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A microcomputer-controlled system for automatic acquisition of kinetic data of gas uptake at constant pressure
0039.9i40/81/070535-04802.00/O Copyright 0 1981 Pergamon Press Ltd
T&u~ro. Vol. 28. pp. 535 to 538. 1981 Pnoted in Great Bntatn. All nghts reserved
A MICROCOMPUTER-CONTROLLED SYSTEM FOR AUTOMATIC ACQUISITION OF KINETIC DATA OF GAS UPTAKE AT CONSTANT PRESSURE HARALD GAMPP Institute
of Inorganic
Chemistry,
University
of Basel, CH-4056
Basel, Switzerland
(Received 14 October 1980. Accepted 12 Nooember 1980) Summary-A
microcomputer-controlled apparatus for measuring the kinetics of gas uptake in a chemical reaction at constant pressure and temperature is described. The slight pressure decrease caused by consumption of the gas is transformed into an electrical signal. A microprocessor stores the time at which the signal occurs and activates a motor-driven burette to restore the original pressure. At the end
of the experiment, the data are recorded on magnetic tape by a cassette recorder and can be transferred from the tape to a desk computer for plotting or further mathematical treatment. The performance of the apparatus
is illustrated
by the example
of the catalytic
In many investigations of catalytic systems, the kinetics of gas absorption are measured at constant pressure and temperature.’ This is usually done with a gas burette, where constant pressure is maintained in a thermostatically controlled reaction vessel by means of a levelling bulb, and the volume of the gas consumed is read visually.’ In many biological and biochemical studies, a Warburg apparatus working at constant volume is used. 3*4Because of the need for constant attention by the operator, these techniques are rather tedious and not suitable for prolonged runs. An automatic apparatus not only frees the operator from taking individual readings but also allows more precise measurements to be made in the case of fast reactions. In recent years several automatic systems have been described,5-7 in which the data are obtained as analogue signals and are displayed on a millivolt recorder. However such systems may not work at constant pressure,5 or may be suitable only for micro amounts of gas,6 or be rather complicated to construct.’ In addition, the data still have to be digitized manually for further treatment. The present paper describes a fully automatic gas
oxidation
of benzoin
in dimethyl
sulphoxide.
volumeter that is simple to construct and can be assembled from readily available laboratory equipment with little extra work. (In fact, the most expensive parts of the system, the microprocessor and the motor-driven burette, are the same as already used in our titrating system.‘) About 1 mmole of gas can be taken up, thus allowing isolation of the products by conventional methods or by chromatography after the kinetic measurement. The apparatus is based on a Zilog 280 microprocessor chip, which maintains the system at constant pressure by means of a motor-driven burette, reads the data and stores them on magnetic tape. For further use the data can be transferred to a desk computer (e.g., an HP 9835 or APPLE II). As a chemical application, we studied the oxidation of benzoin in dimethyl sulphoxide (dmso), catalysed by autoxidized copper(I).’ EXPERIMENTAL
Apparatus The apparatus (Fig. 1) consists of a controlled-temperature reaction vessel, a 20-ml motor-driven burette (Metrohm E 415) capable of adding volume increments of
Dolphtn
Z80
Serial Interface RS 232
Magnetic
tape
Microcorder ZE 601 Fig. 1. Gas volumeter
system with microprocessor. 535
I
HARALD GAMPP
536
Reaction vessel 4
GND
Fig 2. Pressure-control
IRQ
unit. Interface for generating an interrupt in case of pressure decrease in the reaction vessel.
0.1 ml, a magnetic stirrer, a thermostat, a pressure-control unit (Fig. 2), a Dolphin microcomputer system (280 microprocessor, 0.5 kbyte ROM, 1.5 kbyte RAM, 2 kbyte EPROM, 8 lines parallel input/output) with serial interface (Stoppani Ltd.) and a magnetic tape recorder (Zinniker ZE 601) with FSK modem 1200-baud data transfer and an input-output compatible with an RS 232. The reaction vessel is immersed in a thermostatic bath and equipped with the magnetic stirrer. The burette and the pressure control are kept at constant temperature and connected with flexible tubing, which is also kept at constant temperature, to the reaction vessel. As gas is consumed, the pressure in the system decreases and the circuit between the two Pt-Ir contacts is closed by displacement of the aqueous sodium sulphate solution in the U-tube (Fig. 2). In order to avoid electrolysis, only a small alternating current of 18 mA flows through the solution; it has therefore to be transformed by the a.c. switching amplifier into a signal that can be detected by the microprocessor as an interrupt. The use of an aqueous solution instead of mercury in the U-tube manometer has the advantage of higher sensitivity because of the faster response. The motor-driven burette is used to restore the original pressure by diminishing the gas volume in the system by increments of 0.1 ml. The $6 output of the microprocessor is interfaced through an optoelectronically decoupled amplifier of Darlington type to the switch of the burette motor (Fig. 3).
