Equipment Potentiostat

Reference electrodes at the test points may only be needed part of the time, depending on the mode of operation of the protective systems (e.g., for monitoring or for permanent control of potential-controlled protection current equipment). Potentiostatic control is always preferred to galvanostatic systems where operational parameters are changing. 

Potentiodynamic polarisation The characteristics of passive/active conditions for metals can be readily defined using this technique ". Details for laboratory application can be found in ASTM Standard G5 (latest revision). Application in plant is easily performed as portable equipment (potentiostat) is available from several manufacturers, with some models incorporating built-in computer facilities. 

EMIRS spectra were recorded with an EMIRS III grating spectrometer (Hi-Tek, GB), coupled with an electrochemical equipment (potentiostat and waveform generator) allowing a rigourous control of the electrode potential during the spectroscopic experiments. Using a 3-electrode spectro-electrochemical cell, it is thus possible to record voltammograms before and after acquisition of the spectroscopic data... 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

Rapid-Scan Corrosion Behavior Diagram (CBD) Basically, all the same equipment used in the conductance of an ASTM G5 slow-scan polarization study is used for rapid-scan CBDs (that is, a standard test cell, potentiostat, voltmeters, log converters, X-Y recorders, and electronic potential scanning devices). The differences... 

In such conditions, it is recommended that the T-R be equipped with an electrical control circuit, which primarily keeps the potential constant, and, in exceptional circumstances, also the protection current. These pieces of equipment are potentiostats (for controlling potential) and galvanostats (for controlling current) [8]. 

Since usually the reference electrode is not equipped with a capillary probe (see Fig. 2-3), there is an error in the potential measurement given by Eq. (2-34) in this connection see the data in Section 3.3.1 on IR-free potential measurement. The switching method described there can also be applied in a modified form to potential-controlled protection current devices. Interrupter potentiostats are used that periodically switch off the protection current for short intervals [5]. The switch-off phase is for a few tens of microseconds and the switch-on phase lasts several hundred microseconds. 

Corradi and Gasperini claimed that the potentiostatic method was more effective and simpler than the Strauss test for determining intergranular corrosion of stainless steels, and suggested that the method may lend itself for use on finished equipment in service as a non-destructive test. 

If a controlled-potential determination is to be carried out, additional equipment will be required, namely an electronic voltmeter, a potentiostat and a reference electrode. The latter is most commonly a saturated calomel electrode, the construction of which is described in Chapter 14. 

For a practical realization of the foregoing transient modes, special programming units or signal synthesizers are used which are integrated in the potentiostatic or galvanostatic equipment. 

A certain potential is applied to the electrode with the potentiostatic equipment, and the variation of current is recorded as a function of time. At the very beginning a large current flows, which is due largely to charging of the electrode s EDL as required by the potential change. The maximum current and the time of EDL charging depend not only on the electrode system and size but also on the parameters of the potentiostat used. When this process has ended, mainly the faradaic component of current remains, which in particular will cause the changes in surface concentrations described in Section 11.2. 

Figure 2.15 Schematic representation of the equipment necessary to perform linear sweep voltammetry LSV) or cyclic voltammetry CV). WFG waveform generator, P potentiostat, CR chart recorder, EC electrochemical cell, WE working electrode, CE counter electrode, RE...

Electrochemical Equipment. Electrochemical experiments were performed using either a PAR Model 175 universal programmer and a PAR Model 363 potentiostat/galvanostat, or a Pine Instruments RDE-4 bipotentiostat, coupled with a Kipp and Zonen BD 91 X-y-y recorder. The current-time response for the chronoamperometry experiments was recorded with a Nicolet 4094 digital oscilloscope. All potentials were measured vs. a Ag/10"2 M Ag+ reference electrode. 

Electrochemical measurements were performed in an electrochemical cell equipped with quartz windows which fit into the sample compartment of a Cary 14 spectrometer. The cell (CHjCN, 0.1N TEAP vs S.C.E.) employed three electrode (Pt auxiliary electrode) potentiostatic control. A Tacussell PRT Potentiostat and PAR model 175 signal generator were used for the measurements. 

Fig. 3 Scheme of potentiostatic operation for a preparative eiectroiysis, using in principie a simpiified cyciovoitammetry equipment. The potentiai of the working eiectrode is measured by a Luggin capiiiary, coupied with a reference eiectrode (RE, see Sect. 2.5.1.6). The controi circuit in the potentiostat adjusts the ceii current untii the potentiai of the working eiectrode is equai to the voitage at the controi input. 

