troanalysis is in progress in these laboratories and the results will be published. The purpose of the present brief report is to show that the theory is validated by a t least one system. The full curve in the accompanying Figure 1 shows the current us. time curve recorded during a capped-ramp study of the cadmium ion electroreduction from 1.00mM C d Z + in 0.10M K N 0 3 solution at a mercury drop electrode of approximately 0.042 cm2 area. The ramp rate was 100 mV per second from a potential of -490 to -770 mV cs. SCE. The cell was unthermostated at a 21.5 “ C ambient. The dashed curve shows the result of semiintegrating the full curve by the RLO-algorithm. Observe that the semiintegral is finally a constant, as theory predicts since -670 millivolts corresponds to virtually complete concentration polarization. The value, m,of this constant is seen to be approximately 23 microamplombs. [The amplomb is the name given to the unit intermediate between the ampere and
the coulomb; amplornb = ampere second1i2 = coulomb s e c ~ n d - ~ / ~Use ] . of this value in the formula m = n A F C 4 5 of calculate the diffusion coefficient of cadmium ion leads to cmp sec-I, close to the an approximate value of 8 X accepted figure. ACKNOWLEDGMENT It is a pleasure to acknowledge the experimental assistance of Dr. Morten Grenness and the financial assistance of the National Research Council of Canada. KEITHB. OLDHAM Trent University Peterborough Ontario, Canada
RECEIVED for review December 28, 1970. Accepted March 22, 1971.
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The Circulation Cell-A New Device for Electroanalysis, Spectrotitrimetry, and the Study of Fast Reaction Kinetics Anne-Marie Gary, Etienne Piemont, Michelle Roynette, and Jean-Paul Schwing’ Institut de Chimie, I , rue Blaise Pascal, 67-Strasbourg, France
A NEW CELL and its applications to voltammetry, coulometric titrations, fast reaction studies, and spectrotitrimetry is described. The versatility of this “circulation cell” is due t o the fact that the investigated solution is forced, by the action of an integrated centrifugal pump, into an adequate hydrodynamic circuit whose special design allows the establishment of reproducible limiting currents on immersed solid electrodes and the contiruous feeding of an external device as, for instance, a spectrophotometric circulation cell. The described circulation cell is of modular design to allow the practice of the majority of the electroanalytical technique. as listed, for instance, by Kolthoff and Elving ( I ) . Several cells were constructed having volumes between 30 and 100 ml. EXPERIMENTAL Description of the Circulation Cell. Figure 1 shows the realization of this device: the centrifugal pump P is introduced into the cell through the ground joint J6 and gives the solution a rapid movement in the direction of the arrows a. The ground joint J j allows continuous injection of a reactant when the capillary Ca is introduced into Jj. Electrolytic generation of a reactant can be realized when the generating electrode E is introduced into Js, the auxiliary electrode, AE, being placed into J4. The fritted glass disk F separates the auxiliary electrode compartment from the solution. The joints Jl and JZ are intended t o receive the electrodes for endpoint detection or for the preliminary recording of currentvoltage curves. The grounded cylinder C allows the expan-
sion of the solution when a reactant is injected as a solution through Js. If deaeration of the solution is necessary, this can be achieved in the following way: the solution at rest should not be higher than level Lp; the lower part of the cylinder C is raised to level L4 and an inert gas such as nitrogenis admitted through tubing Tq and escapes through T3. The centrifugal pump is then switched on and thus leads to deaeration of the strongly stirred solution. When deaeration is completed, the gas stream through T4 is stopped, and the cylinder C is lowered until the level of the solution rises in the capillary tubing T3 which is then closed. Tubing Tg is used to empty the circulation cell, and tubing T j can be used to connect the cell to a mercury reservoir in order to set up, for certain studies, a mercury pool electrode at the bottom B of the cell. If thermoregulation is necessary, this can be achieved by water circulation inside of the cylinder, C, the water entering through TI and leaving through T2. Thermoregulation can also be realized by a water jacket, J, built around the main body of the cell. When the absorption spectrum of the reacting solution is of interest (spectrotitrimetry, spectrophotometric study of electrochemically reduced or oxidized species), inlet and outlet tubes, shown on Figure 2, can be placed into J1 and Js. The special design of these tubes allows the deviation of a part of the solution, with a small and reproducible delay, through a spectrophotometric observation cell. RESULTS AND DISCUSSION
l
Correspondence to be addressed to this author.
