. Drop-times of a dropping mercury electrode in O.lM potassium chloride at 25”. Potentials were measured with respect to a O.lM potassium chloride/calomel electrode but are reported here with respect to the standard hydrogen electrode rop time, set Potential, V 0.334 0.284 0.234 0.184 0.134 0.084 0.034 -0.016 - 0.066 -0.116 -0.166 -0.216 - 0.266 -0.316 -0.366 -0.416 -0.466 -0.516 -0.566 -0.616 -0.666 -0.716 -0.766 -0.816 -0.866 -0.916 -0.966
Drop 1
Drop 2
Drop 3
Drop 4
Drop 5
5.095
5.094 5.305 5.454 5.575 5.680 5.769 5.850 5.907 5.954 5.989 6.008 6.017 6.013 6.001 5.977 5.948 5.910 5.867 5.813 5.758 5.697 5.621 5.550 5.468 5.385 5.290 5.197
5.098
5.095 5.307 5.452 5.576 5.678 5.773 5.845 5.908 5.954 5.988 6.007 6.015 6.011 6.000 5.981 5.945 5.910 5.869 5.817 5.760 5.697 5.628 5.551 5.469 5.384 5.291 5.196
5.097 5.307 5.449 5.574 5.680 5.770 5.845 5.905 5.951 5.987 6.007 6.015 6.018 6.004 5.977 5.947 5.907 5.860 5.815 5.755 5.695 5.625 5.547 5.468 5.381 5.293 5.194
5.307 5.450 5.578 5.677 5.769 5.844 5.908 5.955 5.990 6.006 6.015 6.008 5.997 5.978 5.944 5.909 5.865 5.818 5.758 5.694 5.625 5.549 5.473 5.383 5.291 5.191
resultant signal being applied to a Schmitt trigger oC2) to provide a clean output pulse for triggering the monostable oscillator (X3). The time constant of IC3 is set at 2 set and during this period the light-emitting diode (D2) is lit and a ‘ITL +5-V oulse is available at the emitter of the buffer stage (TR3).- In this particular case the output is fed to two counting devices: the tirst contains a pair of counters (Newport Universal, Model 613OA), one counting the oddnumbered and the other the even-numbered drops; in the second device, the signal is fed into the first of the one-bit inputs of an Apple He microcomputer fitted with a Mountain computer l-MHz clock (card number MTNOOI). The a.c. signal (10 mV, 100 Hzjapplied to the bridge is derived from either an internal oscillator (IC4) through a buffer stage (TR4) or an external oscillator through C28. The d.c. polarizing voltage is applied to the bridge through Ll, which presents a high impedance to the a.c. signal, thereby preventing this signal from being shunted by the external d.c. supply. To provide an indication of the amplitude of the ac. signal, this is amplified (IC5, TR4) and rectified (IC6), and the magnitude of the resultant d.c. signal can be observed on the meter Ml. The power supply consists of a f 15-V rail with a +5-V regulated supply from a voltage regulator (Ql). The a.c. bridge can be set so that a zero out-of-balance signal is obtained either at the birth of the drop or at the point of its detachment. The first option was chosen in the present studies of the electrocapillary properties of biomedical components where large changes in differential capacity and interfacial tension, and hence in drop times and sixes, were observed. The drop-fall detector is very rugged and has required no maintenance in two years of constant use. It has the advantage that the out-of-balance signal is constantly monitored, allowing the bridge cap-
5.304 5.451 5.573 5.683 5.771 5.847 5.911 5.954 5.985 6.011 6.015 6.011 6.001 5.976 5.948 5.910 5.861 5.813 5.756 5.694 5.624 5.549 5.470 5.383 5.293 5.192
acitors to be switched to maintain optimum operating conditions. With addition of a simple timer/drop-knocker unit, replacement of Cl-C5 with a series of decade capacitors, and display of the Y-out signal on an oscilloscope, the detector may be used to obtain differential capacity measurements by the method of Hills and Payne.”
