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An electronically controlled dual-intermediate coulometric titrator with end-point anticipation
Analytica Chimica Acta. 124 (1981) 85-90 0 Elsevier Scientific Publishing Company, Amsterdam -Printed
in The Netherlands
AN ELECTRONICALLY CONTROLLED DUAL-INTERMEDIATE COULOMETRIC TITRATOR WITH END-POINT ANTICIPATION
JOHN T. STOCK Department of Chemistry,
University of Connecticut, Stan-s. CT 06268
(U.S.A.)
(Received 18th August 1980)
SUMMARY An electronically controlled dual-intermediate coulometric titrator that is constructed from readily-available components and a simple semimicro titration cell are described. The circuitry of the titrator permits most of the titration to be run rapidly, but provides gradual approach to the changeover and end-points. Tbe device was tested by the determination of 4-385 rg of aniline by generation of excess of bromine, followed by backtitration with copper(I). Standard deviations ranged from t 0.5 sg with small amounts of aniline to k3.7 pg with large amounts.
A common titrimetric procedure for substances that react slowly involves the addition of a known amount of reactant, waiting for the reaction to proceed to completion, and back-titration of the excess of reactant with a second reagent *thatreacts quite rapidly with the first one. Buck and Swift [l] determined submillimolar amounts of aniline by coulometric addition of excess of bromine followed by coulometic back-titration with copper(I). These workers used a NaBr-_CuSOe-HCl medium in which bromine generation can be changed to back-titration by mere reversal of the generating current. The equilibria involved in this dual-intermediate process [2, 31 and the optimum conditions for the determination of aniline 143 have been further studied. The dual-intermediate approach has been used for a variety of other species [ 5-121. These titrations were performed manually. Mechanized coulometic titrators began to appear approximately 30 years ago 1131 and recent reviews [14] indicate that the development of these devices is continuing. The present account describes an electronically controlled dual-intermediate coulometric titrator by which the major portions of the titrants are generated rapidly. However, the termination points of the forward addition of reactant and the back-titration are approached by brief generations that alternate with waiting periods. The principle applied here to generation of excess of bromine and back-titration with copper(I) can obviously be modified for cases where the first reactant has reducing action. Modification of the switching system should allow the device to be used for cases that require the use of externally-generated reagents.
86 EXPERIMENTAL
Titration cells
A IO&ml tall beaker was used as the larger of the two titration cells. A rubber stopper, with an opening for sample introduction, supports the working electrode, isolation chamber for the auxiliary electrode, and indicator system 1151. Working and auxiliary electrodes are each 13-mm squares of platinum foil_ The biamperometric indicator electrcdes are two parallel %-gauge platinum wires fixed 3 mm apart by a glass bead that is sealed across the free ends 1161. Each wire has a N-mm exposed length. The smaller cell shown in Fig. 1 was made by cutting down a weighing bottle and supporting it in a spring clamp [ 161. The electrode systems are cemented to a rectangle (A) of unclad electronic circuit board. To allow this assembly to be held on a ring stand, the board is screwed to a 70-mm length of hardwood rod (B). The 9 mm X 5 mm platinum foil working electrode is pinch-sealed into one end of 4-mm diameter soft glass tube (C). A little mercury serves as contact intermediate and is introduced before the upper end of the tube is plugged. The electrode is mounted edge-on to the motion of the solution in the cell. A spiral (E) of platinum wire forms the auxiliary electrode. The upper end of the wire has a connecting lead of subminiature PVC-covered flexible wire with a 1Ocm loop as shown. This allows ready removal of (E) from isolation chamber (D). This is made from E&mm diameter tubing by drawing out to yield an orifice that is approximately 1.5 mm in diameter. The orifice is closed by a short plug (G) of filter paper that has been soaked in the appropriate chamber solution and then well tamped down 1171. With 1 M
Fig. 1. Smd
titration cell.