The electronic components for the measurements consisted of EG Q Model 173 Potentiostat equipped with slow sweep option (0.1 mv/sec) and EG G Model 376 Logarithmic Current Converter. An EG G Model 175 Universal Programmer supplied the waveform for running the polarization experiment. The output from the electrometer of the 173 and the log output of the 376 were connected to a Hewlett-Packard Model 7036B X-Y Recorder and the potential plotted versus log current. 

In general, controlled-current electrolyses need less expensive equipment. Only a controlled-current source in combination with a coulomb integrator is necessary. Therefore, in industry, electroorganic reactions are always performed at a fixed current density. In the laboratory, it is advisable to start with controlled-potential electrolyses using a potentiostat and a three-electrode electrolysis cell (Fig. 22.8). In this way, the reaction can be controlled at the redox potential of the substrate determined analytically, and the selectivity of the process can be studied at different potentials. After determination of the selectivity controlling factors, it is usually possible to change over to current control by proper selection of the current density and the concentration of the substrate. Using a continuous process, the concentration can be fixed at the desired value. Thus, selectivity can also be obtained under these conditions. 

These observations made it possible to think of assays outside the laboratory being carried out in two convenient stages extraction of wool samples and exposure of electrodes to potential inhibitors using a relatively simple field kit, followed by testing of electrodes, possibly in parallel, in a laboratory equipped with potentiostats, etc. A separation in time between exposure and measurement of inhibition had also been used by Brown et al. [50]. 

Fig. 29.3. The Palmsens hand-held potentiostat equipped with the dedicated multiplexer for eight-channel measurements.

EG G PAR (USA), Model 273 A, Potentiostat/Galvanostat volt-ammetric analyzer controlled by PC equipped with a data acquisition and treatment software to record the signal generated in the electrochemical cell for DPY measurements. A 25 mL glass cell at 25°C with the carbon paste biosensor, Ag/AgCl (3.0 mol L-1 KC1) reference electrode, and a platinum wire as auxiliary electrode to perform the volt-ammetric measurements. 

Electropolymerization and cyclic voltammetric experiments are performed with an EG G PARC, Model 173 potentiostat equipped with a Model 175 universal programmer and a Model 179 digital coulometer in conjunction with a Kipp and Zonen BD 91 XY/t recorder. All experiments are carried out using a conventional three-electrode cell. Instrumental setup for amperometric measurements ... 

Set up the potentiostat and data recording equipment to measure the current when a potential of 100 mV vs. Ag/AgCl reference electrode is applied. 

Apparatus Cyclic voltammetry and amperometric current-time curves were obtained with a Pine Instrument Inc., Model RDE4 bipotentiostat and Kipp Zonen BD 91 XYY recorder equipped with a time base module. All measurements were performed in a conventional single-compartment cell with a saturated calomel electrode as the reference electrode and a Pt mesh as the auxiliary electrode at room temperature. Chronoamperometry was made with EG G Princeton Applied Research potentiostat/galvanostat Model 273 equipped with Model 270 Electrochemical Analysis Software. 

Figure 3.5 Schematic representations of typical electrochemical quartz crystal microbalance equipment (a) using a Wenking potentiostat in which the working electrode is at hard ground (b) using a conventional potentiostat in which the current is measured by the voltage drop across a 1 kS2 resistor in series with the counter electrode...

A milder version of the potentiostatic intermittent titration technique (PITT) consists in modifying the initial potential by small steps, may be 1 mV, recording the current after a constant delay, and waiting until it has decreased to almost zero before modifying again the potential (staircase voltammetry). This version gives more information (activity potential and capacity) about the different redox couples present in the electrodes which are successively electroactive, but needs more sensitive equipment. 

The above cells may be mounted and loaded with solution in a glove box, and are then transferred to the FTIR spectrometer, to which a potentiostat/galva-nostat is attached. The external reflectance and ATR modes are performed, using standard accessories from Harrick or Spectra-Tech for these modes. The SIR mode is performed using the horizontal FT-80 or FT-85 grazing angle reflectance accessory from Spectra-Tech, equipped with a polarizer (standard equipment, see Figure 11). 

After assembly, the sandwich-type cell is heated to 50-70°C and maintained at a fixed selected value during acquisition of electrochemical data. The potential is controlled using an RDE 3 Pine potentiostat equipped with a built-in signal generator. Cyclic voltammograms were recorded at scan rates of 5 to 100 mV/s. All potentials are reported with respect to Li[C/R],... 

---