Instrumentation solution resistance effects

Serious errors in corrosion rate measurements may occur in low-conductivity water due to solution resistivity effects. This situation tends to occur at moderate to high corrosion rates and is manifested by indicating lower corrosion rates than actual. The operating range for various LPR instruments can be obtained from the manufacturer. 

The first effect that one notices in the voltammetric response is that quasireversibility induces a separation between the forward and the reverse peaks i.e. the peak-to-peak separation A p) much greater than that of a reversible process (to determine this parameter correctly one must use instrumentation able to compensate for the solution resistance). 

If the three-electrode instrument is equipped with an iR-drop compensator, most of the iT-drop caused by the solution resistance can be eliminated. However, in order to minimize the effect of the iT-drop, a Fuggin capillary can be attached to the reference electrode with its tip placed close to the indicator electrode. Moreover, for a solution of extremely high resistance, it is effective to use a quasi-reference electrode of a platinum wire (Fig. 8.1(a)) or a dual-reference electrode (Fig. 8.1(b)), instead of the conventional reference electrode [12]. 

The bipolar pulse technique for measuring solution resistance minimizes the effects of both the series and parallel cell capacitances in a unique way. The instrumentation for this technique is illustrated in Figure 8.15. The technique consists of applying two consecutive voltage pulses of equal magnitude and pulse width but of opposite polarity to a cell and then measuring the cell current precisely at the end of the second pulse [18]. 

Commercial instruments are generally calibrated directly in corrosion rate units and conversion factors are utilised for probes of metals other than that for which the meter is calibrated [29]. Some instruments also have data capture facilities for "unmanned monitoring. Probes may consist of from two to four elements of which at least one is of the material under test. The higher the solution resistance, the larger the number of elements in the probe, the extra elements are used to assess and nullify the effects of solution resistance. 

In Fig. 8D, we compared the potential applied to the interface during linear potential sweep with and without an uncompensated solution resistance. Clearly, the error is a maximum at the peak, where the current has its highest value. Just before and during the peak, the effective sweep rate imposed on the interphase is much less than that applied by the instrument. The assumption that v = constant, which has been used as one of the boundary conditions for solving the diffusion equation, does not apply. In this sense the experiment is no longer conducted "correctly". 

The redox behavior of species that are adsorbed on the surface is usually activation contrplled and influenced by the remaining number of free sites on the surface. Hence, this maybe conducted in stirred solutions. The typical sweep rates are also in the range (0.01—10) V s but here the lower limit is determined by background currents from residual impurities in solution (and perhaps by the desire of the experimenter to collect more data in a given time) while the upper limit is determined by the uncompensated solution resistance and by instrumentation. The Faradaic current associated with this process is proportional to the sweep rate, and so is the double-layer charging current, so that the relative effect of jdi on the measured current is independent of sweep rate. 

The conductance of a solution is the inverse of its resistance, and conductance has units of ohms 1 or mohs. The higher the conductance of a solution, the lower is its electrical resistance. A conductivity meter and conductivity cell are used to determine the effective resistance of a solution. The conductivity cell consists of a pair of platinized platinum electrodes with an area of approximately 1.0 cm2 with spacers designed to hold the electrodes rigidly parallel and at a fixed distance from each other. The cell can be standardized with solutions of known conductivity to obtain the cell constant, k so that the instrument response R... 

UMEs of 10 pm in diameter and voltammetric instruments for use with such UMEs are commercially available. Electrodes of smaller dimensions can be prepared in the laboratory, although this requires considerable skill [74], In order to use UMEs successfully for high-speed voltammetry in highly resistive solutions, care must be taken concerning the effects of the ohmic drop and the capacitance of the cell system [65 b, 74, 75]. Moreover, two types of voltammograms, i.e. curves (a) and (b) in Fig. 5.23, should be used appropriately, according to the ob-... 

Looking at Fig. lA carefully, one observes that up to a certain potential the current is zero. This is not a matter of limited sensitivity of the measuring instrument. The current is exactly zero (disregarding minor impurity effects), corresponding effectively to an infinite resistance of the interface. The reason for this observation is thermodynamic. Curves such as those shown in Fig. lA are obtained when electrolysis causes a net chemical reaction. Passing a current between two platinum electrodes in a pure solution of sulfuric acid is a good example. The reaction taking place is the electrolysis of water. 

Sample solutions containing low concentrations of several inorganic anions were run at pH 8.5 with increasing concentrations of sodium chloride or lithium sulfate in the BGE. The plots in Fig. 10.6 show several interesting effects. One is that the current increases rapidly with increasing salt concentration and levels out at 280 pA around 200 mM sodium chloride or lithium sulfate. This sharp increase in current can be attributed to less electrical resistance. The maximum current that can be obtained in our instrument is set at 280 pA. In order to maintain this current, the voltage was automatically lowered as the salt concentration in the BGE continued to increase. The full power of the instrument s power supply was then being used. 

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