87
HzS04 in both cell and chamber, the resistance between working and auxiliary electrodes was in the range 300-400 SL. Pinch-sealing is used to secure the platinum wire biamperometric electrode pair in a 4nun diameter tube (F). After sealing on the glass separator bead, the pair is bent twice at right angles, as shown at (c). This allows the exposed wires to be completely submerged when the depth of solution in the cell is small. The upper ends of the wires are threaded through holes in the board and soldered to flexible connecting leads. The stirring-bar (H) is a glassenclosed, 7-mm length of sewing needle [ 161. Samples were introduced into the cells by 2-ml and 0.2-ml Gilmont microburettes. Reagents Standard aniline solution, 0.385 g 1-l in 1 M HCl, was made by direct weighing of freshly-distilled aniline. The concentration was checked by manually-controlled coulometric titration [1] _ The titration medium was made in the cell by introducing equal volumes of 0.25 M NaBr and 0.05 M G&O4 in 2.7 M HCl. The electrolyte used in the isolation chamber was 1 M H,SO,. Opemfingprinciples of the titmtor Only the main controls are mounted on the front panel of the titrator. Subsidiary controls and all other components are mounted on a baseboard that is secured to the panel. The sides of the lift-off cabinet have small labelled openings for screwdriver adjustment of the controls that set the operating parameters of the instrument. Power supplies are mounted on the well-ventilated rear of the baseboard. Operating principles are illustrated in Fig. 2. High and low coulometric constant currents, set at 9.65 mA and 0.965 mA respectively, are selected by a switch that also controls the lighting of the appropriate decimal point of a 3digit, ;I-segment light emitting diode (LED) readout. The constant current lines go to a relay that allows the polarity of the generating system _-_~-~_------_-----_
Fig. 2. Block diagram of titratir system.
_-__:
88 to be reversed_ However, the current can flow only when permitted by the circuit breaker, which is a second relay. When ready for titration, the readout is at zero and a red LED is lit to show that the working electrode is the anode (generation of oxidant) and the generating current is cut off. When the gate is open, the quartz crystal clock feeds 1.00~Hz pulses to the readout, which counts up during oxidant generation and down during back-titration_ The choice of frequency and of coulometric current enables the readout to indicate directly in microequivalents_ Gate and circuit breaker are controlled by the interrupter, which has three states, ON, OFF, and MARKSPACE. The latter is adjustable and was set at approximately 1.1 s on, 3 s off, in the present application_ Pressing the START button of the logic system causes the interrupter to start the titration and to open the gate. The lighting of a yellow LED along with the red one indicates that generation of an oxidant is proceeding. Eventually, the current in the biamperometric indicator circuit begins to rise and this current is proportionately converted to a voltage that is continuously monitored by a set of four comparators. Two comparators are active during the forward generation, while the responses of the other two control the back-titration. The comparator outputs are coupled to the logic system by four opto-isolators. Suppose that the forward generation is to be stopped when the indicator current is 10 r.lA. When this current reaches a presettable value such as 9 PA, the first comparator causes the interrupter to go to the MARK-SPACE state. The yellow LED goes on and off, the readout increases intermittently, and the lO+A point is approached slowly. The triggering of a second comparator at this point causes reversal of the polarity of the generating system, return of the interrupter to the ON state, and change of readout response to “count down”. The red LED goes out and a green one lights to indicate that backtitration is proceeding_ When the indicator current has fallen to a low preset value such as 4 PA, the triggering of the third comparator again brings on the MARK-SPACE action. Finally, the indicator current sinks to the chosen end-point value, which must of course be greater than the residual current value. Titration then ceases and the readout is recorded manually before the STOP button is pressed to prepare the system for the next titration. A new titration can be initiated by merely pressing the START button_ Completecircuit detailswill beprovidedto interestedreaders_ RESULTS
AWD DISCUSSION
The wellestablished Br,-Cu(I) system [ 1, 5-81 was chosen to test the titrator, with aniline as the titrand [ 11. Current-to-voltage conversion was adjusted so that an input of 10 PA (the desired maximum indicator current) gave an output of 2.28 V. An output of this magnitude triggers the change from forward to back titration. Response settings for the other comparators were (a) forward continuous-to-intermittent, 2.04 V; (b) back continuous-to-
89
intermittent, 0.90 V; (c) stop, 0.22 V. Because of the comparative slowness of the bromination of low concentrations of aniline, the forward titration termination requires less “anticipation’? than that of the rapid back-titration. Changes in cell parameters may necessitate alteration of the continuous-tointermittent points. In the present experiments, it was found that setting (b) could be reduced to approximately 0.6 V without causing overshoot of the stop potential_ The forward and back “intermittent” stages should involve approximately equal numbers of microequivalents. A useful effect that was not foreseen in the design stage concerns the switches from continuous to intermittent titration. An arrest of approximately 5 s occurs before the regular MARK-SPACE action ensues. This provides additional time for the equilibration of the solution in the cell. Blank runs were made at 9.65 mA with 50 ml of mixed NaBr-CuSO, solution in the larger cell_ The average readout and standard deviation for 6 runs were -0-05 i 0.06 peq. Corresponding data at 0.965 mA with 5 ml of solution in the smaller cel.l were + 0.05 + 0.03 peq. Actual runs were corrected for these average blanks. Unless otherwise indicated, the results listed in Table 1 were obtained by the successive addition to the cell of equal amounts of aniline solution. A pretitration run with approximately 0.05 ml of this solution was performed each time the cell was emptied and refilled. The total time per run with the largest (385 pg) amount of aniline was approximately 450 s.