RESULTS
Table 1 shows typical drop-times in O.lM potassium chloride, at 25”, as a function of electrical potential. A simple operational relationship between drop-times and interfacial tensions is given in a separate paper.4 A detailed circuit diagram and computer program are available on request. Acknowledgement-This work was sponsored by the United Kingdom Science and Engineering Research Council’s programme on tissue-implant interactions, grant number GR/B/O616.1. REFERENCES
1. P. D. Tyma, J. Elecfrochem. Sec., 1978, 125, 284C. 2. C. Yamitsky, C. A. Wijnhorst, B. Van der Laar, H. Reyn and J. H. Sluyters, J. Electroanal. Chem., 1977,77, 391. 3. G. J. Hills and R. Payne, Trans. Faraalzy Sot., 1965,61, 326. 4. J. B. Craig and C. Mackay, Tafanza, 1988, 35, 365.
Copyright
0039-9140/88 $3.00 + 0.00 0 1988 Pergamon Press plc
SHORT COMMUNICATIONS
A PRECISION POLAROGRAPHIC
DROP-FALL DETECTOR FOR AND ELECTROCAPILLARY WORK
J. B. CRAIG*, D. HOPKINSand R. A. HOWIE Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, Scotland (Received 3 November 1987. Accepted 9 December 1987)
Summary-This paper describes the construction of a rugged yet very precise detector for use in polarographic and electrocapillary work. It employs an a.c. bridge technique to utilize the large and abrupt change in capacitance on drop-detachment. Timing and recording of drop-times is achieved with a laboratory microcomputer or a counter/timer.
In polarographic and electrocapillary experiments the lifetime of a natural drop from a dropping mercury electrode needs to be known with millesimal precision. Currently, drop-fall detectors use a superimposed a.c. voltage, or a light pulse,’ or a frequency-modulated transmitter where the DME forms part of the antenna? This paper describes the construction of a rugged yet sensitive drop-fall detector based on the a.c. bridge technique commonly used in differential capacity measurements. Its output is compatible with a laboratory computer to time, record and manipulate data for consecutive drops. *To whom correspondence should be addressed.
EXPERIMENTAL Apparatus The four arms of the Wheatstone bridge, Fig. 1, are formed by resistors RI and R2, the cell, and the variable resistance VRl in series with switched capacitors Cl-CS, this last arrangement allowing a wide range of C/R ratios to be selected to balance the bridge. The out-of-balance signal appearing across the bridge is fed to a preamplifier stage (TRl) through an isolatory screened transformer (TXl). The signal is further amp&d by an operational amplifier stage (ICl) which has its gain manually controlled by VR2. The Y output signal can be viewed on an oscilloscope, and is also applied to a full-wave rectifier stage (IC7, D5, D6, and associated components). The d.c. voltage appearing across C24 is used to drive the meter M2, thus giving visual indication of the out-of-balance signal. The Y output signal is also fed to a squaring circuit (TR2, Dl), the
n COUNT PULSE OUT
--15v Fig. 1. Block diagram of the drop-fall detector.
