Pulse polarography

J.Electroanal. Chem., 68 (1976) 129--137 @)Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands

129

PULSE POLAROGRAPHY VIII. A NEW DIGITAL TO ANALOG PULSE POLAROGRAPH

J.P. VAN DIEREN, B.G.W. KAARS, J.M. LOS * and B.J.C. WETSEMA Scheikundig Laboratorium, Vri]e Universiteit, De Lairessestraat 174, Amsterdam-l O07 (The Netherlands) (Received 7th July 1975)

ABSTRACT A new pulse polarograph for normal mode instantaneous current sampling with digital control of timing and of potential generation, is described. Pulse times and delay times are independently variable over a wide range. Pulse times can be varied in 10 ms intervals. The performance of the instrument has been tested in the determination of pulse-polarographic diffusion coefficients of T1+, Cd 2+, Co(III)(en)~ + and Co(NH3)~ + . The reproducibility was found to be excellent.

INTRODUCTION In a n u m b e r o f p r e c e d i n g p a p e r s [1] it has b e e n s h o w n t h a t n o r m a l - m o d e pulse p o l a r o g r a p h y with variable pulse t i m e s can b e a p o w e r f u l t e c h n i q u e f o r t h e s t u d y o f t h e m e c h a n i s m o f fast r e a c t i o n s p r e c e d i n g t h e e l e c t r o d e reaction, t h e m o r e so if c u r r e n t s are i n s t a n t a n e o u s l y s a m p l e d . In t h e m e a n w h i l e it b e c a m e q u i t e clear t h a t a m o d e r n i z e d version o f t h e pulse p o l a r o g r a p h b u i l t earlier [2], w i t h t h e s a m e f e a t u r e s b u t w i t h a wide r a n g e o f n a r r o w l y spaced f i x e d pulse t i m e s w o u l d b e necessary. This p r o m p t e d us to build a n e w pulse p o l a r o g r a p h f o r n o r m a l - m o d e i n s t a n t a n e o u s c u r r e n t s a m p l i n g w i t h digital c o n t r o l of t i m i n g and o f p o t e n t i a l g e n e r a t i o n . Its a c c u r a c y a n d stability o f p e r f o r m a n c e will be c h e c k e d b y d e t e r m i n i n g t h e d i f f u s i o n c o e f f i c i e n t s o f T1÷, Cd 2+ and s o m e c o m p l e x ions o f C o ( I I I ) . POLAROGRAPH T h e i n s t r u m e n t consists of t h r e e p a r t s as d e p i c t e d in Fig. 1: (i) P r o g r a m m e r (digital), (ii) P o t e n t i a l g e n e r a t o r (digital/analog), (iii) Measuring circuit (analog). * To whom inquiries should be addressed.

1

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Fig. 1. Block diagram of the pulse polarograph. (i) Programmer, (ii) potential generator, (iii) measuring circuit. In the cell the working, reference and c o u n t e r electrodes are indicated as W.E., R.E. and C.E., respecti~'ely. For details see text.

131

(i) Programmer This unit produces the desired timing scheme, cf. Fig. 2. A 1 MHz crystal oscillator constitutes a clock of great stability. With the aid of seven onedecade counters (type SN 7490) the 1 MHz frequency is split in order to provide an ample range of setpoint possibilities, from 10 - 8 to 10 s. The clock is reset to zero after each delay-time interval and after each pulsetime interval so as to render the duration of both intervals independently adjustable. At the end of the pulse, at to o r t 2 in Fig. 2, the main control n o t only resets the clock, b u t also blocks the sampling gate to the sample and hold circuits SH I and SH II in Fig. 1, activates the electrode knocker and, if required, the short-circuit control. Then it awaits the end, at tl, of the delaytime interval (drop growth prior to pulse application) which occurs u p o n coincidence between the setpoint of the delay time (variable in 100 and 1000 ms steps) and the clock. At that instant a coincidence pulse arrives at the main control which in turn resets the clock, deblocks the sampling gate from the main-control side and awaits the end of the pulse interval. This interval is composed of a variable part (in 10, 100 and 1000 ms steps) and a constant part of 1 ms for current sampling. Upon coincidence between the setpoint of the pulse and the clock the sampling gate opens SH I as well as SH II during this 1 ms. This terminates the cycle. Other functions of the main control are mentioned under (ii). To prevent oscillations in the amplifier system as a consequence of incident spurious signals and to prevent high currents at the beginning of the pulse from overloading the current-measuring amplifier (46 k t y p e in part (iii) of Fig. 1), the latter can be short-circuited during the delay-time inter-