Conchczions Using their high-precision manual technique on amounts of aniline similar to those taken in the present work, Buck and Swift [1] obtained average errors ranging from +l.l to -0.2 pg: corresponding standard deviations were TABLE
1
Results for determination of aniline in standard samples (Regression equation (found (Y) vs. taken (x));y = (1.000 * 0.002)x - 0.7 * 0.4;s,,= r = 0.99998) Number of determinations
?j 6= 5= 11=-d 11qd 7=
Aniline (flg)a Taken
Found
116 385 231
114 385 230
38.5 26.8 19.2 11.5 3.9,
36.9 26.1 18.5 11.6 4.1,
Error (rg)
-2 -10 -1.6 -0.7 -0.7 +0_1 +0.3
0.9;
Std_ dev. (rg)
3.7 3.6 0.5 0.6 0.8 0.6 0.3
=l mole of aniline reacts with 3 moles of bromine_ b50 ml of cell solution, 9.65 mA_ ‘5 ml of solution in smaller cell, 0.965 mA, *7 runs, then 4 runs with refried ceh.
90
21.2 pg and 50.27 pg. The reasonably
accurate results that are rapidly obtainable with the present automatic titrator could probably be improved by modifications such as provision of a 4digit readout and of an anticipation system that senses the rate of change of signal from the indicator electrodes. REFERENCES 1 2 3 4
R. P. Buck and E. H. Swift, Anal. Chem., 24 (1952) 499. P. S. Fanington, D. J. Meier and E. H. Swift, Anal. Chem., 25 (1953) 591. J. J. Lmgane and F. C. Anson, Anal. Chem., 28 (1956) 1871. L. A. Chazova, B. A. Lopatin, G. L. Loshkarev and E. V. Rubanova, Fiz. Khim. Metody Anal. Kontrolya Proizvod., Mezhvus, Sb., 2 (1976) 33. 5 A. P. Zozulya and E. V. Novikova, Zavod. Lab., 29 (1963) 543. 6 F. Kawamura, K. Momoki and S. Suzuki, Bunseki Kagaku, 3 (1954) 29. 7 F. Magnu, Fzumaco Ed. Rat., 22 (1967) 677. 8 F. Magna and M_ Fiorani, Ric_ Sci., 38 (1968) 119. 9 A. J. Bard and J. J. Lingane, Anal. Chim. Acta, 20 (1959) 463. 10 A. J. Bard, Anal. Chem_, 32 (1960) 623_ 11 Hui-yu Yen, Cbao-Pin Chang and Yu-Hsien Li, K’o Hsueh T’ung Pao, 23 (1978) 740. 12 2. Slovak and J. Borak, Chea Prum., 18 (1968) 82. 13 J. J. Lingane, Electroanalytical Chemistry, 2nd edn., Interscience, New York, 1958, p_ 528. 14 J. T. Stock, Anal. Chea, 48 (1976) 1R; 50 (1978) 1R; 52 (1980) 1R. 15 D. T. Sawyer and J. L. Roberts, Experimental Electrochemistry for Chemists. J. Wiley, New York, 1974, p_ 425. 16 J. T. Stock and M. A. Fill, Mikrochim. Acta, (l-2) (1953) 89. 17 N. H. Furman and R. N. Adams, Anal. Chem., 25 (1953) 1564.
in The Netherlands
AN ELECTRONICALLY CONTROLLED DUAL-INTERMEDIATE COULOMETRIC TITRATOR WITH END-POINT ANTICIPATION
JOHN T. STOCK Department of Chemistry,
University of Connecticut, Stan-s. CT 06268
(U.S.A.)
