Potential steady-state

Ion-selective electrodes can be incorporated in flow cells to monitor the concentration of an analyte in standards and samples that are pumped through the flow cell. As the analyte passes through the cell, a potential spike is recorded instead of a steady-state potential. The concentration of K+ in serum has been determined in this fashion, using standards prepared in a matrix of 0.014 M NaCl. ... 

Steady-state potential, negative to saturated calomel half-ceU. 

In the simplest case of one-dimensional steady flow in the x direction, there is a parallel between Eourier s law for heat flowrate and Ohm s law for charge flowrate (i.e., electrical current). Eor three-dimensional steady-state, potential and temperature distributions are both governed by Laplace s equation. The right-hand terms in Poisson s equation are (.Qy/e) = (volumetric charge density/permittivity) and (Qp // ) = (volumetric heat generation rate/thermal conductivity). The respective units of these terms are (V m ) and (K m ). Representations of isopotential and isothermal surfaces are known respectively as potential or temperature fields. Lines of constant potential gradient ( electric field lines ) normal to isopotential surfaces are similar to lines of constant temperature gradient ( lines of flow ) normal to... 

Table 4.18 Steady-state potential of several metals in I M solutions of different electrolytes (vs 0-1 n Calomel, = 0-336 V)...

The standard electrode potential of magnesium is given, along with the potentials of other metals, in Table 4.17 and the steady-state potentials of magnesium in various solutions are listed in Table 4.18. ... 

The critical current and primary passivation potential will not appear on an anodic polarisation curve when the steady-state potential already is higher than In such a case the potentiostat is unable to provide direct data for constructing the full polarisation curve. If that portion of the curve below the steady-state potential is desired, then the potential has to be held constant at several points in this range and corrosion currents calculated from corrosion rates as determined from solution analyses and/or weight losses. 

Steady-state potential comparable with Type 1 reversible electrode Metal in a solution of electrolyte in which ions are produced by a corrosion reaction in an VAf exchange that determines the potential. Zn in NaCI solution Zn in dilute HCI... 

Steady-state potential comparable with Type 2 reversible electrode Potentials of electropositive metals that react with solution to give sparingly soluble salts of the metal. Cu or Ag in NaCl or Ag in HCI giving an M/MX/X type of electrode... 

Steady-state potential comparable with Types 4 and 5 reversible electrodes Potential of metal depends on pH of solution, although the dependence is confined to a limited range of pH and does not conform precisely to the Nernst equation. Ni in H2SO4 (Ni/Hj, H + ) Cu in NaOH (Cu/CujO/OH")... 

Steady-state Potential the potential of an electrode which is independent of time because its reaction occurs at a constant rate. 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

The thermodynamic standard potential of the methanol electrode has a value of + 0.02 V (RHE) that is, it is quite close to the hydrogen electrode potential. The steady-state potential of a platinum electrode in aqueous methanol solutions is about + 0.3 V (RHE). 

The behavior of aluminum in neutral and weakly alkaline solutions resembles the behavior of magnesium, but the negative difference effect is much less pronounced at aluminum. The steady-state potential of aluminum is approximately 1V more positive than the thermodynamic value. Yet unlike magnesium, aluminum will not passivate in strongly alkaline solutions, but undergoes fast dissolution to soluble aluminates. 

It is worthwhile mentioning that the interfacial potential created at the liquid-liquid interface is governed by single ionic or redox equilibrium only in the simple cases. The presence of various, often two, interfacial processes is a source of the steady-state potential, named also the mixed or the rest potential. Its value is situated between the two equilibrium potentials, near that one which corresponds to the higher exchange current... 

The steady-state potential (or current density) is related to a steady growth of the porous oxide into the solution, maintaining a constant number of pores and a constant pore radius. This scheme is supported by electron microscopic observations reported by Xu et a/.102... 

Alternatively, it might be that the underpotentials needed to form atomic layers of the elements were decreasing, shifting closer to the formal potentials for deposition of the bulk elements. This scenario may be a factor, but it is frequently observed that the steady state potentials are more negative then the formal potentials for the elements, where bulk deposits of the elements would be expected to form. 

When the potentials are shifted a little each cycle, steady state potentials are generally achieved after about 25 cycles. The steady state potentials can be maintained, without shifting, through the rest of the deposit, with the amounts deposited remaining constant. 

For a fully dissociated but non-ideal polymer electrolyte (i.e. long range ion interactions are present but not ion association) the following expressions for the steady state potential AV, and current may be derived, again assuming reversible electrode behaviour ... 

Figure 15.10 Selectivity pattern of PPy(NO ) electrodes. The anions tested include (1) nitrate, (2) bromide, (3) perchlorate, (4) sahcylate, and (5) phosphate AB is the difference between the steady-state potential and the starting potential. Reprinted from Hutchins and Bachas (1995). Copyright 1995 American Chemical Society.

What is the quantitative relationship between the steady state, convection-with-diffusion current density and the potential difference across the interface How is the steady-state potential difference at a steady current density related to the zero-current, or equilibrium, potential difference These questions are the relevant ones for steady passage of current in convection-aided situations. 

Let us consider in more detail, using the above concepts, how a photocorrosion process occurs under the illumination of a semiconductor. Suppose that electron transitions at the interface between the semiconductor and solution do not take place in darkness in a certain potential range (the semiconductor behaves like an ideally polarizable electrode). This range is confined to the potentials of decomposition of the semiconductor and/or solution. The steady state potential of a semiconductor is usually determined in this case by chemisorption processes (e.g., of oxygen) or, which is the same in the language of the physics of semiconductor surface, by charging of slow surface states. It is these processes that determine the steady state band bending. 

A. Distributed Resistor Network Model of Steady-State Potentials... 

The absence of any direct, i.e. molecular, means of identifying the adsorbed species in situ rendered the controversy unresolvable and it remained undecided over the ensuing fifteen years. However, in 1981 Beden et til. published EMIRS spectra that were destined to have a major impact on this dispute, as discussed in section 2.1.6. This early paper concluded that C=Oads is the dominant strongly adsorbed species (poison) and it is present at high coverage. Some Pt CO is also present but there is no evidence of COHads under the experimental conditions employed (non steady-state, potential perturbation at 8.5 Hz and with the dissociative chemisorption of methanol slow). The principal assignments of the paper were very quickly verified by Russell et al. (1982) using the IRRAS technique. 

MULTI-ION CASE STEADY-STATE POTENTIAL PROFILE... 

According to the Michaelis-Menten equation, the rate of the reaction depends on the activity of the enzyme, enzyme concentration, and upon several other factors such as the pH, the temperature, availability of cofactors, activators and/or inhibitors, and the concentration of the substrate (analyte). Moreover, the reaction rate depends on the thickness of the slice of tissue or crude extract, and on the size of the dialysis membrane, when used. By contrast, the steady-state potential obtained is dependent only on the substrate concentration and the temperature. 

K(H20)4 ions until their velocities are equal. If the junction is constructed so that a stable diffusion geometry is established, a steady-state potential will be established that is approximately constant with time. 

Figure 4. Potential-time curves from experiments with a thermopile heat conduction calorimeter. A A short heat pulse released at time t,. B A constant thermal power released between time t, and t2. The steady-state potential value, USI is proportional to the released thermal power.

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