The effect of temperature on the electrochemical reduction of quinaldic acid at DME.
The electrochemical reduction of carboxylic acids and its derivatives have been studied widely (Baizer, 1975 and Lund, 1963). The carboxylate group in organic acids can be reduced electrochemically in systems where the energy level of the lowest unfilled orbital is lowered either by conjugation with a further unsaturated group or by the proximity of an electron-withdrawing group. The electro-chemical reduction of carboxyl group of 1-ethyl-4-carboxy-pyridinium bromide was reported by Lund (Iverson et al, 1967 and Brdicka et al, 1947). The rate constants were reported and the results were explained on the basis of theories of Wiesner and Brdicka (Brdicka et al, 1947). The behaviour of pyridine carboxylic acids at dropping mercury electrode was studied by Tompkins and Schmidt (Tompkins et al, 1944). The polarographic reduction of isonicotinic and picolinic acids was also studied (Jellinek, 1954 and Volke et al, 1955). Very few references were found in the literature about the effect of temperature on the electrode kinetics of irreversible reduction of organic depolarisers (Volke et al, 1955 , Ram Ratan et al, 1979 and Veerabhadram, 1984).
The present work is devoted to the study of the polarographic reduction of quinaldic acid to determine the nature of waves, the formal heterogeneous rate constant ([k.sup.0.sub.f,h]) and the other thermodynamic functions in various buffer solutions with the variation of temperature.
The title compound quinaldic acid was obtained from E. Merk Chemical Company Ltd., Germany. The stock solution of the compound (1 x [10.sup.-3] M) was prepared in double-distilled water. The buffer solutions were prepared from Britton-Robinson (Britton, 1955) modified universal buffer solutions. The ionic strength of the solution was maintained constant at 0.6M by adding KCl. The polarographic experiments were carried out after deaerating experimental solutions by passing pure nitrogen gas for about 20 min. The experiments were carried out at various temperatures using a thermostat. Triton X - 100 (5 x [10.sup.-4] %) was used to suppress the polarographic maxima.
The polarograms were recorded with a Recorder Polarograph, d.c. model CL 25 supplied by Elico Pvt. Ltd., Hyderabad. SCE was used as reference electrode. The dropping mercury electrode having characteristics of m=1.425 mg/sec and t = 4.2 seconds, was used as a working electrode.
RESULTS AND DISCUSSION
The electrochemical reduction of 1 mM quinaldic acid (2-quinoline carboxylic acid) has been studied in buffer solutions of pH values 1.20, 3.50, 5.29 and 6.50 at various temperatures viz., 303K, 313K, 323K and 333K. A well defined single cathodic wave at the pH of 1.20, 3.50 and 5.29 and two well defined cathodic waves are observed at the pH values of 6.50 at all these temperatures. The limiting current is diffusion controlled and found to increase with the increase in temperature. The temperature coefficient values of diffusion current as shown in Table 1 and 2 are found to be between 1.00 to 1.56% per degree which is in agreement with the range predicted by Meites for organic molecules (Meites,1967). The temperature coefficient values have been calculated using the following equation.
Temperature coefficient = 2.303/[DELTA]T log [i.sub.2]/[i.sub.1]
where [DELTA]T is the temperature difference, [i.sub.1] is the diffusion current ([mu]A) at lower temperature and [i.sub.2] is the diffusion current ([mu][A) at higher temperature.
The half-wave potential is found to shift to more positive values with increase in the temperature and the temperature coefficient of [E.sub.1/2] is found to be 0.9 mV/degree, indicating the electrochemical reduction to take place at more positive potential with the increase in temperature.
The plots of--[E.sub.d.e] vs log i / ([i.sub.d]-i) are linear at all pHs and temperatures but their slopes are not in agreement with theoretical values (0.030 V for 2e reduction) of a reversible wave. This indicates the nature of the electrode reaction to be irreversible. The [[alpha].sub.na] values tabulated in Tables 1 and 2 suggest a value of < 0.5 for '[alpha]' ([alpha] = transfer coefficient), supporting the irreversible nature of the electrode process.
The formal heterogeneous rate constants ([k.sup.0.sub.f,h]) have been calculated from various methods such as Meites-Israel (Meites et al, 1961), Oldham-Parry (Oldham et al, 1968) and Gaur-Bhargava (Gaur et al, 1973) in order to find the validity of the results obtained. Meites and Israel have extended the koutecky's graphical method into a comparatively more precise mathematical form. Oldham and Parry have shown that polarographic curves of irreversible process can be analysed by considering curvature effect resulting from electrode sphericity. Gaur and Bharghava have extended the Koutecky's treatment for an irreversible wave by considering the diffusion to the electrode surface is spherical and not linear process as assumed earlier. The diffusion of formal heterogeneous rate constant ([k.sup.0.sub.f,h]) at various temperatures has been calculated from stokes-Einstein's equation (Einstein, 1905).
The values of [k.sup.0.sub.f,h] are shown in Table 1 and 2. It is observed that for each system in each treatment the values of [k.sup.0.sub.f,h] have shown similar trend and found to increase with the increase in the temperature suggesting a decrease in irreversibility with the increase in temperature. The increase in '[alpha][n.sub.a]' with temperature also confirms the increase in irreversibility with the raise in temperature. This is attributed to the sole increase in the value of transfer coefficient (Ram Ratan, 1979).
The thermodynamic function viz., [DELTA]Hp, [DELTA]Hv, [DELTA]G and [DELTA]S follow the same trend as that of [k.sup.0.sub.f,h] with respect to the mode of reduction to the reducible species and are tabulated in Tables 1 and 2.