(1) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,’’ Interscience, New York, N.Y., 1965, Vol. 4, p 2225. 198
Figure 3 shows the current-voltage curve obtained for the rapid system 12/1-, in acidic medium (0.05M H2SO4) when a three-electrode system is used. In this case, the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
I Spectrophotometric
I
Figure 2. Attachment for the deviation of a part of the solution through a spectrophotometric observation cell. Flow accelerated by Venturi effect
’6
S
Figure 1. Schematic representation of the circulation cell with some accessories working electrode consists of a platinum wire ending in a small sphere and is introduced into J1 in such a manner that the platinum wire comes between level Lz and L3 (Figure 1) where the flowing of the liquid is the most nearly regular. The reference electrode (saturated calomel electrode) is placed in J2 and the auxiliary electrode, AE, in J4. It is interesting t o compare the small current fluctuations shown on Figure 3 with the important fluctuations observed by Vetter ( 2 ) when reduction of the same system is studied on a platinum electrode immersed in a solution stirred with a conventional magnetic stirrer. Furthermore, we have found excellent reproducibility of the reduction current of I1 t o I-, nine consecutive currentvoltage curves being indistinguishably superposed. The limiting current i, for this reduction, is related t o the number N of rotations per minute of the centrifugal pump (varied between 1000 and 4000 rpm) by the relation : i
= k”&575
(1)
similar to the relation : i
=
kU“
(2)
given by Adams (3), where k is a constant characteristic of the shape of the electrode for a given set of experimental conditions and U represents the velocity of the liquid flow. The value 0.575 of the exponent of N , being higher than 0.5, indicates a turbulent flow a t the surface of the working elecelectrode. A speed of 2600 rpm has been retained for further work as it gives very stable limiting currents and also allows the feeding of an external spectrophotometric circulation cell (2) K. J. Vetter, “Electrochemische Kinetik,” Springer Verlag, New York, N.Y., 1961, p 307. (3) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker, New York, N.Y., 1969.
Figure 3. Voltammetric curve of the system 12/1-in 0.05M H,S04medium, recorded with the circulation cell with the aid of the attachment shown on Figure 2. We have also shown that very satisfying proportionality (deviations of the order of l z ) is observed between limiting current and concentration. This is an important feature of the circulation cell as it justifies its use in fast reaction kinetics. Application of the Circulation Cell to Electroanalysis and Spectrotitrimetry. The circulation cell was tested with the well known titration of As (111) with Iz in 0.5M sodium hydrogen carbonate medium and of Mn2+ with M n 0 4 - in 0.4M pyrophosphate medium at p H 6. These two titrations were chosen to compare, for each of them, three different indicating methods-Le., amperometric titration with one or two polarized electrodes and spectrophotometric end-point detection. The titration curves were recorded and showed well defined breaks near the end point. The preliminary recording of the voltammetric curves for these systems gave the values of the tension t o be applied between the detection electrodes. Figure 4 represents the position of the electrodes for biamperometric end-point detection, and Figure 2 shows the attachment for spectrotitrimetry. The reactants were injected as solutions, through J5 (Figure 1). For the spectrophotometric end-point detection of the titration of As (111), with Is, a Beckman 1211 photometer was used with a green filter. A more monochromatic light beam had to be used for the spectrotitrimetry of Mn2+ with MnO,-: in
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199
Figure 4. Electrodes for biamperometric end-point detection this case a Beckman D. B. spectrophotometer was used at 345 nm. As a general result the three end-point detection methods were of comparable precision for the titration of As(II1) with 12 (+ 1%, i.e., the precision of the buret used) whereas for the titration of Mn2+ with Mn04- the spectrophotometric detection was much more precise and reliable (=t 1%) than the amperometric methods used. This is probably due t o the fact that in this latter case the surface state of the detection electrodes is modified in a nonreproducible way during the titration. Thus it appears that a comparative study of end-point detection methods will be useful in each case: the modular design of the circulation cell does facilitate such a study