Reagents
The benzoin used (Fluka, purum) was recrystallized from hot ethanol, CU(CH,CN)~BF~ was synthesized,” and the dmso (Merck, uvasol) was freshly distilled over calcium hydride under reduced pressure in a nitrogen atmosphere. Operation
In the catalytic experiments, the reaction vessel was a round-bottomed flask with a dropping funnel and a magnetic stirrer. Under a stream of oxygen, a solution of the substrate was placed in the flask and a solution of the catalyst was put in the dropping funnel, then the apparatus was closed. After thermal equilibrium was reached, which took about 15min, atmospheric pressure was restored in the system with the aid of a syringe. The reaction was started by mixing the two solutions, by opening the stopcock between funnel and flask. At the same time the microprocessor program, which had been loaded from magnetic tape, was started by reading in the experiment number. Thereafter the measurement proceeded automatically. Program*
The software was written in CALM Z80 assembled by hand in machine language, stored on magnetic tape and read into RAM before the start of the experiment. The program (Fig. 4) 150 bytes long, consists of a main program, for generation and display of the reaction time (in
Fig. 3. Interface for burette-motor * Programs are available on request.
activation,
Automatic acquisition of gas-uptake data
537
t At
Display
yes E
Store At on stock 1
I
Activate burette
Start internal time generator 1 Enter:
Expt
NO.
no
Store data on tape
I
End
0 Fig. 4. Data-acquisition
program flow-chart for kinetic measurements.
tenths of minutes), and of a subroutine for storage of the data and activation of the burette. In addition, monitor subroutines stored in EPROM are used, which process the data between the tape recorder and the memory of the microprocessor. Once the program has been started by reading in the experiment number from the keyboard, the time-counter is initialized and the program enters the time-generating loop. As soon as an interrupt from the pressure-control unit is detected, the program branches to the interrupt subroutine, where it stores the actual time in the RAM and switches on the burette motor by giving a signal on the $6 output of the microprocessor. Each addition of 0.1 ml takes 0.9 set, determined by the motor. Next, the program tests whether the whole content of the burette has already been added, and if this is not yet the case returns to the main loop. After all the data are collected, they are transferred from the RAM to the magnetic tape. If the reaction has ended before all the 20 ml of gas have been consumed, the data can be stored on tape by operating a switch on the keyboard of the microprocessor. The data can be read from the recorder by any computer with an RS 232 interface. A BASIC program has been written for an HP 9835 desk computer, which reads the data and creates a data file on the HP tape. The experimental points in the file are then plotted with an appropriate program.