(Received 18th August 1980)
SUMMARY An electronically controlled dual-intermediate coulometric titrator that is constructed from readily-available components and a simple semimicro titration cell are described. The circuitry of the titrator permits most of the titration to be run rapidly, but provides gradual approach to the changeover and end-points. Tbe device was tested by the determination of 4-385 rg of aniline by generation of excess of bromine, followed by backtitration with copper(I). Standard deviations ranged from t 0.5 sg with small amounts of aniline to k3.7 pg with large amounts.
A common titrimetric procedure for substances that react slowly involves the addition of a known amount of reactant, waiting for the reaction to proceed to completion, and back-titration of the excess of reactant with a second reagent *thatreacts quite rapidly with the first one. Buck and Swift [l] determined submillimolar amounts of aniline by coulometric addition of excess of bromine followed by coulometic back-titration with copper(I). These workers used a NaBr-_CuSOe-HCl medium in which bromine generation can be changed to back-titration by mere reversal of the generating current. The equilibria involved in this dual-intermediate process [2, 31 and the optimum conditions for the determination of aniline 143 have been further studied. The dual-intermediate approach has been used for a variety of other species [ 5-121. These titrations were performed manually. Mechanized coulometic titrators began to appear approximately 30 years ago 1131 and recent reviews [14] indicate that the development of these devices is continuing. The present account describes an electronically controlled dual-intermediate coulometric titrator by which the major portions of the titrants are generated rapidly. However, the termination points of the forward addition of reactant and the back-titration are approached by brief generations that alternate with waiting periods. The principle applied here to generation of excess of bromine and back-titration with copper(I) can obviously be modified for cases where the first reactant has reducing action. Modification of the switching system should allow the device to be used for cases that require the use of externally-generated reagents.
86 EXPERIMENTAL
Titration cells
A IO&ml tall beaker was used as the larger of the two titration cells. A rubber stopper, with an opening for sample introduction, supports the working electrode, isolation chamber for the auxiliary electrode, and indicator system 1151. Working and auxiliary electrodes are each 13-mm squares of platinum foil_ The biamperometric indicator electrcdes are two parallel %-gauge platinum wires fixed 3 mm apart by a glass bead that is sealed across the free ends 1161. Each wire has a N-mm exposed length. The smaller cell shown in Fig. 1 was made by cutting down a weighing bottle and supporting it in a spring clamp [ 161. The electrode systems are cemented to a rectangle (A) of unclad electronic circuit board. To allow this assembly to be held on a ring stand, the board is screwed to a 70-mm length of hardwood rod (B). The 9 mm X 5 mm platinum foil working electrode is pinch-sealed into one end of 4-mm diameter soft glass tube (C). A little mercury serves as contact intermediate and is introduced before the upper end of the tube is plugged. The electrode is mounted edge-on to the motion of the solution in the cell. A spiral (E) of platinum wire forms the auxiliary electrode. The upper end of the wire has a connecting lead of subminiature PVC-covered flexible wire with a 1Ocm loop as shown. This allows ready removal of (E) from isolation chamber (D). This is made from E&mm diameter tubing by drawing out to yield an orifice that is approximately 1.5 mm in diameter. The orifice is closed by a short plug (G) of filter paper that has been soaked in the appropriate chamber solution and then well tamped down 1171. With 1 M
Fig. 1. Smd
titration cell.
