I CORRESPONDENCE Cyclic Voltammetric Investigation of a Chemical Reaction Preceded by a Reversible Charge Transfer: Reaction of Pyridinyl Radical with 4Nitrobenzyl Chloride Sir: Nicholson and Shain ( I ) , in their paper on the theory of stationary electrode polarography have discussed t h e reversible, irreversible, a n d kinetic system. For t h e kinetic system, O t n e S R
(1)
R k -
(2
They have derived a n equation relating t h e peak potential, the half wave potential, the rate constant k and rate of scan a, as RT E , = E,,2 - q[0.78 - ln(k/~)*'~] (31
The E112 for 1-ethyl-4-carbomethoxy pyridinium in 1,2dimethoxy ethane (with 0.1M TBAP) was -1.07 V in t h e absence of 4-NBC1 and t h a t of 4-NBC1 (in t h e absence of the pyridinium salt) was -1.22 V vs. Ag/AgC104 reference electrode. From the shift in the peak potential E , - E 1 / p N 15 millivolt and the scan rate 2 V/min, k', the pseudo-first-order rate constant was obtained from Equation 3 which, on dividing by the concentration of 4-NBC1, yielded the secondorder rate constant hp as 4 X IO4 M - l sec-l (at 25 "C) (Equation 7 ) .
Py* T h e disappearance of R could be a first-order decay as in Equation 2 above or a pseudo-first-order decay (Equation 4). 0 +ne a R
R
t
[A]
k'
(4
where A is present in large excess. Thus, for a reaction as given in Equation 4, 12' can be obtained using Equation 3. From this pseudo-first-order rate constant, h', t h e secondorder rate constant can be easily obtained. T o test the validity of t h e application of Equation 3 t o a system described by Equation 4, reaction of pyridinyl radical with 4nitrobenzyl chloride (the homogenous reaction of which had been studied earlier (2)) was studied with the help of' triangular wave cyclic voltammetry. The free radical was generated on the surface of a hanging mercury drop electrode by reducing 1-ethyl-4-carbomethoxy pyridinium perchlorate (Py+ C104-1 in 1,P-dirnethoxyethane (with 0.1M tetra-n- butylammonium perchlorate as supporting electrolyte and Ag/AgC104 as reference electrode).
Py'
+
e
q=?
Py.
(5)
The reversible E 1 / p was recorded from the cyclic voltammogram at a scan rate of 2 V/min (potential corresponding Q obtained was the to 85% of the peak current, the E ~ , thus same as recorded polarographically). T h e cyclic voltammogram (at 2 V/min) was again recorded in the presence of 5 x 10-4M 4-nitrobenzyl chloride (4-NBC1). T h e reduction peak for the pyridinium ion (Equation 5 ) was found to be shifted anodically and at the same time the anodic wave, which was recorded earlier in the absence of 4-NBCl, completely disappeared. T h e anodic shift in E , is thus caused by the disappearance of Pya radical by a chemical reaction which can be shown as
Py' t e Py.
+
G?