RESULTS AND DISCUSSION The
accuracy
and
sensitivity
of the
Table 1. Accuracy and sensitivity of the apparatus as a function of the total gas volume (f0 replicates) Total gas volume, ml
Volume of gas removed with syringe, ml
Mean volume of gas added by burette, ml
Standard deviation of the mean ml
Sensitivity,* ml
35 130 230
5.0 5.0 5.0
4.97 4.99 4.99
0.048 0.074 0.099
0.05 0.08 0.12
* The minimum decrease in gas volume that can be detected by the pressure-control unit. TM.. 28;7s-E
apparatus
depend on its gas-tightness, on the inertia of the pressure control, on the constancy of temperature, and on the dead volume of the system (3timl for‘burette and tubing plus the volume of the reaction vessel). As the differences in pressure between the atmosphere and the system will be small, there is no problem in keeping the apparatus gas-tight. Apart from the U-tube of the manometer, the temperature of the whole system can be kept within f0.1” or better when working at room temperature. The use of a salt solution instead of mercury in the U-tube gives a higher sensitivity. Depending on the dead volume, amounts of gas as small as 0.05 ml can be detected (see Table 1, last entry). The accuracy as a function of the total gas volume was tested by removing 5 ml of gas with a gas-tight syringe from reaction vessels of 5, 100 and 200ml capacity, respectively. For each dead volume, the amount of gas added by the burette was determined 10 times, and the mean values and their standard deviations are given in Table 1. In all cases
538
HARALD GAMPP
the mean values are very close to the theoretical value and their standard deviation is about the same as the experimentally determined sensitivity. Hence, the accuracy of the results obtainable with our apparatus is limited by the burette, which can only add volume increments of 0.1 ml. A gas consumption of 20 ml can thus be resolved into 200 steps, so the maximum error from this source should be 0.5%. A better resolution can be obtained with a smaller burette and smaller reaction vessel. Chemical example
In our investigation of the oxidation of benzoin to benzil, catalysed by autoxidized copper(I) in dmso,’ we obtained a large number of kinetic data with the apparatus described. For example, lOm1 of 0.0054M Cu(CH3CN),BFd in dmso were stirred under oxygen for a time t,. Then 10 ml of O.OSMbenzoin in dmso were added. The curves obtained (Fig. 5) show that with the autoxidized copper(I), there was a nearly linear uptake of oxygen with time, after the induction period. The maximum rate of gas consumption, u,.~, which can be related to the concentration of the active catalyst,’ ’ decreases with increasing age of the autoxidized solutions. The mathematical analysis of the dependence of u,,, on t, suggests that a copper species which can no longer be regenerated as a catalyst under the conditions of Fig. 5 is formed according to a second-order rate law.9
t , min Fig. 5. Oxidation of benzoin, catalysed by autoxidized &(I) in dmso. Molar ratio of O2 consumed to initial substrate, vs. time. The age of the catalyst (in minutes) is given in the figure.
only one of the many available, but it was found best for our purposes. Light-activated switches are readily available and could be used equally, and a computertriggered valve could be used to start the reaction.
Acknowledgements-The author thanks Mr. R. Kissner for design and construction of the interfaces, and the Swiss National Science Foundation (grant No. 2.9240.77) and the Stiftung der Portlandcementfabrik Laufen for financial support. He also thanks the referee for some interesting and useful suggestions for further development of the equipment.
CONCLUSIONS REFERENCES
apparatus can be used to measure the kinetics of gas uptake at rates up to 6.5 ml/min. A total volume of 20 ml of gas can be taken up, with resolution to 0.5%. The apparatus is simple to construct and consists mainly of readily available laboratory equipment. Its operation is simple and, apart from mixing the reactants, fully automatic. The system is flexible and can easily be expanded to operate under pressures other than atmospheric or to measure the uptake of volumes from 5 to 50ml. The data can be stored on magnetic tape, and read from the recorder by any computer with an RS 232 interface, or transferred directly to the desk computer through the RS 232 interface, but the tape-recording allows several sets of data to be processed in a single batch. Any microprocessor or microcomputer capable of interrupt handling can be used to control the system and acquire the data. The pressure transducer system is The
1. H. Gampp and A. D. Zuberbilhler, in Metal Ions in Biological Systems, H. Sigel (ed.), Vol. XII. Dekker, New York, 1981. 2. A. I. Vogel, Quantitative Inorganic Analysis, 3rd Ed., p. 1054 ff. Longmans, London, 1966. 3. W. W. Umbreit, R. H. Burris and J. F. Stauffer, Mmometric Techniques, Burgens, Minneapolis, 1957. 4. 0. Warburg aid G. Kippahl, Ann.,‘1957,604,94. 5. J. W. Timberlake and J. C. Martin. Rev. Sci. Instr.. 1973.44, 151. 6. D. D. Davis and K. L. Stephenson, J. Chem. Educ., 1977,54. 394. 7. C. D. Thompson and N. Hackerman, Rev. Sci. Instr., 1973.44, 1029. 8. H. Gampp, M. Maeder, A. D. Zuberbllhler and Th. A. Kaden, Talanta, 1980, 27, 513. 9. H. Gampp and A. D. Zuberbllhler, in preparation. 10. German Patent No. 1230025, 1966; Chem. Abstr., 1967, 19, 488. 11. H. Gampp and A. D. Zuberbiihler, J. Mol. Cat., 1980, 7, 81.