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132 val and, in 1, 10 and 100 ms steps shortly afterwards. As shown in Fig. 1 this is arranged by the setpoint short circuit and the short-circuit control. If n o t needed, this short-circuiting system can be switched off. All setpoints are adjustable with t h u m b wheels in BCD code (Philips 4311 027 8 2 5 1 1 ) .

(ii) Potential generator Potential generation occurs in a combination of a digital to analog converter, D/A, and a booster amplifier, B. The linearity, accuracy and rapidity of pulse application are fixed by the qualities of these two elements. B was constructed in our laboratory. Its slew rate is 50 V ps -1 at a maximum load of 400 mA. The D/A converter is from Analog Devices, type DAC 14 • QG. Bin., with external amplifier also from Analog Devices, type 45 k. The potential generation proceeds as follows: Through the setpoint delay potential the desired value of this potential is loaded by a push b u t t o n in the memory of the pulse potential. This m e m o r y consists of four presettable synchronous u p / d o w n decade counters, type SN 74192. The value of the delay potential is normally constant during a complete voltage scan. The values in the setpoint of the delay potential and in the pulse-potential memory (usually but not necessarily equal at the beginning of a scan) are both presented to a 2-line-to-l-line multiplexer. These values, in the order mentioned, are alternatively passed on to the BCD/Binary converter during the intervals to--t1 and t l - - t 2 (Fig. 2), respectively, as directed by the main control. At t 2 the main control also resets a one-decade counter, t y p e SN 7490, which then starts counting 0.1 MHz pulses till coincidence is achieved with the setpoint A-pulse potential. This setpoint consists of one t h u m b wheel in BCD, so the maximum number of pulses counted is nine. The actual number of pulses is transmitted by means of a switch for ease of operation located at the setpoint A-pulse potential, to the first, second or third decade of the pulse-potential memory, corresponding to 1, 10 and 100 mV steps, respectively. A c o u n t u p / d o w n control fixes the scan direction. The m o m e n t a r y value of the pulse potential can be read from a solid-state display apart from an additional constant value (see below). The cycle from to till t2 is repeated in the way described until the pulse potential matches the preset value of the setpoint pulse limiter. The D/A converter had a m a x i m u m o u t p u t of 10 V at an input o f 14 bits, so the least significant bit would correspond to 104/214 mV ~- 0.6 mV. By raising the m a x i m u m voltage of the D/A converter to 16.384 V its o u t p u t per least significant bit was raised to 1 mV in accordance with the decimal read-out of the t h u m b wheels. The D/A converter is used from --5 to +5 V and the constant value mentioned above is --5000. Recently D/A converters with BCD input have become available and a BCD/Binary converter is henceforth superfluous.

