Controlled Current Techniques 1 Part
Controlled Current Techniques 1 Part
Controlled Current Techniques 1 Part
Basic Terminology
Electrolyte: Medium that contain free ions making it electrically conductive Typically ionic solutions, but molten and solid electrolytes too exist Solute dissociates into ions, and its tendency to dissociate governs the strength of the electrolyte Electrolyte drinks contain Na, K replenish body waters
Electrode: A electric conductor through which electric current is passed. A collector or emitter of electric charge. Used in forms of plates, rods, wires, etc. Metals like Cu, Ag, Pb, Zn and nonmetals like Carbon
1. Working Electrode: At which prinicple electrochemical reaction (oxidation/reduction) takes place Cathodic/Anodic depending on the nature of the reaction Behavior (Potential) is studied with respect to a standard reference electrode
2. Counter/Auxiliary Electrode: Closes the current circuit in the cell Works as complimentary to WE Surface area larger in order not to limit process at WE Supplies the current required by WE Current carrying electrode that completes cell circuit Prone to corrosion - safe 3. Reference Electrode: Provides fixed potential that doesn't vary during operation and against which other potentials are measured.
Potentiostat: Electrical instrument that controls and measures voltage. Accurately control the potential between WE and CE
Galvanostat:
Electrical instrument that controls and measures the current flow through electrolytic cell Capable of maintaining constant current flow even when under load variations itself Controls current flow between WE and CE Consists of a high voltage source producing a constant voltage V with a resistor Rx connected in series . To maintain almost constant current through load , this resistor shall be much higher than the load resistor Rload = +
(1)
First consider constant-current chronopotentiometry for anthracene (An). The steady current, i, applied to the electrode causes the (An) to be reduced at a constant rate to the anion radical An- .
The potential of the electrode varies with time as the An/Anconcentration ratio changes at the electrode surface.
The process can be regarded as a titration of the An in the vicinity of the electrode by the continuous flux of electrons, resulting in an E-t curve like that obtained for a potentiometric titration . The time after application of the constant current till this potential transition occurrence is called the transition time, .
The shape and location of the E-t curve is governed by the reversibility, or the heterogeneous rate constant, of the electrode reaction.
=t, a current ramp). this technique, called programmed current chronopotentiometry . (3) If The current is reversed after some time (current reversal chronopotentiometry) at, or before the transition time, the An formed during the forward step will start oxidizing. The potential will move in a positive direction as the An/An concentration ratio increases. When the An concentration falls to zero at the electrode surface, a potential transition toward positive potentials occurs, and a reverse transition time can be measured. (4) if the current continuously reversed at each transition. it is resulting in cyclic chronopotentiometry .
- Boundary conditions involving the concentration gradient allows the diffusion problem to be solved without reference to the rate of electron transfer reaction, in contrast with the concentration-potential boundary conditions required for controlled potential methods. - On applying Laplace transform yields,
- These integrals forms are convenient for solving controlled current problems
Concentration profile of O and R various values of t/ during constant current electrolysis are shown in graph.
- The measured value of at known i can be used to determine C0* or D0. A lack of constancy of the transition time constant, i1/2/C*, with i or C0* indicates complications to the electrode reaction from coupled homogeneous chemical reactions, adsorption etc.
Response Function:
= Any response R is a function of the input (controlled current, ) and the system properties S. For the semi infinite linear diffusion scenario, response function at x=0 can be given as
Reversal Techniques: i. Consider a situation where current i is supplied for a time t1. At t=t1, the direction of current is reversed to i. Response function at x = 0 thus turns out to be
If 2 is the time at which CR at the R electrode falls to 0, relation between t1 and 2 can be derived as follows:
At t = t1 + 2 , CR = 0, which gives
Multicomponent systems: Consider a system containing two reducible substances O1, O2 1 + 1 1 1 + 2 2 Response functions can be written individually as:
From 0 < t < t1, i2 = 0 (due to insufficient negative potential), during which O1 reduces alone,
Derivative Method: By rather straightforword in instrumental approach the deriative of the chronopotonetiogram that is a curve of dE/dt vs. t
While finding from the maximum of the derative curve is possible . for the Nernstian process occur at t= 4 /9
Determination of the by this approach is free from problem of double layer charging because it evaluated at a position in the curve before the transition time region where an appreciable charging current contribution exist However the large charging current contribution at salt of the chronopotentiogram still contributes The derative approach does suffer from need for knowledge about the degree of thr reversibility of the electrode reaction and it is the reversible knowing the
CHEM 5390
CHEM 5390
CHEM 5390
CHEM 5390
q=0.01C
The time required for this charge injection will depend on the cell resistance, R. This injected charge causes the potential of the electrode to deviate from its original value Eeq to a value E(t = 0),
The charge on Cd now discharges through the faradaic impedence, and the open circuit potential moves back towards as decreases to zero. If no faradaic reaction reaction is possible, Cd remains charged and the potential will not decay.
is linear with a zero intercept. This method has been suggested for the determination of small concentrations of electroactive materials, but not been widely applied because it requires recording of E-t curve.