T&u~ro. Vol. 28. pp. 535 to 538. 1981 Pnoted in Great Bntatn. All nghts reserved
A MICROCOMPUTER-CONTROLLED SYSTEM FOR AUTOMATIC ACQUISITION OF KINETIC DATA OF GAS UPTAKE AT CONSTANT PRESSURE HARALD GAMPP Institute
of Inorganic
Chemistry,
University
of Basel, CH-4056
Basel, Switzerland
(Received 14 October 1980. Accepted 12 Nooember 1980) Summary-A
microcomputer-controlled apparatus for measuring the kinetics of gas uptake in a chemical reaction at constant pressure and temperature is described. The slight pressure decrease caused by consumption of the gas is transformed into an electrical signal. A microprocessor stores the time at which the signal occurs and activates a motor-driven burette to restore the original pressure. At the end
of the experiment, the data are recorded on magnetic tape by a cassette recorder and can be transferred from the tape to a desk computer for plotting or further mathematical treatment. The performance of the apparatus
is illustrated
by the example
of the catalytic
In many investigations of catalytic systems, the kinetics of gas absorption are measured at constant pressure and temperature.’ This is usually done with a gas burette, where constant pressure is maintained in a thermostatically controlled reaction vessel by means of a levelling bulb, and the volume of the gas consumed is read visually.’ In many biological and biochemical studies, a Warburg apparatus working at constant volume is used. 3*4Because of the need for constant attention by the operator, these techniques are rather tedious and not suitable for prolonged runs. An automatic apparatus not only frees the operator from taking individual readings but also allows more precise measurements to be made in the case of fast reactions. In recent years several automatic systems have been described,5-7 in which the data are obtained as analogue signals and are displayed on a millivolt recorder. However such systems may not work at constant pressure,5 or may be suitable only for micro amounts of gas,6 or be rather complicated to construct.’ In addition, the data still have to be digitized manually for further treatment. The present paper describes a fully automatic gas
oxidation
of benzoin
in dimethyl
sulphoxide.
volumeter that is simple to construct and can be assembled from readily available laboratory equipment with little extra work. (In fact, the most expensive parts of the system, the microprocessor and the motor-driven burette, are the same as already used in our titrating system.‘) About 1 mmole of gas can be taken up, thus allowing isolation of the products by conventional methods or by chromatography after the kinetic measurement. The apparatus is based on a Zilog 280 microprocessor chip, which maintains the system at constant pressure by means of a motor-driven burette, reads the data and stores them on magnetic tape. For further use the data can be transferred to a desk computer (e.g., an HP 9835 or APPLE II). As a chemical application, we studied the oxidation of benzoin in dimethyl sulphoxide (dmso), catalysed by autoxidized copper(I).’ EXPERIMENTAL
Apparatus The apparatus (Fig. 1) consists of a controlled-temperature reaction vessel, a 20-ml motor-driven burette (Metrohm E 415) capable of adding volume increments of
Dolphtn
Z80
Serial Interface RS 232
Magnetic
tape
Microcorder ZE 601 Fig. 1. Gas volumeter
system with microprocessor. 535
I
HARALD GAMPP
536
Reaction vessel 4
GND
Fig 2. Pressure-control
IRQ
unit. Interface for generating an interrupt in case of pressure decrease in the reaction vessel.
0.1 ml, a magnetic stirrer, a thermostat, a pressure-control unit (Fig. 2), a Dolphin microcomputer system (280 microprocessor, 0.5 kbyte ROM, 1.5 kbyte RAM, 2 kbyte EPROM, 8 lines parallel input/output) with serial interface (Stoppani Ltd.) and a magnetic tape recorder (Zinniker ZE 601) with FSK modem 1200-baud data transfer and an input-output compatible with an RS 232. The reaction vessel is immersed in a thermostatic bath and equipped with the magnetic stirrer. The burette and the pressure control are kept at constant temperature and connected with flexible tubing, which is also kept at constant temperature, to the reaction vessel. As gas is consumed, the pressure in the system decreases and the circuit between the two Pt-Ir contacts is closed by displacement of the aqueous sodium sulphate solution in the U-tube (Fig. 2). In order to avoid electrolysis, only a small alternating current of 18 mA flows through the solution; it has therefore to be transformed by the a.c. switching amplifier into a signal that can be detected by the microprocessor as an interrupt. The use of an aqueous solution instead of mercury in the U-tube manometer has the advantage of higher sensitivity because of the faster response. The motor-driven burette is used to restore the original pressure by diminishing the gas volume in the system by increments of 0.1 ml. The $6 output of the microprocessor is interfaced through an optoelectronically decoupled amplifier of Darlington type to the switch of the burette motor (Fig. 3).
