Electrodes lifetime

The response of the immobilized enzyme electrode can be made independent of the enzyme concentration by using a large excess of enzyme at the electrode surface. The electrode response is limited by the mass transport of the substrate. Using an excess of enzyme often results in longer electrode lifetimes, increased linear range, reduced susceptibiUty to pH, temperature, and interfering species (58,59). At low enzyme concentrations the electrode response is governed by the kinetics of the enzyme reaction. 

The concept of a biocatalytic membrane electrode has been extended to the use of a tissue slice as the catalytic layer. An example of this approach is an electrode for AMP which consists of a slice of rabbit muscle adjacent to an ammonia gas electrode. NHj is produced by enzymatic action of rabbit muscle constituents on AMP The electrode exhibits a linear range of 1.4 x 10 to 1.0 x 10 M with a response time varying from 2.5 to 8.5 min, depending on the concentration. Electrode lifetime is about 28 days when stored between use in buffer with sodium azide to prevent bacterial growth. Excellent selectivity enables AMP to be determined in serum. 

A very important aspect is corrosion, concerning a possible contamination of electrolyte and products - particularly in case of toxic materials - and with respect to the electrode lifetime. Last, not least, the price can be decisive, especially if a commercial appKcation is planned. 

Solution movement has the tendency to remove species from the membrane surface, i.e. membrane deterioration is faster than in stationary solution, and there is a consequent shortening of electrode lifetime. Periodic calibration becomes extremely important. 

A non-linear wall-stabilized non-transferred arc is shown in Fig. 4 8. It consists of a cylindrical hollow cathode and coaxial hollow anode located in a water-cooled chamber and separated by an insulator. Gas flow blows the arc column out of the anode opening to heat a downstream material, which is supposed to be treated. In contrast to transferred arcs, the treated material is not supposed to operate as an anode. Magnetic 7x5 forces cause the arc roots to rotate around electrodes (Fig. 4-48), which provides longer electrode lifetime. The generation of electrons on the cathode is provided in this case by field emission. An axisymmetric version of the non-transferred arc, usually referred to as the plasma torch or the arc jet, is illustrated in Fig. 4-49. The arc is generated in a conical gap in the anode and pushed out of this opening by gas flow. The heated gas flow forms a very-high-temperature arc jet, sometimes at supersonic velocities. 

If the electrode is stable for 1 h, durability testing should be repeated for longer testing times, e.g., 24 h, 2(X) h, 10(X) h, and eventually toward 5000 h (see Chapter PEC Characterization Flowchart ) for a more conclusive determination of stability. A simple extrapolation to determine electrode lifetime (as done above) is... 

The high pH at the negative electrode surface promotes calcium carbonate or magnesium hydroxide deposition, and regular cleaning is necessary. Polarity reversal, every 2—8 h depending on the water hardness, will remove the deposits, but this operation decreases the electrode lifetime (see below). If the electrode material does not allow polarity reversal (titanium or stainless steel cathodes), frequent acid cleaning is necessary. 

The BMSA electrodes have many advantages, among others lack of the inner, reference solution limits the washing out of the membrane components, which prolongs the electrode lifetime and improves the stability of potential in time (lower penetration of water into the membrane). Moreover, the electrodes are relatively fast and easy to prepare. BMSA electrodes are maintenance-free sensors and do not have to be stored in an upright position. 

Although absence of any redox reactions is ideal for electrode lifetime, these reactions are very hard to avoid. As soon as the electrode materials come in contact with an ionic solution, lots of different reactions may occur, even if no current flows into the electrode. Consider a piece of iron immersed in saline solution. After enough time, rust appears on the iron surface. Corrosion occurs following simultaneous reduction and oxidation reactions on the surface. While the metal is oxidized, oxygen (O2) or H+ ions are reduced at the same time to complete the redox reaction. 

The Effect of the Counter Electrode on Stimulation Electrode Lifetime... 

A new topic covered here is the effect of the counter electrode material on the faradaic reactions and galvanic corrosion in a two electrode monopolar stimulation system. It was explained how selecting a proper material for the counter electrode can enhance the electrode lifetime. 

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