WO1999009419A2

WO1999009419A2 - Method for voltametric measurement of very low voltage using few components

Info

Publication number: WO1999009419A2
Application number: PCT/DE1998/002469
Authority: WO
Inventors: Peter SCHÖNWEITZ; Dietrich Wabner
Original assignee: Schoenweitz Peter; Dietrich Wabner
Priority date: 1997-08-20
Filing date: 1998-08-19
Publication date: 1999-02-25
Also published as: WO1999009419A3; DE19736224A1

Abstract

The invention relates to a simplified and improved method to carry out very sensitive voltametric measurements. The pre-determined voltage between the electrolyte and the collector is not obtained by readjusting the electrolyte potential, but by an autonomous regulating circuit which maintains the potential grounded. This provides the advantage that the slightest difference in potential is detected in the operational amplifiers and that the regulator adjusts an equivalent current by means of the load resistor. Said circuit has a limited use in fast cyclovoltametric applications since the regulating distance of the integrator used is longer in comparison with direct compensation.

Description

Description:

Process for measuring the smallest electrical currents in voltammetry with low component expenditure

The present invention relates to a method according to claim 1, with which voltammetric measurements can be carried out with low component complexity (two operational amplifiers) in current ranges up to picoampere.

The current state of the art allows current measurements in very complex circuits down to the lower picoampere range (10 ^"12 amps) for voltammetric measurements and current density potential measurements on two or three electrode systems. In principle, operational amplifiers are switched as I / U converters or the potential on a load resistor The I / U converter with operational amplifiers is one of the most important circuits for this area, and the relationship between the current and voltage converter (see FIG. 1-b) results directly from the virtual ground

U ₀ = - R l _e (Eq. 1)

Since the potential changes due to the chemistry taking place at the working electrode, the additional voltage (FIG. 2) tracks the differential voltage across the counterelectrode.

In these mentioned and common methods, the potential difference that is measured between the working electrode and ground is effectively added to the specified potential by the D / A converter, in order to obtain exactly reproducible cyclo-graphs.

The disadvantage that, in terms of circuit technology, that the potential at the working electrode changes even further after the potential has been set by the counterelectrode, was used as an advantage in this implementation.

The interface (see FIG. 3) and the potential gradient between the electrolyte and the electrode surface were considered electrotechnically. Since the potential on the working electrode would like to change, an independent control system was designed that keeps the potential of the working electrode at ground. The opposite potential required for this was applied to a load resistor on the working electrode (FIG. 4). According to Ohm's law, this resulted in the current that flows through the working electrode.

As can be seen from the circuit (FIG. 2 and FIG. 5) and the description of the prior art, an even greater effort is being made to measure currents down to the picoampere range.

Literature.

[1] H. Wupper, professional circuits with operational amplifiers, Franzis Verlag 1994.

[2] H. Wupper, U. Niemeyer, Electronic Circuits 2, Operational Amplifiers, Digital Circuits, Connection Lines, Springer

Verlag, Berlin, Heidelberg 1996, 82.

[3] Hamann / Vielstich, Elektrochemie II, Electrode Processes, Applied Electrochemistry, Verlag Chemie 1981, 142-155.

[4] Dennis E. Tallman, A wide band with computer based potentiostat for fast voltammentry at microelectrodes, J. Electroanal. Chem., 1990, 280: 327-340

It is therefore an object of the present invention to provide a reliable and simple electronic circuit (FIGS. 6-8) for voltammetric measurements. Furthermore, surprisingly, in addition to the high sensitivity and the low component complexity (two operational amplifiers, FIG. 7 and FIG. 8), there was the possibility of eliminating disturbing noise by simply adding circuitry with resistors (FIG. 8).

This object is achieved by the features of claim 1, a method for voltammetric measurement in which the potential at the working electrode is kept at ground by a control system. According to Ohm's law, the potential against ground tapped at the measuring point corresponds to the current that flows through the load resistor and the working electrode.

Furthermore, the use of an integrator reduced the background noise and at the same time smoothed the measurement signals. There was no need for capacitive components as well as active and passive filters to reduce noise.

A voltage divider (R1 / R2) enabled the resolution to be adapted to the A / D converter despite the already high resistance R _LAST of 100 MOhm. The resistance R _Gl of 100 ohms and less smoothes unwanted voltage peaks after the voltage tracker.

Further advantages and features of the present invention result from the description and the drawings:

The OPA111 from BurrBrown and the instrument amplifier INA116 from BurrBrown were used and tested as operational amplifiers. Despite their different fields of application, both produced very satisfactory results in the area of voltammetric measurements. As can be seen from FIGS. 2 and 5, the component outlay has so far been significantly higher than with the new circuit. Because the integrator can set a potential at the non-inverting input to which the working electrode is to adapt, it is also possible to dispense with the potential-setting side for the electrolyte (FIG. 2, see counterelectrode). From the resulting two-electrode system, only the counter electrode or reference electrode has to be grounded. The corresponding electrode can still be dispensed with. Abbreviations

A / D analog-to-digital converter

AE working electrode

AUX counter electrode

BE reference electrode

CE counter electrode

D / A digital-to-analog converter

GE counter electrode

E potential

1 electric current

OP operational amplifier

R electrical resistance

REF reference electrode

U voltage / potential difference

WKG working electrode

UAE potential working electrode

UAD potential measuring point

UOP potential operational amplifier input

UDA potential DA converter

CORRECTED SHEET (RULE 91)

ISA / EP

Claims

Expectations:

1. Method with which voltammetric measurements can be carried out on a two- or three-electrode system,

characterized in that the potential of the working electrode is drawn through a separate control circuit via a load resistance to the mass of the measuring system in order to maintain the potential difference between the electrolyte and the working electrode, in order to determine the current exclusively thereby.

2. current measuring system according to claim 1,

characterized in that the control loop consists of two operational amplifiers, one taking over the potential-detecting function (voltage tracker) and the other taking over the controller function (inverting integrator).