Constant-current electrolysis techniques

Potential sweep technique with cyclic voltammetry is usually the most efficient for electropolymerization of monomers. Potenliostatic (constant potential) electrolysis and galvanostatic (constant current) electrolysis are also used. 

Pulsed-current techniques can furnish electrochemical kinetic information and have been used at the RDE. With a pulse duration of 10-4 s and a cycle time of 10-3 s, good agreement was found with steady-state results [144] for the kinetic determination of the ferri-ferrocyanide system [260, 261], Reduction of the pulse duration and cycle time would allow the measurement of larger rate constants. Kinetic parameter extraction has also been discussed for first-order irreversible reactions with two-step cathodic current pulses [262], A generalised theory describing the effect of pulsed current electrolysis on current—potential relations has appeared [263],... 

Controlled-current electrolysis in flowing solution has been extremely useful for analytical purposes. The prevalent techniques are constant-current coulometry and coulometric titrations, which are discussed in Chapter 25. 

In this technique, the COD of the electrolyte is measured at regular intervals (At) during constant current (galvanostatic) electrolysis and the instantaneous current efficiency (ICEcod) is calculated using the relation ... 

Transition time (for chronopotentiometry) — Electrolysis time required before the surface concentration of a redox species drops to zero in - chronopotentiometry. In constant-current chronopotentiometry (see also -> constant-current techniques), the transition time r is given by the - Sand equation ... 

Another coulometric approach is based on controIled-current technique. In this case, a constant current (iapp < iL(t = 0)) is performed, with corresponding potential varied with time span of the electrolysis in Fig. 6a. While the iapp is higher than iL(t > 0), a secondary (side) reaction occurs during the electrolysis (Fig. 6b [2]). As a result, the current efficiency of the analyte falls below 100 %, due to the potential drift with increasing time. 

In the Oxygen Flow Rate technique (ICEofr)> a two compartment electrochemical cell is used, and the oxygen flow rate is measured continuously in the anodic compartment during constant current (galvanostatic) electrolysis. The instantaneous current efficiency is then calculated using the relationship ... 

As an example of the technique, we may consider the reduction of penta-chlorophenol (PCP). The reactor consisted of two compartments, each 10 cm long, 2 cm wide and 0.5 cm deep, the cathode being a packed bed of carbon fibre, which was separated from the anode by a cation-exchange membrane (Nation 426). In the batch-recycle mode, constant current (10 A) electrolysis of 1 dm of 50 mg dm PCP in 0.1 mol dm " Na2S04 plus 0.1 mol dm NaOH removed PCP to a level < 0.5 mgdm within 30 min. The overall current efficiency for complete dechlorination was approximately 1 %, the energy consumption being 36 kWhm"l... 

Experimental Procedure Before the electrolysis, a solution of trifluoromethane-sulfonic acid in methanol (0.125 M, 0.5 ml) is introduced at the inlet to fill the anodic and cathodic chambers. After that, a 0.05 M solution of p-methoxytoluene (31 mg, 0.25 mmol) in methanol (5 ml) is fed into the inlet of the anodic chamber (flow rate 2 ml/h). The electrolysis is carried out under constant current conditions (11mA, 4.0 F/mol and 22 mA, 8.0 F/mol based on the amount of p-methoxytoluene) at room temperature (cooled in a water bath). The outlet solution is discarded until it becomes neutral, and then the aliquots (1 ml) of the outlet solution are collected at intervals and analyzed by GC to determine the amount of the product, p-methoxybenzaldehyde dimethyl acetal. For preparative purpose, the outlet solution is collected in a flask and the product is isolated using a conventional technique. 

These techniques are dependent on an electrolysis that attempts to oxidize or reduce completely all of an electroactive analyte species in a volume of sample solution. These methods have the distinct advantage of requiring no calibration since they are dependent on total electron transfer, which can be quantitatively related to the total mass of the analyte. Coulometry refers to the integration of electric current with time as electrolysis proceeds either at a constant current or at a constant voltage. The latter parameters are chosen to optimize analysis time and limit interferences. For constant current, the number of coulombs of electrons consumed or produced, Q, is related to the current I and time t by... 

Controlled-potential electrolysis (CPE) represents an improvement over the previous constant-potential method this is attained by the application of an emf across the electrodes that yields a cathodic potential as negative as is acceptable in view of current density limitations and without taking the risk that the less noble metal is deposited hence the technique requires non-faradaic control of the cathodic potential versus the solution. 

The first application of the quartz crystal microbalance in electrochemistry came with the work of Bruckenstein and Shay (1985) who proved that the Sauerbrey equation could still be applied to a quartz wafer one side of which was covered with electrolyte. Although they were able to establish that an electrolyte layer several hundred angstroms thick moved essentially with the quartz surface, they also showed that the thickness of this layer remained constant with potential so any change in frequency could be attributed to surface film formation. The authors showed that it was possible to take simultaneous measurements of the in situ frequency change accompanying electrolysis at a working electrode (comprising one of the electrical contacts to the crystal) as a function of the applied potential or current. They coined the acronym EQCM (electrochemical quartz crystal microbalance) for the technique. 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

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