402
SHORT
COMMUNICATtONS
Table 1. Drop-times of a dropping mercury electrode in O.lM potassium chloride at 25”. Potentials were measured with respect to a O.lM potassium chloride/calomel electrode but are reported here with respect to the standard hydrogen electrode rop time, set Potential, V 0.334 0.284 0.234 0.184 0.134 0.084 0.034 -0.016 - 0.066 -0.116 -0.166 -0.216 - 0.266 -0.316 -0.366 -0.416 -0.466 -0.516 -0.566 -0.616 -0.666 -0.716 -0.766 -0.816 -0.866 -0.916 -0.966
Drop 1
Drop 2
Drop 3
Drop 4
Drop 5
5.095
5.094 5.305 5.454 5.575 5.680 5.769 5.850 5.907 5.954 5.989 6.008 6.017 6.013 6.001 5.977 5.948 5.910 5.867 5.813 5.758 5.697 5.621 5.550 5.468 5.385 5.290 5.197
5.098
5.095 5.307 5.452 5.576 5.678 5.773 5.845 5.908 5.954 5.988 6.007 6.015 6.011 6.000 5.981 5.945 5.910 5.869 5.817 5.760 5.697 5.628 5.551 5.469 5.384 5.291 5.196
5.097 5.307 5.449 5.574 5.680 5.770 5.845 5.905 5.951 5.987 6.007 6.015 6.018 6.004 5.977 5.947 5.907 5.860 5.815 5.755 5.695 5.625 5.547 5.468 5.381 5.293 5.194
5.307 5.450 5.578 5.677 5.769 5.844 5.908 5.955 5.990 6.006 6.015 6.008 5.997 5.978 5.944 5.909 5.865 5.818 5.758 5.694 5.625 5.549 5.473 5.383 5.291 5.191
resultant signal being applied to a Schmitt trigger oC2) to provide a clean output pulse for triggering the monostable oscillator (X3). The time constant of IC3 is set at 2 set and during this period the light-emitting diode (D2) is lit and a ‘ITL +5-V oulse is available at the emitter of the buffer stage (TR3).- In this particular case the output is fed to two counting devices: the tirst contains a pair of counters (Newport Universal, Model 613OA), one counting the oddnumbered and the other the even-numbered drops; in the second device, the signal is fed into the first of the one-bit inputs of an Apple He microcomputer fitted with a Mountain computer l-MHz clock (card number MTNOOI). The a.c. signal (10 mV, 100 Hzjapplied to the bridge is derived from either an internal oscillator (IC4) through a buffer stage (TR4) or an external oscillator through C28. The d.c. polarizing voltage is applied to the bridge through Ll, which presents a high impedance to the a.c. signal, thereby preventing this signal from being shunted by the external d.c. supply. To provide an indication of the amplitude of the ac. signal, this is amplified (IC5, TR4) and rectified (IC6), and the magnitude of the resultant d.c. signal can be observed on the meter Ml. The power supply consists of a f 15-V rail with a +5-V regulated supply from a voltage regulator (Ql). The a.c. bridge can be set so that a zero out-of-balance signal is obtained either at the birth of the drop or at the point of its detachment. The first option was chosen in the present studies of the electrocapillary properties of biomedical components where large changes in differential capacity and interfacial tension, and hence in drop times and sixes, were observed. The drop-fall detector is very rugged and has required no maintenance in two years of constant use. It has the advantage that the out-of-balance signal is constantly monitored, allowing the bridge cap-
5.304 5.451 5.573 5.683 5.771 5.847 5.911 5.954 5.985 6.011 6.015 6.011 6.001 5.976 5.948 5.910 5.861 5.813 5.756 5.694 5.624 5.549 5.470 5.383 5.293 5.192
acitors to be switched to maintain optimum operating conditions. With addition of a simple timer/drop-knocker unit, replacement of Cl-C5 with a series of decade capacitors, and display of the Y-out signal on an oscilloscope, the detector may be used to obtain differential capacity measurements by the method of Hills and Payne.”
RESULTS
Table 1 shows typical drop-times in O.lM potassium chloride, at 25”, as a function of electrical potential. A simple operational relationship between drop-times and interfacial tensions is given in a separate paper.4 A detailed circuit diagram and computer program are available on request. Acknowledgement-This work was sponsored by the United Kingdom Science and Engineering Research Council’s programme on tissue-implant interactions, grant number GR/B/O616.1. REFERENCES
1. P. D. Tyma, J. Elecfrochem. Sec., 1978, 125, 284C. 2. C. Yamitsky, C. A. Wijnhorst, B. Van der Laar, H. Reyn and J. H. Sluyters, J. Electroanal. Chem., 1977,77, 391. 3. G. J. Hills and R. Payne, Trans. Faraalzy Sot., 1965,61, 326. 4. J. B. Craig and C. Mackay, Tafanza, 1988, 35, 365.