133

(iii) Measuring circuit

A double-throw switch, 2 el/3 el in Fig. 1, adapts the circuitry to usage with two- or three-electrode cells. This switch is put in corresponding positions when calibrating the system with the lower precision I k~2 resistor in the dummy. Potential measurement occurs by way of an amplifier type 45 k from Analog Devices, used as an impedance converter. The cell current is converted to a voltage by the amplifier t y p e 46 k from Analog Devices, used as a current to voltage converter with decade resistor R1, which is variable from 10--107 ~2, corresponding to a current range from 100 nA--100 mA. Each voltage, in the order just given and during the 1 ms access period governed by the sampling gate, is supplied to sample and holds SH I and SH II which in turn continuously feed the X- and Y-axes, respectively, of a recorder (Hewlett Packard HP 7005B). Both sample and holds are constructed in our laboratory and have a capacitance of 1 pF. In the sampling mode the resistance of the M.O.S. switch is 60 ~2, corresponding with an RC time of 6 × 1 0 - 5 s . In the hold mode the leak resistance is 1011 ~2 which corresponds with an RC time of 10 +5 s. The instrument can also be used as a classical d.c. polarograph. In that case the multiplexer is fixed in one position to supply the BCD/Binary converter continuously with the value of the pulse-potential memory. The present polarograph has a flat potential (E)--time (t) characteristic, contrary to conventional polarographs. This difference is depicted in Fig. 3a for d.c. and in Fig. 3b for pulse polarographs. The advantage of this flat E - - t feature lies in the partial cancellation of the double-layer charging (capacity) current, ic, which is i¢ = AC z d E / d t + K I ( E -- E z ) d A / d t

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where A is the electrode-drop area, Cz and K 1 are the differential and integral

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134

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Fig. 4. I n s t a n t a n e o u s d.c. p o l a r o g r a m s of 0 . 8 6 8 × 10 _ 3 M Tl(I) in 0.2 M KC1 w i t h scan rates o f 2 m V s- 1 ( ) a n d 100 m V s- 1 (~); (a) n e w p o l a r o g r a p h , d r o p t i m e 0 . 5 1 1 s; (b) P A R 170, d r o p t i m e 1 . 0 0 3 s, s a m p l i n g t i m e 1 0 0 ps. m _~ 1 mg s- 1 .

double-layer capacitances per unit area, respectively and E~ is the potential of zero charge. For the present polarograph dE/dt = 0. This permits the use of high overall scan rates. In Fig. 4a we show a T1+ d.c. diffusion wave at two scan rates: 2mV s - 1 (full line) and 100 mV s - 1 (squares). There is no difference, which implies that the response of the new polarograph is fast. If one really wants to work under circumstances of zero charging current i¢, a pendent drop electrode should be employed: dA/dt = 0. That would also make the present polarograph the ideal apparatus for use in the differential mode, at least if quantitative differential recordings are required. The same applies of course to recordings of cyclic voltammograms, for which we are making provisions at the moment. EXPERIMENTAL

A cell with a mercury-pool counter electrode was used with the polarograph in the two-electrode configuration. The dropping mercury electrode (DME) was of the cylindrical type and siliconized inside the capillary only. The internal bore diameter of the latter was always about 60 pm and the rate of flow of mercury always between 0.6 and 0.9 mg s - 1. A PAR Model 172 was used for the electrode-knocker device. The interval between the beginning of knocker activation (t 2 in Fig. 2) and the severing of the drop was about 6 ms. This was deducted from the formal delay time to give the true delay time. The current response of the polarograph was further checked by replacing

135 the cell b y a Bleeker decade-resistance box. Calibration of t he time intervals were carried out with a Hewlett Packard Model 5245L-electronic c o u n t e r provided with a time-interval unit Model HP 5263A. Experiments were carried out at 2 5 . 0 0 ( -+ 0.05) ° C. Oxygen was rem oved f r o m the cell by passing very pure, water-saturated nitrogen t hrough the solution and, during a run, over it. Depolarizer salts and supporting electrolytes were of reagent-grade quality. The supporting electrolytes were also twice recrystallized from water. The water and mercury used were bot h very pure [2]. RESULTS AND DISCUSSION Diffusion coefficients were calculated f r om the instantaneous limiting currents measured, id, at 25.0 °C, f r om eqn. (23) of ref. 2, which we now rewrite in S.I. units. id {1+

