Microelectrodes profiles

Figure 11. Microelectrode profiles in biofouling deposits on ennobled 316L stainless steel coupons. Horizontal line indicates biofouling/solution interface. (Reprinted from Ref 12 with permission from NACE International.)...

Figure 6 Microelectrode profiles of dissolved O2 and ApH obtained by Hales and Emerson (1996) at 2.3 km depth on the Ontong-Java Plateau in the western equatorial Pacific. On the right are model curves showing the pH trend expected if none of the CO2 released during the consumption of the O2 was neutralized by reaction with sediment CaCOs (dashed curve) and a best model fit to the measured ApH trend (solid curve). The latter requires that much of the respiration CO2 reacts with CaCOs before it escapes into the overlying bottom water.

Table 6 CaCOs dissolution rates at sites above the calcite saturation horizon (i) in situ microelectrode profiling (NW Atlantic and Ceara Rise) and (ii) in situ whole-core squeezer (Ceara Rise and Cape Verde Plateau).

Fig. 3.22 Diffusive boundary layer at the sediment/bottom water transition measured with oxygen microelectrodes. Profiles are shown in both representations on the left. The two representations on the right show the respective spatial patterns. In the above right corner, the surface is shown, whereas the subsurface of the diffusive boundary layer is shown below (after Gundersen and Jorgensen 1990).

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

SECM is a scaiming-probe teclmiqiie introduced by Bard et aJ in 1989 [49, and M ] based on previous studies by the same group on in situ STM [ ] and simultaneous work by Engstrom et aJ [53 and M], who were the first to show that an amperometric microelectrode could be used as a local probe to map the concentration profile of a larger active electrode. SECM may be envisaged as a chemical microscope based on faradic current changes as a microelectrode is moved across a surface of a sample. It has proved iisefiil for... 

Profiles of DO and HjOj concentrations and microelectrode Ecm were measured within biofouling deposits on coupons. The microelectrodes, mounted in a commercial three-axis micromanipulator, were positioned above a deposit by adjusting the X-Y micromanipulator controls... 

In the Dickinson et al. studies, the results of microelectrode measurements indicated that eimohlement was not caused by elevated levels of dissolved oxidants. Measurements at various heights above the substratum and at numerous sites over the coupon surface showed HjOj concentrations < 2 pM and no significant variation in for the stainless steel microelectrode at any site. DO profiles in the same regions showed saturation levels at all sites. Representative profiles are shown in Fig. 11. 

The reaction was followed by the local measurement of chloride ions, at a potentio-metric Ag/AgCl microelectrode probe, positioned in the aqueous receptor phase, as DCE droplets containing TPMCl were grown (feeder phase). The reaction was shown unambiguously to occur interfacially, and was first-order in TPMCl with a hydrolysis rate constant of 6.5 x 10 cms. A typical concentration-distance profile determined in these experiments is shown in Fig. 18. 

FIG. 18 Chloride concentration profile recorded by a microelectrode probe during the hydrolysis of TPMCl at a DCE drop-aqueous interface (O)- The concentration of TPMCl in the organic phase was 50 mmol dm, the drop time from formation to contact with the probe was 4.80 s, and the final drop radius was 0.55 mm. The solid lines represent theoretical time-dependent concentration profiles, from top to bottom, generated for k = 3.50 x 10 , 3.25 x 10 , and 3.00 x 10 molcm s . A value of 1.8 X 10 cm s was employed for the diffusion coefficient of chloride. (Reprinted from Ref. 73. Copyright 1997, American Chemical Society.)... 

Antonenko and Bulychev [84] measured local pH changes near BLM surfaces using a variably positioned 10 pm antimony-tip pH microelectrode. Shifts in pH near the membrane surface were induced by the addition of (NH4)2S04. As the neutral NH3 permeated, the surface on the donor side of the BLM accumulated excess H+ and the surface on the acceptor side of the membrane was depleted of H+ as the permeated NH3 reacted with water. These effects took place in the UWL. From measurement of the pH profile as a function of distance from the membrane surface, it was possible to estimate /raq as 290 pm in the stirred solution. 

The electrodes used in the above studies were double-barreled glass pH sensitive microelectrodes, and the spatial retinal pH profile was recorded by withdrawing the microelectrode tip at a rate of 1 //m/s or lOOpm/step across the retina in vivo or in vitro. In a typical retina pH profile (Fig. 10.9), measured in cat retina by the microelectrode, started from the choroids (Ph = 7.41, at distance Ojum). The pH steadily decreased to a minimum value (a maximum [H+] concentration) in the proximal portion of the outer nuclear layer (pH = 7.14 at —140 jum), then increased to —7.28 (at —310 pm) at the vitreous retinal border. The peak [H+] concentration in this layer indicated that a net production of proton occurred across the avascular outer retina [76],... 

XJ. Zhang, A. Fakler, and U.E. Spichiger, Design of pH microelectrodes based on ETHT 2418 and measurement of pH profile in instant noodles. Anal. Chim. Acta 445, 57-65 (2001). 

Fig. 7.35. Development of diffusion concentration profiles in ensembles of microelectrodes. Concentration distortions at very short times during chronoamperometry or fast sweep rates during (a) cyclic voltammetry, (b) intermediate times or sweep rates, and (c) long times or slow sweep rates. Voltam-metric responses are shown schematically. (Reprinted from B. R. Scharifker, Microelectrode Techniques in Electrochemistry, in Modem Aspects of Electrochemistry, Vd. 22, J. O M. Bockris, B. E. Conway, and R. E. White, eds., Plenum, 1992, p. 505.)...