87
HzS04 in both cell and chamber, the resistance between working and auxiliary electrodes was in the range 300-400 SL. Pinch-sealing is used to secure the platinum wire biamperometric electrode pair in a 4nun diameter tube (F). After sealing on the glass separator bead, the pair is bent twice at right angles, as shown at (c). This allows the exposed wires to be completely submerged when the depth of solution in the cell is small. The upper ends of the wires are threaded through holes in the board and soldered to flexible connecting leads. The stirring-bar (H) is a glassenclosed, 7-mm length of sewing needle [ 161. Samples were introduced into the cells by 2-ml and 0.2-ml Gilmont microburettes. Reagents Standard aniline solution, 0.385 g 1-l in 1 M HCl, was made by direct weighing of freshly-distilled aniline. The concentration was checked by manually-controlled coulometric titration [1] _ The titration medium was made in the cell by introducing equal volumes of 0.25 M NaBr and 0.05 M G&O4 in 2.7 M HCl. The electrolyte used in the isolation chamber was 1 M H,SO,. Opemfingprinciples of the titmtor Only the main controls are mounted on the front panel of the titrator. Subsidiary controls and all other components are mounted on a baseboard that is secured to the panel. The sides of the lift-off cabinet have small labelled openings for screwdriver adjustment of the controls that set the operating parameters of the instrument. Power supplies are mounted on the well-ventilated rear of the baseboard. Operating principles are illustrated in Fig. 2. High and low coulometric constant currents, set at 9.65 mA and 0.965 mA respectively, are selected by a switch that also controls the lighting of the appropriate decimal point of a 3digit, ;I-segment light emitting diode (LED) readout. The constant current lines go to a relay that allows the polarity of the generating system _-_~-~_------_-----_
Fig. 2. Block diagram of titratir system.
_-__:
88 to be reversed_ However, the current can flow only when permitted by the circuit breaker, which is a second relay. When ready for titration, the readout is at zero and a red LED is lit to show that the working electrode is the anode (generation of oxidant) and the generating current is cut off. When the gate is open, the quartz crystal clock feeds 1.00~Hz pulses to the readout, which counts up during oxidant generation and down during back-titration_ The choice of frequency and of coulometric current enables the readout to indicate directly in microequivalents_ Gate and circuit breaker are controlled by the interrupter, which has three states, ON, OFF, and MARKSPACE. The latter is adjustable and was set at approximately 1.1 s on, 3 s off, in the present application_ Pressing the START button of the logic system causes the interrupter to start the titration and to open the gate. The lighting of a yellow LED along with the red one indicates that generation of an oxidant is proceeding. Eventually, the current in the biamperometric indicator circuit begins to rise and this current is proportionately converted to a voltage that is continuously monitored by a set of four comparators. Two comparators are active during the forward generation, while the responses of the other two control the back-titration. The comparator outputs are coupled to the logic system by four opto-isolators. Suppose that the forward generation is to be stopped when the indicator current is 10 r.lA. When this current reaches a presettable value such as 9 PA, the first comparator causes the interrupter to go to the MARK-SPACE state. The yellow LED goes on and off, the readout increases intermittently, and the lO+A point is approached slowly. The triggering of a second comparator at this point causes reversal of the polarity of the generating system, return of the interrupter to the ON state, and change of readout response to “count down”. The red LED goes out and a green one lights to indicate that backtitration is proceeding_ When the indicator current has fallen to a low preset value such as 4 PA, the triggering of the third comparator again brings on the MARK-SPACE action. Finally, the indicator current sinks to the chosen end-point value, which must of course be greater than the residual current value. Titration then ceases and the readout is recorded manually before the STOP button is pressed to prepare the system for the next titration. A new titration can be initiated by merely pressing the START button_ Completecircuit detailswill beprovidedto interestedreaders_ RESULTS
AWD DISCUSSION
The wellestablished Br,-Cu(I) system [ 1, 5-81 was chosen to test the titrator, with aniline as the titrand [ 11. Current-to-voltage conversion was adjusted so that an input of 10 PA (the desired maximum indicator current) gave an output of 2.28 V. An output of this magnitude triggers the change from forward to back titration. Response settings for the other comparators were (a) forward continuous-to-intermittent, 2.04 V; (b) back continuous-to-
89
intermittent, 0.90 V; (c) stop, 0.22 V. Because of the comparative slowness of the bromination of low concentrations of aniline, the forward titration termination requires less “anticipation’? than that of the rapid back-titration. Changes in cell parameters may necessitate alteration of the continuous-tointermittent points. In the present experiments, it was found that setting (b) could be reduced to approximately 0.6 V without causing overshoot of the stop potential_ The forward and back “intermittent” stages should involve approximately equal numbers of microequivalents. A useful effect that was not foreseen in the design stage concerns the switches from continuous to intermittent titration. An arrest of approximately 5 s occurs before the regular MARK-SPACE action ensues. This provides additional time for the equilibration of the solution in the cell. Blank runs were made at 9.65 mA with 50 ml of mixed NaBr-CuSO, solution in the larger cell_ The average readout and standard deviation for 6 runs were -0-05 i 0.06 peq. Corresponding data at 0.965 mA with 5 ml of solution in the smaller cel.l were + 0.05 + 0.03 peq. Actual runs were corrected for these average blanks. Unless otherwise indicated, the results listed in Table 1 were obtained by the successive addition to the cell of equal amounts of aniline solution. A pretitration run with approximately 0.05 ml of this solution was performed each time the cell was emptied and refilled. The total time per run with the largest (385 pg) amount of aniline was approximately 450 s.