and also gives access t o the important voltammetric curves of the electrochemical systems in the medium used for the titration. Application of the Circulation Cell to the Spectrophotometric Study of Electrochemically Oxidized or Reduced Species. We have used the circulation cell for the spectrophotometric study of the cathodic reduction products of Wv’, in 8M HCI, on a mercury cathode. A constant current of 3 mA was imposed between a mercury cathode set up a t the bottom B of the cell and the auxiliary electrode compartment placed into Jd. The spectrum of the solution was recorded every 2000 seconds using the attachment shown in Figure 2. The results obtained are very similar t o those of Lingane and Small ( 4 ) who studied the cathodic reduction of Wv’ in 1 2 M hydrochloric acid. The use of the circulation cell avoids any contact between atmospheric oxygen and the strongly reducing cathodic reduction products. The whole study can be done without manipulation of test portions. This feature of the circulation cell might be of interest in the spectrophotometric study of electrochemically oxidized or reduced radioactive solutions. Application of the Circulation Cell to the Study of Fast Reaction Kinetics. We have examined the kinetics of bromination of cyclohexene in methanol a t 25 “C which had previously been studied by Dubois et al. ( 5 ) using a rotating electrode for stabilization of limiting currents. The rate constant that Mouvier (6) has calculated by the “concentrostat” method was determined by two methods: the first one follows bromine consumption by the olefin us. time, the second one maintains a steady-state of bromine concentration. The re(4) J. J. Lingane and L. Small, J. Amer. Chem. SOC.,71,973, (1949). (5) J. E. Dubois, P. Alcais, and G. Barbier, J . Electrounul. Cltenz., 8, 359 (1964). (6) G. Mouvier, Thkse, No. 5093, University of Paris, 1964. 200
sults of the different methods have been compared for this reaction. EXPERIMENTAL ARRANGEMENT. Bromine concentration was detected amperometrically, a 350-mV potential difference being applied between two identical platinum wires (Figure 4) introduced through ground joint JI. Bromine is produced by anodic oxidation of bromide ion on electrode E placed in ground joint Jz. The cathode dips into a cathodic compartment filled with 0.5M sulfuric acid, isolated from the cell with a fritted glass disk and agar-gel. This compartment is placed in the ground joint J4. The reactant is introduced with a Hamilton microsyringe through a rubber diaphragm covering Js. The mixing time is minimized by injecting very close to the centrifugal pump rotating at 3000 rpm. The temperature is maintained at 25 “C with water circulated from a thermostat through jacket J. The limiting reduction current of Brz to Br- can be recorded directly between el and e2 (Figure 4) or after damping with a low-pass filter, FIRST METHOD : RECORDING OF THE CONSUMPTION OF BROMINE BY THE ALKENE US. TIME. A volume of 100 ml of sodium bromide solution (0.2M) in methanol was introduced in the circulation cell and deaerated. A preelectrolysis is necessary to produce a certain amount of bromine in order to drive t o completion the reaction of bromine on the solvent used. This phenomenon has appeared even with good grade redistillated solvents. Between D and A (Figure 5 ) an immediate consumption of bromine by the methanol has been observed. After a 700-sec electrolysis at 3 mA, the electrolysis was stopped, a steady level was recorded, and then a current-intensity of 3 m A for 100 sec was imposed so that a known variation of the concentration of bromine could be introduced. Thereafter, it was possible to determine in several successive measurements, the average ratio A[BrJAi (i being the diffusion current of bromine). This result allows the calculation of the consumption of bromine by cyclohexene during the reaction. The concentration of bromine remaining in the solution after preelectrolysis and standardization is determined with accuracy by using a low-pass filter. When the ratio A[Brz]/Ai is known, the value of the initial concentration of bromine can be obtained. It was approximately equal t o 1.4 x lO-4M for all our experiments. At time ti (Figure 5) a volume of 1 p1 of cyclohexene (Fluka purissimum) is rapidly injected with a microsyringe through the rubber diaphragm. The concentration of olefin is lower than that of bromine; it has a n average value of 10-4M. A fast rate of recording (60 mm/min) and a simple potentiometric circuit were used t o follow the consumption of bromine by cyclohexene. When the reaction had been completed, the filter was re-connected so that the level of the