The planar, cylindrical, and spherical forms of Fick s second law, and combinations of those forms, are sufficient to describe diffusion to most microelectrode geometries in use today. Just as was illustrated in Chapter 2, the appropriate form of Fick s second law is solved, subject to the boundary conditions that describe a given experiment, to provide the concentration profile information. The sought-after current-time or current-voltage relationship is then obtained by evaluating the flux at the electrode surface. 

Fig. 2.18 Chronoamperometric profiles showing oxidative faradaic transients of gold nanoparticles at potentials of (a) 0.8 V and (b) 1.1 V at a Glassy Carbon microelectrode of 11 pm of radius. Reproduced from reference [62] with permission...

A voltammetric experiment in a microelectrode array is highly dependent on the thickness of the individual diffusion layers, <5, compared with the size of the microelectrodes themselves, and with the interelectrode distance and the time experiment or the scan rate. In order to visualize the different behavior of the mass transport to a microelectrode array, simulated concentration profiles to spherical microelectrodes or particles calculated for different values of the parameter = fD Ja/r s can be seen in Fig. 5.17 [57] when the separation between centers of... 

Most electrodes are of the planar type and can be affected by such factors as convectional mass transport within the sample in which they are immersed. Any type of turbulence will tend to increase the supply of ions to an electrode surface, above and beyond that of diffusional supply. This can have an effect on the magnitude of sensor response and give rise to erroneous results. Diffusion to a large planar electrode can be approximated to be perpendicular to the electrode surface. However, when electrodes become very small, the diffusion profile is hemispherical, mass transport is greatly increased (Fig. 9) and diffusion becomes much less of a limiting factor in the sensor response [109]. Thus, turbulence has a much smaller effect and this means that very small microelectrodes display stir-independent responses. Also the small size of micro-... 

Anion detection at microelectrodes has not been studied widely. Amongst the first was the work of de Beer et al. [ 111 ] who manufactured a nitrite sensor with a tip just a few microns in diameter, which could detect nitrite ions down to 1 pM. This proved to be suitable for profiling the concentrations of nitrite anion within biofilms less than 1-mm thick inside water treatment plants. Other workers have found that use of an interdigitated microelectrode array [ 112] allows measurement of iodide via monitoring of its redox peak down to sub-micromole levels, making it a suitable technique for analysing mineral water. Carbon nanotubes coated onto Pt microdiscs have been utilised to make a nitrite sensor [113,114] with detection levels of 0.1 pM. Sulphide has also been detected at nickel microdiscs (50 pm diameter) [115]. 

Microelectrodes often display superior properties to those of larger planar electrodes. They experience hemispherical diffusion profiles which can confer stir independence on the results, which may be important if measuring turbulent systems such as in a river or in the bloodstream. They also allow measurements in high-resistance media, which may be experienced if electrolyte concentrations are very low. 

Fig. 35. Detail of the conductivity profile measured close to the anode of an electrocolored Fe-doped SrTiCb polycrystal (E = 103 V cm1, electrocoloration with Au electrodes at about 493 K for 60 min). The corresponding area of the sample with evaporated microelectrodes (dmt = 10 pm) is depicted below the diagram. The dotted lines indicate grain boundaries the arrow, the investigated electrode line. The profile was measured at 473 K.

In summary, all these local measurements demonstrate the power of microelectrodes i) to determine local bulk conductivities in ionic solids and ii) to study an important phenomenon in solid state ionics, namely the occurrence of nonstoichiometry profiles in mixed conducting solids. 

The applicability of microelectrodes in various fields of solid state ionics has been demonstrated in four examples i) Local conductivity measurements on SrTiC>3 revealed pronounced conductivity profiles after high-field stress and confirmed that non-stoichiometry effects due to blocked ion exchange at the electrodes cause the phenomenon of resistance degradation in perovskite-type electroceramics, ii) Micro-... 

Even if rate measurements in sediments are made using whole core incubations, e.g., when the inhibitor is a gas, it is still difficult to obtain a depth distribution of the rate (usually, an areal rate is obtained). A sophisticated measurement and model based system that avoids direct rate measurements has been used to overcome this problem. Microelectrodes, which have very high vertical resolution, are used to measure the fine scale distribution of oxygen and NOs" in freshwater sediments. By assuming that the observed vertical gradients represent a steady state condition, reaction-diffusion models can then be used to estimate the rates of nitrification, denitrification and aerobic respiration and to compute the location of the rate processes in relation to the chemical profiles (e.g., Binnerup et ai, 1992 Jensen et ai, 1994 Meyer et ai, 2001 Rysgaard et ai, 1994). Recent advances and details of the microelectrode approach can be found in the Chapter by Joye and Anderson (this volume). 

With the advent of rapid-scan and high-frequency pulse methods, more direct approaches for evaluating annihilation mechanisms and dynamics have been developed. Early work of van Duyne, using triple potential steps with very short step times, allowed estimation of the annihilation rate constant for DPA anion and cation radicals [29]. More recently, Wightman and coworkers have used multicycle generation of ECL at microelectrodes to determine annihilation rate constants and ECL efficiencies [41, 42]. Figure 7 shows the normalized ECL intensity from DPA at a 1-pm Pt disk as a function of time (t/tf) at different oscillation frequencies. The intensity increases rapidly after the potential is switched, and then decays as the reactants are depleted. As the oscillation frequency is increased, the annihilation occurs closer to the electrode surface, the intensity-time profile broadens and... 

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