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where z is the charge n u m b e r of the cell reaction, m is the rate of flow of mercury, D is the diffusion coefficient of the depolarizer, c* is t he bulk concentration o f th e depolarizer, t I is the delay time, t is the pulse time and 0 is the reduced pulse time t/t 1. When 0 < 0.5 the last square braces in eqn. (2) can be replaced by 1/2 (1 + 0), which is correct within one per cent of the total current. In the calculations all diffusion coefficients differing more than twice the standard deviation of the individual values f rom the average were rejected. This was repeated until no f u r t h e r rejects occurred on this basis, but the actual numbet of results discarded was very small. A good check on t he proper functioning of the pulse polarograph was obtained f r o m measuring the diffusion coefficient of T1÷ (0.5 mM T1NO~) in 0.2 M KC1 under largely varying conditions. Pulse times varied from 21 ms to 321 ms and d r o p times f r om 0.5 s to 2.5 s. This resulted in D T ! = 1 7 . 9 2 (+0.05) × 1 0 - 6 c m 2 s - 1 as the average of 55 polarograms. This is in excellent agreement with the pulse-polarographic diffusion coefficient previously obtained in this lab o r at or y [3] : D = 17.86 (+0.12) × 10 - 6 cm2s - 1 and also with the tracer value o f Wang and Polestra [4] : D = 17.9 (-+0.2) × 10 - 6 cm2s - 1 . During these measurements it became clear t hat the new pulse polarograph yields very reproducible values for its time intervals: delay times, pulse times and drop times. All of these were always identical with their nominal (face) values within 0.015% and are reproducible over a long period of time, within the accuracy of the c o u n t e r used. Preceding this work we had made an analogous check with the same 0.5 mM

136 solution of T1NO3 in 0.2 M KC1 on the PAR Model 170 Electrochemistry System. This instrument, in its normal pulse-polarographic mode, has a pulse time of 45 ms plus 5, 10, 15 or 20 ms for current sampling. A small modification made it possible to choose from 11 pulse times between 20 and 900 ms. Another modification reduced the sampling duration to about 50 ps. We found this apparatus to behave in a somewhat erratic way, both in its original configuration and after alteration. When employing the sampling times of the original apparatus we used eqn. (2) in the integrated form analogous to eqn. (25) of ref. 2. When short drop times (e.g. 0.5 or 1 s) are combined with a long sampling period (10, 20 ms) of the PAR 170, the calculated Dwl values tended to be low. On the other hand a drop time of 2 s (pulse time before current sampling: 23.3 ms) gave too high results, the more so the shorter the sampling time: e.g. sampling times of 0.05, 10 and 20 ms gave values of DT1 under said conditions equal to 19.2, 18.4 and 18.1 X 10 - 6 cm2s - 1 , respectively. This is clearly due to overloading of the amplifier at the beginning of the pulse. Sluggishness of the amplifier system then produces too high a current during sampling and therefore too high a diffusion coefficient. When lifting the overloading of the amplifier by reducing the amplification by a factor 10 and simultaneously expanding the current axis of the PAR 170 recorder by the same factor, the diffusion coefficients of T1÷ obtained under the conditions mentioned above dropped to smaller values: 17.7, 18.1 and 17.9 X 10 - 6 cm2s - 1 in the same order, but now in good agreement with the proper value. Analogous to the slow and fast voltage scan, described earlier, with the new pulse polarograph (Fig. 4a), d.c. polarograms were recorded with the PAR 170 System at the same scan rates: Fig. 4b. The latter polarograms were taken at a sampling duration of 100 ps, comparable with the instantaneous current recording of the new polarograph. Fig. 4b shows the PAR 170 to give erroneo u s results at fast scan rates, presumably also on account of its sluggish amplifier. The original sampling duration of 5 ms gave the same result. Similar objections have been raised against the PAR Model 174 polarographic analyzer [5]. Diffusion coefficients of Cd 2+ in aqueous 1 mM solutions of CdC12 in 0.1 and 0.5 M KC1 have also been determined with the new pulse polarograph, using the same wide range of pulse times and drop times. For reasons unknown the diffusion coefficients derived in the 0.1 M KC1 solution from short pulse times (21 and 41 ms) were significantly too high and therefore discarded. In 0.5 M KC1 this effect was not present. The diffusion coefficients of Cd 2+ are given in Table 1. Both are smaller than the value of 7.91(+0.11) X 10 -6 cm2s- 1 given previously [2]. We think it likely that the latter value is in error. In a very recent paper Bolzan [ 6] investigated the trend of pulse-polarographic diffusion coefficients of Cd 2+ in solutions of different supporting electrolytes at varying concentrations, inclusive 0.1 and 0.5 M KC1. This author finds Dcd = 7.40 X 10 - 6 cm 2 s- ] in 0.1 M KC1 and 7.65 X 10 -6 c m 2 s~- 1 in 0.5 M KC1 at 25°C, in satisfactory agreement with our data.

