Electrode potential drift

However, most of the time, the corrective term to E° is unfortunately unknown and beyond the error associated with potential measurements on an absolute (NHE) scale, mainly because of junction potential and reference electrode potential drifts. Thus in actual experiments Ej/2 and E° (or E°) are generally considered identical [94]. 

Glass pH electrodes are convenient in the lab but are not well suited to longterm environmental measurements because electrode potentials drift We compensate for drift in the lab by frequent calibration in standard buffers. To monitor pH in natural waters, a spectrophotometer measures the ratio of absorbance of light at wavelengths corresponding to the two colored forms of an indicator. This ratio provides stable measurements without calibration. The precision (repeatability) of shipboard spectrophotometric pH measurements is 0.000 4 pH units. 

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. 

This figure demonstrates that also under potentiometric conditions (- no external current flow) electrochemical net reactions occur. The EMF of the zinc-amalgam in a given Zn2 -ion solution depends on the current-voltage characteristic of other ions (in this example, Cu2 and Pb2 are interfering ions with respect to the Zn2 equilibrium potential) at the amalgam electrode. EMF drifts are thus explainable. 

Other situations may also occur that allow a simple determination of the sensitivity factor. When, for example, a sufficiently negative electrode potential forces all minority carriers to drift into the interior of the semiconductor electrode, where they recombine subject to the bulk lifetime Tfr we will see a limiting PMC signal (given a sufficiently thick electrode). Knowing the photonflux /0 (corrected for reflection), we may expect the following formula to hold ... 

Unfortunately, in the presence of detectable polyions in the solution a strong potential drift is normally observed due to the instability of the ion concentration gradients. Moreover, the main disadvantage of polyion-selective potentiometric electrodes lies in the intrinsic irreversibility of the underlying response mechanism. The target polyions eventually displace the counter-ions in the membrane phase and consequently the sensor loses its response. 

The behavior of potentiometric and pulsed galvanostatic polyion sensors can be directly compared. Figure 4.11 shows the time trace for the resulting protamine calibration curve in 0.1 M NaCl, obtained with this method (a) and with a potentiometric protamine membrane electrode (b) analogous to that described in [42, 43], Because of the effective renewal of the electrode surface between measuring pulses, the polyion response in (a) is free of any potential drift, and the signal fully returns to baseline after the calibration run. In contrast, the response of the potentiometric protamine electrode (b) exhibits very strong potential drifts. 

The first and very simple solid contact polymeric sensors were proposed in the early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-selective polymeric membrane [94], These coated wire electrodes (CWEs) had similar sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but suffered from severe potential drifts, resulting in poor reproducibility. The origin of the CWE potential instabilities is now believed to be the formation of a thin aqueous layer between membrane and metal [95], The dominating redox process in the layer is likely the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and p02 changes in a sample. Additionally, the ionic composition of this layer may vary as a function of the sample composition, leading to additional potential instabilities. 

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. 

The fluoride ion selective electrode is the most popular means of fluoride ion determination after sample destruction by any method but it does have limitations. It can be used either directly to measure the fluoride potential6 or as an end-point detector in a potentiometric titration with a lanthanum(l II) reagent as titrant.4,7 Problems can be experienced with potential drift in direct potentiometry, especially at low fluoride ion concentrations. Titration methods often yield sluggish end points unless water miscible solvents are used to decrease solubilities and increase potentia 1 breaks and sulfate and phosphate can interfere. End-point determination can be facilitated by using a computerized Gran plotting procedure.4... 

Ideally, a reference electrode should have a stable potential with respect to time. However, time stability requirements depend on the objective of the electrochemical measurement. For example, for long-term monitoring of the corrosion potential a time stable reference electrode is a must. Otherwise, it will be almost impossible to distinguish between drift in reference electrode potential and drift in corrosion state. 

It is good practice to check carefully the electrochemical potential of the embeddable reference electrode against an accurate reference (SCE or Ag/AgCl), preferably in a laboratory, before the electrode is embedded in concrete. Normally, a saturated Ca(OH)2 solution is used as a test solution. By prolonging the exposure time in the solution, the magnitude of shortterm potential drift can be detected (be aware of temperature dependence). Potential values should always be compared with data provided by the supplier of the reference electrode. It is recommended that the functional and/or calibration check procedures given by the supplier are followed. 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

In addition, because the net reaction at converts Fe to Fe, the measured potential exhibits a slow drift. Such mixed potentials are of little worth in determining equilibrium values. Many important redox couples in natural waters are not electroactive. No reversible electrode potentials are established for N03T-N0 -NH4-H2S or CH4-CO2 systems. Unfortunately, many measurements of Eh (or pe) in natural waters represent mixed potentials not amenable to quantitative interpretation. 

The open-circuit potential (OCP) recorded immediately after bringing in contact the C0-c(4x2)/Ni(lll) specimen with the 0.1 M KOH electrolyte was found to be about 0.24 V vs DHE (The electrode potential of Che DHE in 0.1 M KOH vs Reversible Hydrogen Electrode (RHE) was calculated to be 0.06 V. All of the electrode potentials in this thesis are cited vs DHE)) drifting rapidly over about 2 minutes to a fairly stable value of 0,20 V. This behavior was found to be very reproducible, yielding for... 

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