-
Py. k'
[4-NBCl]
(6)
where 4-NBCl is in large excess. 958
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
+
4-NBC1
k2
(7)
T h e homogenous reaction of pyridinyl radical and 4NBCl involves electron transfer reaction and is very sensitive to solvent polarity ( 2 ) . We have used Z-value as criteria for solvent polarity (2, 3 ) . The polarity of solvent is quite sensitive to the presence of salt and tetrabutyl ammonium perchlorate (TBAP) raises the solvent polarity (Z value) of 1,2-dimethoxyethane ( 4 ) . In fact, 2 value of 1,2dimethoxyethane (DME) in the presence of 0.1M T B A P (the condition under which electrochemical studies are carried out) is 66. The linear relationship between the secondorder rate constant for the reaction between the 4-NBC1 and Py. radical predicts (2) a rate constant of 1 X lo3 A4-l sec-' a t Z value 66. This means t h a t a second-order rate constant ( h 2 ) of about 1 X lo3 M-1 sec-l can be expected if a homogenous reaction of 4-KBC1 and Pya is studied in the presence of 0.1M TBAP. The rate constant obtained by the electrochemical method, and after dividing by a factor of 2 ( 2 ) , is 2 X lo4 M - l sec-I ( a t Z = 66). However, this discrepancy is not too high considering the inherent errors in determining the peak potential accurately, the approximation in deriving Equation 3. and some scattering in Z valuehp relationship, e.g., the Z value of dimethylformamide is 68 but hp is 1.2 X l o 4 A4-l sec-l. Generally, for a process given in Equations 2 or 4, if h' is quite small then the measurement of the ratio ianodiclicathodic gives rather accurate value for h' (2, 5 ) . However, when h' is 10 sec-' (or higher), then ianodiclicathodic measurement is neither very practical nor does it give accurate values for h' (6). As explained by Nicholson and Shain (21, whenever the ratio of ia/ic falls below 0.4, then h ' ~is difficult to measure. In such a case, a lower limit of h' can be obtained from ia/ic ratio, whereas t h e expression in Equation 3 is expected to give better value for h' if a reversible El/r (Equation 1) can be measured. In the present case, ia/ic could not be measured with any accuracy. A lower limit for hp was obtained from cyclic voltammetry as follows: If we assume that a t 2 V/min scan rate, on reversing the scanning after the peak potential is reached, 66% of pyridinyl already reacted and only 33% or less remained on the electrode (i.e., half life 1 sec or less
and 7 about 2 sec or less). Then, from the pseudo-firstorder constant (0,’i sec-’ or more), the lower limit of the second-order rate constant of 2 x IO3 M - l sec-l is obtained. T h e second order rate constant 2 X lo4 (using Equation 3), 1 x 103 from 2 value-ha plot, and 2 x IO3 M - l sec-l from reversal of scanning, all confirm t h a t Equation 3 can be used for reactions described by Equations 4 and 7 . T h e importance of such a study is apparent: the rate of protonation of anion or radical anions and similar fast reactions can be studied. The pseudo-first-order condition for the reaction (Equation 4) was approximately maintained by considering ( a t 2 V/min) the peak current (about 5 FA) and the volume of cm3, plus t h a t of the hanging mercury drop (about 4 X the diffusion layer around the electrode. The concentration of the free radical generated, thus, comes out to be less mole/liter. (The concentration of 4-NBC1 than 2 X was 5 x 10-4M.) The experimental details have already been described (2). The concentration of pyridinium salt was 1 X 10-4M.
ACKNOWLEDGMENT T h e author gratefully acknowledges the facility provided by the Chemistry Department, State University of New York, Stony Brook, NY.
LITERATURE CITED (1) R . S. Nicholson and I. Shain, Anal. Chem., 36,706 (1964). (2) M. Mohammad and E. M. Kosower, J. Am. Chem. Soc., 93,2709, 2713 (1971). (3) E. M. Kosower, “An Introduction to Physical Organic Chemistry,” Sec. 2.8, Wiley, New York, NY, 1968. (4) M. Mohammad and E. M. Kosower, J. Phys. Chem., 74, 1153 (1970). (5) M. Mohammad, J. Hajdu, and E. M. Kosower, J. Am. Chem. Soc., 93, 1792 (1971). (6) M. Mohammad, unpublished results.
Mahboob Mohammad Department of Chemistry University of Islamabad Islamabad, Pakistan
RECEIVEDfor review August 12,1974. Accepted December 20, 1974.
