Drift of potential

An incongruous set of results often indicates that decomposition is occurring during titration. Sometimes the first few readings give concordant pKs, after which decomposition causes a drift of potential. Those substances which are easily decomposed by acid or alkali are often attacked by each drop of titrant, even in well stirred... 

In a vacuum (a) and under the effect of a potential difference of V volts between two electrodes (A,B), an ion (mass m and charge ze) will travel in a straight line and reach a velocity v governed by the equation, mv = 2zeV. At atmospheric pressure (b), the motion of the ion is chaotic as it suffers many collisions. There is still a driving force of V volts, but the ions cannot attain the full velocity gained in a vacuum. Instead, the movement (drift) of the ion between the electrodes is described by a new term, the mobility. At low pressures, the ion has a long mean free path between collisions, and these may be sufficient to deflect the ion from its initial trajectory so that it does not reach the electrode B. 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Field studies are required to provide a more reaUstic picture of the dissipation of the parent compound and those degradates determined to be significant. Under field conditions pesticides are exposed simultaneously to the individual dissipation processes that were examined separately in the laboratory studies. Thus, in field studies, some dissipation processes may be altered due to competition and interaction. Requirements for spray drift data were outlined in draft Subdivision R, but the EPA agreed that data generated on a generic basis by an industry consortium could represent the potential for drifting of individual pesticides. 

The main chemico-analytical properties of the designed ionoselective electrodes have been determined. The work pH range of the electrodes is 1 to 5. The steepness of the electrode function is close to the idealized one calculated for two-charged ions (26-29 mV/pC). The electrode function have been established in the concentration range from 0.1 to 0.00001 mole/1. The principal advantage of such electrodes is the fact that thiocyanate ions are simultaneously both complexing ligands and the ionic power. The sensitivity (the discovery limits), selectivity (coefficient of selectivity) and the influence of the main temporal factors (drift of a potential, time of the response, lifetime of the membranes) were determined for these electrodes. 

Draper WM, Street JC. 1981. Drift from a commercial, aerial application of methyl and ethyl parathion An estimation of potential human exposure. Bull Environ Contam Toxicol 26 530-536. 

M. Fibbioli, W.E. Morf, M. Badertscher, N.F. de Rooij, and E. Pretsch, Potential drifts of solid-contacted ion-selective electrodes due to zero-current ion fluxes through die sensor membrane. Electroanalysis 12, 1286-1292 (2000). 

The integrated planar silver chloride electrode uses a thin layer of 150 pm polymer that consists of a heat curing epoxy resin poly-hydroxy-ethylmethacrylate (PHEMA) to immobilize the KC1 electrolyte. The potential drift of the reference electrode reduced to 59 pV/h after a conditioning phase of several hours. However, this reference electrode was only used for P02 measurement, while an external reference electrode was used for pH measurement. 

Draper, W.M., R.D. Gibson, and J.C. Street. 1981. Drift from and transport subsequent to a commercial aerial application of carbofuran an estimation of potential human exposure. Bull. Environ. Contam. Toxicol. 26 537-543. 

Obviously the optimum potential for detection of analyte X in this mobile phase is a compromise a higher potential will increase the peak height, but will also increase the baseline offset and thus the baseline noise and drift. Lower potentials will decrease the peak height, but also decrease the noise and drift. 

On the other hand, it is possible to measure even in non-linear regions of the calibration curve if the optimal measuring conditions are carefully maintained, the relative error of measurement usually does not exceed about 20%. At very low concentrations, semiquantitative procedures can be employed for example, the sample is compared with standards and the direction of the drift of the unstabilized potential indicates whether the sample concentration is higher or lower than that in the standard [147, 162). It is necessary to bear in mind that the ISE response at very low concentrations is generally slow and the potential is unstable, so that potential values read after a certain, fixed time interval must often be used instead of stabilized values. 

Richert R, Pautmeier L, Bassler H (1989) Diffusion and drift of charge-carriers in a random potential - deviation from Einstein law. Phys Rev Lett 63 547... 

There are several difficulties in the application of this technique to the analysis of sodium barrier properties of these polyimide films. First, as we have seen above, large shifts in the surface potential characteristics of MPOS structures can be associated with electronic conduction in the polyimide and charging of the polyimide-oxide interface. These shifts are not readily separable from any that might be caused by the inward drift of sodium ions. Second, the effect of the electronic charging process is to buck out the electric field in the polyimide which is needed to drive the ion drift mechanism. As seen in Figure 6, the electric field is reduced to very small values in a matter of minutes or less, particularly at the higher temperatures where ion drift would normally be measured. 

A Criterion of Thermodynamic Equilibrium between Two Phases Equality of Electrochemical Potentials. It has been stated that ihe total driving force responsible for Ihe flow or transport of a species j is the gradient d lj/dx of its electrochemical potential. However, when there is net flow or flux of any species, this means that the system is not at equilibrium. Conversely, for the system to be at equilibrium, it is essential that there be no drift of any species—hence, that there should be zero gradients for the electrochemical potentials of all the species. It follows, therefore, that, for an interface to be at equilibrium, the gradients of electrochemical potential of the various species must be zero across the phase boundary, i.e.,... 

Because the diffusive flux is enhanced by this drift of a charge under the influence of the coulomb potential [as represented in eqn. (142)], the partially reflecting boundary condition (127) has to be modified to balance the rate of reaction of encounter pairs with the rate of formation of encounter pairs [eqn. (46)]. However, the rate of reaction of ion-pairs at encounter is usually extremely fast and the Smoluchowski condition, eqn. (5), is adequate. The initial and outer boundary conditions are the same as before [eqns. (131) and (128), respectively], representing on ion-pair absent until it is formed at time t0 and a negligibly small probability of finding the ion-pair with a separation r - ... 

There is the influence of potential ignition sources. These are minimal in non-built-up areas and differ in nature between industrial and residential areas. The result is that cloud drift tends to be larger in the more open areas, but that the larger time delay before ignition decreases the turbulence level in the cloud... 

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