Solution resistance effects, potential control

The electrical characteristics of the cell and electrode will comprise both capacitative and resistive components, but for simplicity the former may be neglected and the system can be represented by resistances in series (Fig. 19.36 > and c). The resistance simulates the effective series resistance of the auxiliary electrode A.E. and cell solution, whilst the potential developed across by the flow of current between the working electrode W.E. and A.E. simulates the controlled potential W.E. with respect to R.E. 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

Figure 7.1 (A) Typical controlled-potential circuit and cell OA1, the control amplifier OA2, the voltage follower (Vr = Er) OA3, the current-to-voltage converter. (B) Equivalent circuit of cell Rc, solution resistance between auxiliary and working electrodes Ru, solution resistance between reference and working electrodes, Rs = Rc + Ru and Cdl, capacitance of interface between solution and working electrode. (C) Equivalent circuit with the addition of faradaic impedance Zf due to charge transfer. Potentials are relative to circuit common, and working electrode is effectively held at circuit common (Ew = 0) by OA3.

Potentiostat — A potentiostat is an electronic amplifier which controls the potential drop between an electrode (the -> working electrode, (WE)) and the - electrolyte. The WE is normally connected to ground potential the potential of the electrolyte is measured by a special probe, the -> reference electrode (RE). Effects of the -> counter electrode (CE), (e.g., potential drop at the CE electrolyte interface) and the electrolyte (esp. the solution resistance) can be suppressed by this technique. Potentiostats are based on -> operational amplifiers (OPA) the simplest circuit is given in Fig (a). The difference between the desired potential Ureference electrode potential Ure is amplified, resulting in currents via counter and working electrode until this difference becomes (almost) zero. 

Supporting electrolytes are required in controlled-potential experiments to decrease the resistance of the solution, to eliminate electromigration effects, and to maintain a constant ionic strength (i.e., swamping out the effect of variable... 

Fig. 7. Synergistic action of PUFAs. (A) Voltage dependence of steady-state inactivation of the sodium current in the control solution (open symbols) after perfusion with 15 pAf carbamazepine (CBZ, gray symbols) and after the addition of 0.5 pAf EPA (black symbols). Data are fit with a Boltzmann equation. Like PUFAs, CBZ induces a shift in I4 and increases the slope factor V. The addition of 0.5 fiM EPA induces an extra shift in V), but without affecting the V, whereas 0.5 pAf EPA alone has no discernible effect on V. (B) The shift in induced by addition of 0.5 pAf EPA on top of 15 [iM CBZ is given against the shift induced by 15 pAf CBZ alone for CAl neurons from healthy rats (open circles), for CAl neurons from patients with pharmaco-resistant temporal lobe epilepsy (filled circles), and for neocortical neurons from epilepsy patients (squares). The potentiating effect of subthreshold concentrations of EPA is similar for all groups.

Whatever the measurement mode, proper control of the potential of the working electrode is necessary and a return path for the current from the electrolyte Solution must be provided. This second connection to the electrolyte, the reference electrode, must be capable of carrying the maximal current to be measured, without losing the constancy of the potential jump, Sg to the electrolyte. Even if we can find a reference electrode that fulfills these requirements, the solution between the electrodes will generate a voltage drop, due to its ohmic resistance, the so called in drop. This effect can be described by ... 

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