Reagents
The benzoin used (Fluka, purum) was recrystallized from hot ethanol, CU(CH,CN)~BF~ was synthesized,” and the dmso (Merck, uvasol) was freshly distilled over calcium hydride under reduced pressure in a nitrogen atmosphere. Operation
In the catalytic experiments, the reaction vessel was a round-bottomed flask with a dropping funnel and a magnetic stirrer. Under a stream of oxygen, a solution of the substrate was placed in the flask and a solution of the catalyst was put in the dropping funnel, then the apparatus was closed. After thermal equilibrium was reached, which took about 15min, atmospheric pressure was restored in the system with the aid of a syringe. The reaction was started by mixing the two solutions, by opening the stopcock between funnel and flask. At the same time the microprocessor program, which had been loaded from magnetic tape, was started by reading in the experiment number. Thereafter the measurement proceeded automatically. Program*
The software was written in CALM Z80 assembled by hand in machine language, stored on magnetic tape and read into RAM before the start of the experiment. The program (Fig. 4) 150 bytes long, consists of a main program, for generation and display of the reaction time (in
Fig. 3. Interface for burette-motor * Programs are available on request.
activation,
Automatic acquisition of gas-uptake data
537
t At
Display
yes E
Store At on stock 1
I
Activate burette
Start internal time generator 1 Enter:
Expt
NO.
no
Store data on tape
I
End
0 Fig. 4. Data-acquisition
program flow-chart for kinetic measurements.
tenths of minutes), and of a subroutine for storage of the data and activation of the burette. In addition, monitor subroutines stored in EPROM are used, which process the data between the tape recorder and the memory of the microprocessor. Once the program has been started by reading in the experiment number from the keyboard, the time-counter is initialized and the program enters the time-generating loop. As soon as an interrupt from the pressure-control unit is detected, the program branches to the interrupt subroutine, where it stores the actual time in the RAM and switches on the burette motor by giving a signal on the $6 output of the microprocessor. Each addition of 0.1 ml takes 0.9 set, determined by the motor. Next, the program tests whether the whole content of the burette has already been added, and if this is not yet the case returns to the main loop. After all the data are collected, they are transferred from the RAM to the magnetic tape. If the reaction has ended before all the 20 ml of gas have been consumed, the data can be stored on tape by operating a switch on the keyboard of the microprocessor. The data can be read from the recorder by any computer with an RS 232 interface. A BASIC program has been written for an HP 9835 desk computer, which reads the data and creates a data file on the HP tape. The experimental points in the file are then plotted with an appropriate program.
