Heated wire electrodes, temperature

Analytical solution of the differential equations given above is not useful or even impossible. The problems of heat transport as well as of particle diffusion can be solved much better by digital simulation. This way, the temperature profiles d77dx as well as the concentration profiles dc/dr have been calculated for the convectimi-free period existing at a heated wire electrode of 25 pm in diameter. For this purpose, the solutimi surrounding the wire is separated in cylindric shells, so as if it would cOTisist of nested valves (Fig. 5.3). The dimensions of individual shells should be small enough to allow utilisation of linear equation forms of heat conduction and of diffusion, respectively. Linear transport of heat or matter into every ring as well as away from it is calculated for consecutive short time intervals At by means of oidy two equations ... 

As stated above, the layer containing the temperature profile seems to possess a complex substructure. If, at a permanently heated wire electrode, a voltammetric experiment is performed, then, as mentioned above, a constant limiting current is found. It seems that inside the region with T Tq exists a stagnant layer, although just at places nearest to the electrode surface should act the strongest forces to cause... 

Fig. 6.7 Effect of scan rate on cyclic voltammograms at permanently heated wire electrodes. CVs of reversible redox couples at different scan rates [(a) 20 V/s (b) 1 V/s (c) 100 mV/s]. Left-. [Ru (NHs) ] (1 mmolA) in aqueous KCl solution (0.1 molA) at a Pt wire 0.25 pm in diameter, reference electrode SCE, temperature 49 °C. Right Ferrocene (2 mmol/l) in DMF with suppmting electrolyte tetrabutylammonium fluoroborate (NBU4BF4) 0.5 mol/l, temperature 56 °C, reftaence electrode Ag/0.01 MAg /0.1 M NBu4BF4/MeCN/A).l M NBu4BF4/DMF. From [14], with permission...

Sigmoidal voltammograms at permanently heated wire electrodes proved very useful for analytical application since their limiting current can be determined easily. The latter is strictly proportional over some orders of magnitude to bulk concentration of the electrolysed species in solution. An extra benefit is the increased temperature which improves kinetic behaviour. Since temperature is increased only close to the microelectrode, constituents of bulk volume keep unaffected. This proved useful for analysis of dissolved gases and sensitive substances. 

Appendix A A Calculation Procedure for Temperature Profiles at Heated Wire Electrodes... 

A heated wire electrode attains a temperature profile with maximum temperature at the surface, and minimum in the bulk solution, sufficiently far from the surface. The surface temperature Tsurf is not constant, but tends to increase, if heating energy is fed continuously. As a result, the thickness of the temperature profile Axu,enn is growing into solution. The profile can be calculated for the simplifying assumption that heat is spreading by conduction alone, without contribution of radiation. This assumption is a good approximation as long as the wire temperature does not exceed that of the bulk solution by more than ca. 100 K. 

The Ru single crystal was oriented by Laue x<-ray back-scattering to within 1° of the Ru(001) plane, cut by a diamond saw and mechanically polished. After being etched in hot aqua regia for about 15 min, the crystal was spot welded to two tantalum heating wires which were connected to two stainless steel electrodes on a sample manipulator. The temperature was monitored by a Pt/Pt-10% Rh thermocouple which was spot welded to the back of the crystal. 

The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically heated, fine platinum wire which is immersed into the flow. As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect on the wire electrode occurs, causing its electrical resistance to change. In a constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance. In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, the resistance constant. 

Another problem with the DC arc is that volatile elements will selectively vaporize and enter the plasma before the less volatile elements in a sample. The electrode temperature increases slowly from the initiation of the arc. If the sample is the anode, which heats more rapidly and to a higher temperature than the cathode, volatile elements will rapidly enter the arc. For example, tungsten powder for light bulb wire contains added potassium... 

Fig. 5.1 Temperature rise and decay for a horizontally oriented wire electrode (Pt, 25 pm diameter) heated by constant power of different magnitudes. From [1], with permission...

Fig. 5.8 Obtained diffusion coefficients Zfobt from the peak or steady-state currents of cyclic voltammograms with different scan rates (10 mV s to 200 Vs ) of 1 mM piu(NH3)6] in aqueous KCl solution (0.1 M) at a continuously heated Pt-wire electrode (/ = 1 cm, d = 25 )rm) at different temperatures (25-81 °C). From [6], with permission...

---