Preparation and Properties of a Strontium-Selective Electrode Sir: A simple, rapid method for trace strontium determination was required to support process development for our nuclear waste management program. Levins ( I , 2) in his report describing a barium-selective electrode, mentions an analogous strontium-selective electrode. The preparation and properties of the strontium-selective electrode suggested by Levins were investigated. The electrode, a liquid-membrane type, is based on a complex of strontium with polyethylene glycol, which acts as a neutral carrier. T h e electrode is selective a t 0.1M concentrations for strontium in the presence of calcium and other divalent ions (with the exception of barium and mercury), and in the presence of alkali metals, except cesium. T h e electrode is limited in determining trace strontium in the presence of other cations, however, because of diminished selectivity inherent a t lower concentrations. EXPERIMENTAL R e a g e n t s . Deionized water and reagent-grade chemicals were used to prepare solutions of strontium and other salts. “Igepal” CO-880(Trademark, General Aniline and Film Corporation). which is nonylphenoxypoly(ethyleneoxy)ethanol, was purchased from Varian Instrument Division a n d used without further purification. T h e 4-ethylnitrobenzene ( E N B ) , from Aldrich Chemical Co., Inc., was purified by vacuum distillation. T h e neutral carrier complex salt was prepared as follows: 1.5 g (1 mmole) of “Igepal” CO-880was dissolved in 100 ml of H20; 5 ml of p H 6 buffer (60 ml glacial HC2H302, 270 g NaC2H302.3H20 per liter) were added, followed by 1 ml of 1M Sr(NO& or SrC12. T h e insoluble salt was precipitated by adding dropwise with stirring 10 ml of 0.5Msodium tetraphenylboron. After aging a t room temperature overnight, t h e clumps of white precipitate were washed with water, filtered onto a sintered glass filter of medium porosity, a n d subsequently vacuum-dried overnight. Without t h e p H adjustment, a finely divided unfilterable product was formed. Product a t room temperature was stable for severvacuum-dried over P 2 0 ~ al months; product vacuum-dried a t 50 “ C tended t o decompose within a few weeks. as evidenced by a benzene aroma. T h e strontium product obtained was assumed to be analogous to t h e barium compound described by Levins ( 1 ) and hence t o have t h e formula Sr . “Igepal’‘ CO-880 2B(C&)4. A solution of 0.2 to 0.5 g of the strontium compound in 5 ml of E N R served as the organic “exchanger” in t h e liquid membrane electrode. T h e particular concentration did not affect t h e response of t h e electrode. T h e most concentrated solution was thixotropic, which necessitated occasional agitation of t h e electrode.
A p p a r a t u s a n d P r o c e d u r e . T h e strontium-selective electrode was assembled in an Orion (Orion Research) liquid membrane electrode body No. 920000. Orion porous membranes were soaked in petroleum ether t o remove t h e proprietary treatment and were air-dried before use. T h e organic “exchanger” in t h e electrode was t h e solution of strontium complex in E N B . T h e internal aqueous solution was 0.1M SrC12 saturated with AgC1. T h e reference electrode was a double junction reference electrode, Orion No. 900200, with the outer chamber filled with 3M lithium trichloracetate. T h e electrodes were placed in -25 ml solution t h a t was slowly stirred magnetically. Potential measurements were made a t room temperature (-23 “C) with a n Orion Model 801 Digital pH/mV Meter.
RESULTS AND DISCUSSION Electrode Response. The measured potential as a function of strontium ion activity and concentration is shown in Figure 1. At >10-5M, the response is -27 mV per decade of activity. From 10-5 to 10-6M, the response is smaller, yet sufficiently reproducible to enable semiquantitative determinations in this region. Several electrodes were assembled, with different Orion porous membranes and with organic solutions of different concentrations. All had Eo values within 10 mV of one another. The lifetime of an electrode was several weeks, with a change in Eo, usually to a more-negative potential reading, of perhaps 5 mV. Electrodes were stored upright in air. Better response was maintained if the electrodes were never rinsed with water, but rather were blotted dry after use. During the working day, the Eo varied less than 1 mV. In pure SrC12 solutions, uncertainty in the measured potential was -0.2 mV, which would correspond to an uncertainty of -2% in strontium activity (or concentration). In 1O-I to 10-3M solutions, stable readings were usually obtained within 1 minute. With more-dilute solutions, a 5 minute period was required. At 10-6M, sometimes as long as 15 minutes was necessary to obtain a stable reading. Stability of readings was better with slow, rather than rapid, stirring. Effect of Other Ions. Selectivity constants (Ks,M) of the electrode for strontium over other cations were determined by the conventional method (3)from potentials ( E s , and E M ) measured in the respective solutions of 0.1M ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1 9 7 5
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