RESULTS AND DISCUSSION The
accuracy
and
sensitivity
of the
Table 1. Accuracy and sensitivity of the apparatus as a function of the total gas volume (f0 replicates) Total gas volume, ml
Volume of gas removed with syringe, ml
Mean volume of gas added by burette, ml
Standard deviation of the mean ml
Sensitivity,* ml
35 130 230
5.0 5.0 5.0
4.97 4.99 4.99
0.048 0.074 0.099
0.05 0.08 0.12
* The minimum decrease in gas volume that can be detected by the pressure-control unit. TM.. 28;7s-E
apparatus
depend on its gas-tightness, on the inertia of the pressure control, on the constancy of temperature, and on the dead volume of the system (3timl for‘burette and tubing plus the volume of the reaction vessel). As the differences in pressure between the atmosphere and the system will be small, there is no problem in keeping the apparatus gas-tight. Apart from the U-tube of the manometer, the temperature of the whole system can be kept within f0.1” or better when working at room temperature. The use of a salt solution instead of mercury in the U-tube gives a higher sensitivity. Depending on the dead volume, amounts of gas as small as 0.05 ml can be detected (see Table 1, last entry). The accuracy as a function of the total gas volume was tested by removing 5 ml of gas with a gas-tight syringe from reaction vessels of 5, 100 and 200ml capacity, respectively. For each dead volume, the amount of gas added by the burette was determined 10 times, and the mean values and their standard deviations are given in Table 1. In all cases
538
HARALD GAMPP
the mean values are very close to the theoretical value and their standard deviation is about the same as the experimentally determined sensitivity. Hence, the accuracy of the results obtainable with our apparatus is limited by the burette, which can only add volume increments of 0.1 ml. A gas consumption of 20 ml can thus be resolved into 200 steps, so the maximum error from this source should be 0.5%. A better resolution can be obtained with a smaller burette and smaller reaction vessel. Chemical example
In our investigation of the oxidation of benzoin to benzil, catalysed by autoxidized copper(I) in dmso,’ we obtained a large number of kinetic data with the apparatus described. For example, lOm1 of 0.0054M Cu(CH3CN),BFd in dmso were stirred under oxygen for a time t,. Then 10 ml of O.OSMbenzoin in dmso were added. The curves obtained (Fig. 5) show that with the autoxidized copper(I), there was a nearly linear uptake of oxygen with time, after the induction period. The maximum rate of gas consumption, u,.~, which can be related to the concentration of the active catalyst,’ ’ decreases with increasing age of the autoxidized solutions. The mathematical analysis of the dependence of u,,, on t, suggests that a copper species which can no longer be regenerated as a catalyst under the conditions of Fig. 5 is formed according to a second-order rate law.9
t , min Fig. 5. Oxidation of benzoin, catalysed by autoxidized &(I) in dmso. Molar ratio of O2 consumed to initial substrate, vs. time. The age of the catalyst (in minutes) is given in the figure.
only one of the many available, but it was found best for our purposes. Light-activated switches are readily available and could be used equally, and a computertriggered valve could be used to start the reaction.
Acknowledgements-The author thanks Mr. R. Kissner for design and construction of the interfaces, and the Swiss National Science Foundation (grant No. 2.9240.77) and the Stiftung der Portlandcementfabrik Laufen for financial support. He also thanks the referee for some interesting and useful suggestions for further development of the equipment.
CONCLUSIONS REFERENCES
apparatus can be used to measure the kinetics of gas uptake at rates up to 6.5 ml/min. A total volume of 20 ml of gas can be taken up, with resolution to 0.5%. The apparatus is simple to construct and consists mainly of readily available laboratory equipment. Its operation is simple and, apart from mixing the reactants, fully automatic. The system is flexible and can easily be expanded to operate under pressures other than atmospheric or to measure the uptake of volumes from 5 to 50ml. The data can be stored on magnetic tape, and read from the recorder by any computer with an RS 232 interface, or transferred directly to the desk computer through the RS 232 interface, but the tape-recording allows several sets of data to be processed in a single batch. Any microprocessor or microcomputer capable of interrupt handling can be used to control the system and acquire the data. The pressure transducer system is The
1. H. Gampp and A. D. Zuberbilhler, in Metal Ions in Biological Systems, H. Sigel (ed.), Vol. XII. Dekker, New York, 1981. 2. A. I. Vogel, Quantitative Inorganic Analysis, 3rd Ed., p. 1054 ff. Longmans, London, 1966. 3. W. W. Umbreit, R. H. Burris and J. F. Stauffer, Mmometric Techniques, Burgens, Minneapolis, 1957. 4. 0. Warburg aid G. Kippahl, Ann.,‘1957,604,94. 5. J. W. Timberlake and J. C. Martin. Rev. Sci. Instr.. 1973.44, 151. 6. D. D. Davis and K. L. Stephenson, J. Chem. Educ., 1977,54. 394. 7. C. D. Thompson and N. Hackerman, Rev. Sci. Instr., 1973.44, 1029. 8. H. Gampp, M. Maeder, A. D. Zuberbllhler and Th. A. Kaden, Talanta, 1980, 27, 513. 9. H. Gampp and A. D. Zuberbllhler, in preparation. 10. German Patent No. 1230025, 1966; Chem. Abstr., 1967, 19, 488. 11. H. Gampp and A. D. Zuberbiihler, J. Mol. Cat., 1980, 7, 81.