137 TABLE 1 Pulse-polarographic diffusion coefficients at 25.0 ° C N is the number of pulse polarograms. Values in brackets are standard deviations of the mean Depolarizer

Supporting electrolyte

106D/cm 2 s- 1

N

T1+ Cd 2+ Cd 2+ Co(en) 3+ Co(NH3)3+

0.2 M KC1 0.1 M KC! 0.5 M KC1 0.1 M (NH4)2SO 4 0.1 M (NH4)2SO 4

17.92(_+ 0.05) 7.51(+ 0.04) 7.62(+_0.04) 5.30(_+ 0.05) 6.37(+_0.03)

55 27 23 15 43

Also listed in Table 1 are diffusion coefficients of t h e ions C o ( I I I ) ( e n ) 3+ and Co(III)(NH3)~+ in 0.1 M(NH4)2SO4 originating f r o m I m M solutions o f Co(III)(en)3C13 • 3 H 2 0 and Co(III)(NH3)6(C104)3, respectively. F o r establishing these D values, pulse and d r o p times were varied t o t h e same e x t e n t as for T1÷ and Cd 2+ and the wave C o ( I I I ) -* C o ( I I ) with z = 1 was used for the calculations. In view o f t h e great variation o f pulse-polarographic c o n d i t i o n s a n d the s u b s e q u e n t small error in t h e calculated d i f f u s i o n coefficients, it m a y safely be c o n c l u d e d t h a t t h e pulse p o l a r o g r a p h described in this p a p e r is an instrum e n t o f great stability, capable o f p r o d u c i n g very reliable results. It has also been used successfully in c o m b i n a t i o n with a r o t a t i n g p l a t i n u m disk electrode, e.g. with H 3 0 ÷ ion as depolarizer. A f t e r each c a t h o d i c pulse t h e p l a t i n u m elect r o d e was c o m p l e t e l y r e s t o r e d during t h e n e x t d e l a y - t i m e interval. REFERENCES 1 A.W. Fonds, J.L. Molenaar and J.M. Los, J. Electroanal. Chem., 22 (1969) 229; A.W. Fonds and J.M. Los, ibid., 36 (1972) 479; J.M. Los, A.A.A.M. Brinkman and B.J.C. Wetsema, ibid., 56 (1974) 187. 2 A.W. Fonds, A.A.A.M. Brinkman and J.M. Los, J. Electroanal. Chem., 14 (1967) 43. 3 A.W. Fonds, Thesis, Vrije Univer~iteit, Amsterdam, 1969, p. 26. 4 J.H. Wang and F.M. Polestra, J. Amer. Chem. Soc., 76 (1954) 1584. 5 J.H. Christie, J. Osteryoung and R.A. Osteryoung, Anal. Chem,, 45 (1973) 210. 6 J.A. Bolzan, J. Electroanal. Chem., 59 (1975) 303.