Limiting Currents increase

Example 3-5 Polarogram A was obtained for a lOmL lead-containing sample. The limiting current increased (to B) after adding 100 pL of a 0.10 m lead standard to the 10 ml sample. Calculate the original lead concentration in the sample. 

Unlike solid electrodes, the shape of the ITIES can be varied by application of an external pressure to the pipette. The shape of the meniscus formed at the pipette tip was studied in situ by video microscopy under controlled pressure [19]. When a negative pressure was applied, the ITIES shape was concave. As expected from the theory [25a], the diffusion current to a recessed ITIES was lower than in absence of negative external pressure. When a positive pressure was applied to the pipette, the solution meniscus became convex, and the diffusion current increased. The diffusion-limiting current increased with increasing height of the spherical segment (up to the complete sphere), as the theory predicts [25b]. Importantly, with no external pressure applied to the pipette, the micro-ITIES was found to be essentially flat. This observation was corroborated by numerous experiments performed with different concentrations of dissolved species and different pipette radii [19]. The measured diffusion current to such an interface agrees quantitatively with Eq. (6) if the outer pipette wall is silanized (see next section). The effective radius of a pipette can be calculated from Eq. (6) and compared to the value found microscopically [19]. 

From Eqs. (4.221) and (4.222), it is clear that both limiting currents increase when the electrode radius decreases and, when the chemical reaction is irreversible (K = 0). the anodic limiting current / pvu s independent of the electrode size. 

Addition of hydroxide ions results in formation of ageminal diol anion -CH=0 + OH-=CH(OH)CT (ix). As shown for substituted benzaldehydes, resulting anion undergoes in alkaline media electrooxidation to a car-boxylate anion. The height of the anodic wave of this process is controlled by the rate of addition of the OH-ions. The limiting current increases with increasing activity of OH- ions until it reaches a diffusion-controlled value, corresponding to a two-electron process. 

This limiting current increases linearly with the bulk concentration and decreases with the square root of time. Accordingly, this current should decrease by a factor 10 within a time interval from 1 to 100 s. In most experiments such a variation in current is not observed because of some convection of the liquid. 

In studies of the (100) face, two distinct types of dissolution behavior were observed. In the center of the face, dislocation etching revealed that the average dislocation spacing was considerably less than the size of the UME probe (50), i.e., in the SECM configuration the electrode would sit over a constant source of dissolution sites once the process had been initiated. This was reflected in the shape of experimental SECM chronoamper-ometric transients (3), such as those depicted in Figure 17 at different tip-substrate separations. In all cases, the current rapidly attained a steady state, indicative of a uniform, constant rate of dissolution. As the UME was moved closer to the crystal surface, the limiting current increased, consistent with an increase in the flux of Cu2+ from the crystal surface to the tip electrode. 

Note that the steady-state limiting current increases with the rotation speed... 

For all the detectors, the limiting current increases with Reynolds number. Therefore it is desirable to construct detectors with small cross-sectional areas (or high electrolyte speeds) in order to increase the mean linear fluid velocity, and to choose an electrode geometry with a high Reynolds exponent... 

The limiting current increases, thus reducing the mass transport or concentration overvoltage losses. This is because of the absence of nitrogen gas, which is a major contributor to this type of loss at high current densities (see Section 3.7). 

FIGURE 13.8 Limiting cuirent measured as a function of the number of air holes in a 675-size zinc/air cell. Limiting current increases with increased air access. (Courtesy of Rayovac, Inc.)... 

The cyclic voltamogramms evidenced the Ni (II) reduction at about -1.05 V/ Ag evsi-ref. for low working temperatures (30°C) which shifts towards more pwsitive values with temperature increase, towards -0.75h--0.88 V/Ag evasi-ref at 80cathodic limiting current increases with temperature. The anion nature does not significantly influence the cathodic process. 

Figure 13.17 at different tip/substrate separations. In all cases, the current rapidly attained a steady state, indicative of a uniform, constant rate of dissolution. As the UME was moved closer to the crystal surface, the limiting current increased, consistent with an increase in the flux of Cu + from... 

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