Conchczions Using their high-precision manual technique on amounts of aniline similar to those taken in the present work, Buck and Swift [1] obtained average errors ranging from +l.l to -0.2 pg: corresponding standard deviations were TABLE
1
Results for determination of aniline in standard samples (Regression equation (found (Y) vs. taken (x));y = (1.000 * 0.002)x - 0.7 * 0.4;s,,= r = 0.99998) Number of determinations
?j 6= 5= 11=-d 11qd 7=
Aniline (flg)a Taken
Found
116 385 231
114 385 230
38.5 26.8 19.2 11.5 3.9,
36.9 26.1 18.5 11.6 4.1,
Error (rg)
-2 -10 -1.6 -0.7 -0.7 +0_1 +0.3
0.9;
Std_ dev. (rg)
3.7 3.6 0.5 0.6 0.8 0.6 0.3
=l mole of aniline reacts with 3 moles of bromine_ b50 ml of cell solution, 9.65 mA_ ‘5 ml of solution in smaller cell, 0.965 mA, *7 runs, then 4 runs with refried ceh.
90
21.2 pg and 50.27 pg. The reasonably
accurate results that are rapidly obtainable with the present automatic titrator could probably be improved by modifications such as provision of a 4digit readout and of an anticipation system that senses the rate of change of signal from the indicator electrodes. REFERENCES 1 2 3 4
R. P. Buck and E. H. Swift, Anal. Chem., 24 (1952) 499. P. S. Fanington, D. J. Meier and E. H. Swift, Anal. Chem., 25 (1953) 591. J. J. Lmgane and F. C. Anson, Anal. Chem., 28 (1956) 1871. L. A. Chazova, B. A. Lopatin, G. L. Loshkarev and E. V. Rubanova, Fiz. Khim. Metody Anal. Kontrolya Proizvod., Mezhvus, Sb., 2 (1976) 33. 5 A. P. Zozulya and E. V. Novikova, Zavod. Lab., 29 (1963) 543. 6 F. Kawamura, K. Momoki and S. Suzuki, Bunseki Kagaku, 3 (1954) 29. 7 F. Magnu, Fzumaco Ed. Rat., 22 (1967) 677. 8 F. Magna and M_ Fiorani, Ric_ Sci., 38 (1968) 119. 9 A. J. Bard and J. J. Lingane, Anal. Chim. Acta, 20 (1959) 463. 10 A. J. Bard, Anal. Chem_, 32 (1960) 623_ 11 Hui-yu Yen, Cbao-Pin Chang and Yu-Hsien Li, K’o Hsueh T’ung Pao, 23 (1978) 740. 12 2. Slovak and J. Borak, Chea Prum., 18 (1968) 82. 13 J. J. Lingane, Electroanalytical Chemistry, 2nd edn., Interscience, New York, 1958, p_ 528. 14 J. T. Stock, Anal. Chea, 48 (1976) 1R; 50 (1978) 1R; 52 (1980) 1R. 15 D. T. Sawyer and J. L. Roberts, Experimental Electrochemistry for Chemists. J. Wiley, New York, 1974, p_ 425. 16 J. T. Stock and M. A. Fill, Mikrochim. Acta, (l-2) (1953) 89. 17 N. H. Furman and R. N. Adams, Anal. Chem., 25 (1953) 1564.