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Russian Journal of Applied Chemistry, Vol. 74, No. 5, 2001, pp. 750 !754. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 5,
2001, pp. 728!732.
Original Russian Text Copyright + 2001 by Shaidarova, Zaripova, Tikhonova, Budnikov, Fitsev.
AND CORROSION PROTECTION OF METALS
Electrocatalytic Determination of Oxalate Ions on Chemically
L. G. Shaidarova, S. A. Zaripova, L. N. Tikhonova, G. K. Budnikov, and I. M. Fitsev
Kazan State University, Kazan, Tatarstan, Russia
Received May 18, 2000; in final form, December 2000
Abstract-The electrocatalytic activity with respect to oxalate ions of spongy osmium immobilized within
a carbon-paste electrode was studied. A procedure is proposed for electrocatalytic determination of oxalate
ions on a chemically modified electrode based on spongy osmium under conditions of flow-through3injection
The necessity for analytical monitoring of oxalate
ions is due to the biological importance of these com-
pounds. For example, oxalic acid and its salts play an
important part in the human body , quite a num-
ber of diseases being associated with disruption of the
oxalate exchange. Therefore, it is advisable to use
simple, proximate, and accurate analytical techniques
in clinical diagnostics.
Analytical laboratories more frequently employ
classical methods of analysis, gravimetry and titrim-
etry, for determining oxalate ions, with potentiometric
titration with ion-selective or glass electrodes  oc-
casionally used among electrochemical methods.
Voltammetric techniques have not found use for this
purpose. This is due to the fact that anions, including
oxalates, are oxidized or reduced on indicator elec-
trodes with a high overvoltage. However, as reported
in , the overvoltage effects can be diminished by
means of the so-called redox mediators. That is why
interest has been aroused recently in electrochemical
analytical techniques based on the use of chemically
modified electrodes (CMEs) possessing electrocatalyt-
ic properties . These electrodes are already being
used successfully for electrochemical detection of
a wide variety of compounds, such as oxygen, hydro-
gen peroxide, anions, alcohols, aldehydes, carbohy-
drates, amino acids, etc., including application in
flow-through analysis systems .
A search for new redox mediators for CME and
their use in flow-through3injection analysis (FIA) of
biologically important compounds for the purpose of
introducing automation and improving the efficiency
of determination is a currently pressing problem.
The present study is concerned with the possibility
of electrocatalytic determination of oxalic acid and
oxalate ions, using carbon-paste electrodes (CPE)
modified with spongy osmium under FIA conditions.
Carboxylic acids and their salts are oxidized at
graphite electrodes with high overvoltage . When
using a CME based on a redox mediator, the overvolt-
age can be made lower. It was found that CPE modi-
fied with spongy osmium exhibits electrocatalytic
activity toward oxalic acid. To elucidate the electro-
catalysis mechanism, electrooxidation of the modifier
and substrate on the CPE and CME was studied.
Oxalic acid is oxidized at CPE at E > 1.2 V (Fig. 1,
curve 1). The anodic voltammogram of the supporting
electrolyte, recorded with CME in the absence of sub-
Fig. 1. Voltammograms of oxalic acid oxidation at (1) CPE
and (2) osmium-based CME with borate buffer solution
with pH 9.0 as supporting electrolyte. Concentration of
oxalic acid 1 0 10
M; the same for Fig. 2. (E) Potential.
Background curve: (3) at CPE and (4) at CME.