The enthalpy of activation '[DELTA][H.sub.p]' is calculated from the slope of the plots of log [k.sup.0.sub.f,h] vs 1/T. The heat of activation at constant volume '[DELTA][H.sub.v]' is evaluated using the equation :
[DELTA][H.sub.p] = [DELTA][H.sub.v] + RT
The activation free energy change, [DELTA]G is determined using the relationship (Delahay, 1951)
[k.sup.0.sub.f,h] = kT/h [r.sub.0] exp [DELTA]G/RT
where k = Boltzmann constant, h = plank constant
[r.sub.0] = mean distance between two depolariser ions in bulk solution.
In general the value of [r.sup.0] is taken as 2 x [10.sup.-8] cm (Delahay, 1951).
The positive value of the [DELTA]G for all the systems suggest the difficult reduction of reducible species. A little positive increase in the value of [DELTA]G in all the systems with the increase in the temperature may be because of the inconsistency in the value of r0 with the change in temperature (Mehta et al, 1978)
The entropy of activation, [DELTA]S is calculated using Helmholtz-reaction
[DELTA]S = [DELTA][H.sub.y] - [DELTA]G/T
The negative values of [DELTA]S obtained in each system suggest that the activated state has more rigid structure than the initial state (Mehta et al, 1978). The approximate radius of the reduced species (r) has been calculated for various systems are given in Tables 1 and 2.
The values of radius, 'r' and diffusion coefficient 'D' are in reverse order obeying stokes-Einstein equation(Einstein, 1905). The decrease in the value of 'r' with the increase in temperature may indicate that the reduced species are solvated, the extent of decrease in the value of 'r', is different at different pH values, may be due to the difference in the extent of solvation (Mehta et al, 1979).
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(2.) Lund, H. 1963 .Acta Chem. Scand., 17: 972.
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(4.) Brdicka, R. and Wiesner, K. 1947 .Collect. Czech. Chem. Communs. 12 : 138.
(5.) Tompkins, P.C. and Schmidt, C.L.A. 1944. Univ. California Pub. Physiol, 8 : 229.
(6.) Jellinek, H.H.J. and Urwin, J.R. 1954. J. Phys. Chem., 58: 168.
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(10.) Herbert T.S. Britton. 1955. Hydrogen ions, Chapman and Hall Limited, London, Fourth Edition, 353.
(11.) Meites, L. Polarographic Techniques, 1967. Interscience publishers, New York, Second Edition, 139.
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Y. Ravi Kumar and G. Veerabhadram
Department of Chemistry, Osmania University Hyderabad-500 007--India
Table--1: Values of kinetic and thermodynamic parameters of 1 mM quinaldic acid reduction at pH 3.50 in various treatments Parameters 303k 313k 323k 333k (1) (2) (3) (4) (5) Meites-Israel [K.sup.0.sub.f,h] x 1.74 2.38 3.27 4.32 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] --5.72-- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 5.12 5.10 5.08 5.06 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 19.18 19.64 20.09 20.55 [mole.sup.-1]) -[DELTA]S (e.u) 46.40 46.45 46.47 46.57 Oldham - Parry [K.sup.0.sub.f,h] x 1.65 2.25 3.10 4.10 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] -- 6.40 -- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 5.8 5.78 5.76 5.74 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 19.22 19.67 20.12 20.58 [mole.sup.-1]) -[DELTA]S (e.u) 44.29 44.37 44.45 44.58 [K.sup.0.sub.f,h] x 2.65 3.62 4.98 6.59 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] -- 5.95 -- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 5.35 5.33 5.31 5.29 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 18.93 19.38 19.82 20.27 [mole.sup.-1]) -[DELTA]S (e.u) 44.82 44.89 44.92 44.98 -E 1/2, Volts 0.850 0.843 0.835 0.829 [alpha][n.sub.a] 0.573 0.589 0.606 0.622 Temperature coefficient - 1.27 1.12 1.00 of diffusion current (% [deg.sup.-1]) R x [10.sup.8], cm 1.88 1.78 1.70 1.63 Table--2: Values of kinetic and thermodynamic parameters of 1 mM quinaldic acid reduction at pH 5.29 in various treatments Parameters 303k 313k 323k 333k (1) (2) (3) (4) (5) Meites-Israel [K.sup.0.sub.f,h] x 1.39 1.80 2.34 2.97 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] --4.80-- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 4.20 4.18 4.16 4.14 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 19.30 19.81 20.30 20.79 [mole.sup.-1]) -[DELTA]S (e.u) 49.83 49.93 49.96 50.00 Oldham - Parry [K.sup.0.sub.f,h] x 1.25 1.69 2.24 2.91 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] --5.49-- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 4.89 4.87 4.85 4.83 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 19.38 19.85 20.33 20.81 [mole.sup.-1]) -[DELTA]S (e.u) 47.82 45.87 47.92 47.97 [K.sup.0.sub.f,h] x 2.01 2.74 3.61 4.52 [10.sup.9] (cm.[sec.sup.-1]) [DELTA][H.sub.p] --5.72-- (K.cal.[mole.sup.-1]) [DELTA][H.sub.v] 5.12 5.10 5.08 5.06 (K.cal.[mole.sup.-1]) [DELTA]G (K.cal. 19.10 19.56 20.03 20.51 [mole.sup.-1]) -[DELTA]S (e.u) 46.14 46.20 46.28 49.39 -E 1/2, Volts 0.940 0.931 0.924 0.917 [alpha][n.sub.a] 0.509 0.525 0.540 0.555 Temperature - 1.56 1.34 1.19 coefficient of diffusion current (% [deg.sup.-1]) r x [10.sup.8], cm 2.97 2.65 2.41 2.30
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|Author:||Kumar, Y. Ravi; Veerabhadram, G.|
|Publication:||Bulletin of Pure & Applied Sciences-Chemistry|
|Date:||Jul 1, 2006|
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