final concentration and thus the value of the concentration of remaining bromine might be determined with best accuracy. From eight measurements, the average value of the rate constant kz was found to be 4.87 X l o 4 1. mole-’ min-I for the bromination of cyclohexene in methanol at 25 “C. The value found by Mouvier (6) with the “concentrostat” method was 4.71 X l o 41. mole-’ min-’. SECONDMETHOD: USING A STEADY-STATE OF BROMINE CONCENTRATION. This method consists of recording the reduction current of bromine during the competition between the rate of bromine production by electrolysis and the rate of its consumption by the excess of cyclohexene. This competition can be described by the general equation:
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
d[Br~l- i dt 2FV
kz [cyclohexene] [bromine]
(3)
if
,
Reelectrdysis 7 m s 3rnA
Standardization
iwi
i
I
3mA
,
mi
3mA
700s
DA
unption of bromine
/cons
iws 3mA
i 30
ti
60
90
t(s)
Figure 5. Instantaneous bromine concentration during preelectrolysis, standardization, and reaction with cyclohexene
t
t
mv Standardizatcn
mv Steady-date concentration
Figure 6. Bromine concentration during preelectrolysis, standardization, and after attainment of the steady-state where F is the Faraday, V the volume of the solution, and 1 the electrolysis current. If the value of i is low and if cyclohexene is present in a large excess, the concentration of olefin can be taken as constant and equal to "a" for a given r. After a certain period of time a steady-state is reached and bromine concentration in the solution tends towards the constant value: i [Brzlst = ___ 2 FVkza
(4)
Thus, the rate constant kz is given by : kz
i 2 FVu [Br2lst
The height of the competition level leads to the value of [Brzlst. The first steps of this method are the same as those of the former method. However, as the knowledge of time is not necessary for steady-state methods, all of the measurements were made with the low-pass filter connected. Figure 6 represents these different steps. After preelectrolysis and standardization 4 p1 of cyclohexene were injected with a Hamilton microsyringe (accuracy 2 %) through the rubber diaphragm. This amount of cyclohexene corresponds to an excess of about 3 X 10-4M. A sensitivity fifty times higher than that used for the first steps of this method (or for the coulamperometric method) was used t o detect the competition level. Before anodic production of bromine, a residual current iR about seven times greater than that recorded for the sodium bromide solution alone, at the same sensitivity, appeared. This residual current is not proportional t o the concentration of cyclohexene nor to that of dibromocyclohexane: iR has t o be subtracted from the steady-state current. Electrolysis with a current of 1 m A of the sodium bromide
solution in methanol containing dibromocyclohexane and a large excess of cyclohexene leads t o a competition level corresponding to a bromine concentration of approximately 2 x lO-'M. Even under the best conditions, a slight increase of this level during the electrolysis time can be observed in all recordings. This leads t o an inaccuracy in the height of the level of about i5 %. The cyclohexene and the microsyringe had t o be thermostated t o improve the reproducibility of the results. In six recordings the competition levels were identical. Their height increased from 1.1 cm (for t = 1 min) t o 1.2 cm (for t = 4 min). This leads t o two values of the rate constant kz = 5.05 X lo4 1. mol-1 min-' and k2 = 4.64 X lo4 1. mole-' min-I. These values are included in the 95% confidence interval determined in the first method. The two kinetic methods described in this paper will probably be applicable to a large number of rapid reactions involving electroactive species as reactants or products. Because of the fast mixing of reactants, and the possibility of electrolytic reactant generation, the circulation cell would probably also be a convenient tool for the study of reactions leading to the formation o r t o the consumption of ionic species for which reversible electrodes are known. If, for certain electrodes, the potential would not be rapidly established in comparison with the rate of the reaction, this would evidently imply the use of the steady-state method. ACKNOWLEDGMENT
The authors thank L. B. Rogers for reading the manuscript. They thank also F. Antoni, E. Goesel, and s. Wechsler for technical assistance during this work. RECEIVED for review July 1, 1971. Accepted September 10, 1971. This work was supported in part by the Centre National de la Recherche Scientifique E.R.A. No. 166.
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