Fouling of electrodes

In applications of voltammetry to biological samples, it is often the sample rather than the sensitive voltammetric analyzer that is the limiting factor. Getting the sample into a form that can take full advantage of the instrument capability may be the hardest part of the analysis. For this reason, the sample is usually treated prior to analysis. Such treatment releases the trace metals bound to sample components, and minimizes fouling of electrode (by adsorption of certain sample components) or background currents (from other electroactive constituents). The precision and bias of the data obtained by voltammetric analysis of biological samples will be more dependent on how well the sample is decomposed than with many other analytical techniques (e.g., atomic absorption spectroscopy which relies on atomization of the metal from the solution). 

As a consequence of the short time involved in gast scan experiments, total electrolysis is much smaller at microelectrodes, and fouling of electrodes by electrolysis products is minimized. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Fouling of the pH sensor may occur in solutions containing surface-active constituents that coat the electrode surface and may result in sluggish response and drift of the pH reading. Prolonged measurements in blood, sludges, and various industrial process materials and wastes can cause such drift. Therefore, it is necessary to clean the membrane mechanically or chemically at intervals that are consistent with the magnitude of the effect and the precision of the results requited. 

Fouling of conductivity electrodes and other measurement devices... 

The transfer of an automated analysis from the laboratory to the plant will often require special precautions for instance, while turbidities in a process stream can cause a loss of selective absorptivity in a spectrophotometric measurement, in potentiometric methods fouling of the electrodes, potential leakage in metal containers or tubing and loss of signal in remote control may occur (see later). 

An ultrasonic transducer is installed on the pH probe mount in the acid-alkali treatment module [T-31], This prevents fouling of the electrodes and provides a more closely controlled pH in the effluent discharged to the precipitator. 

Fouling of the reference electrode or the reference side of a combination electrode is a common problem in soil pH measurements. Fouling can be caused by salts, organic matter, and clay. Each electrode manufacturer will provide specific cleaning procedures that help to keep electrodes functioning properly however, in many cases, no amount of cleaning is effective and the electrodes need to be replaced. 

Biosensors fabricated on the Nafion and polyion-modified palladium strips are reported by C.-J. Yuan [193], They found that Nafion membrane is capable of eliminating the electrochemical interferences of oxidative species (ascorbic acid and uric acid) on the enzyme electrode. Furthermore, it can restricting the oxidized anionic interferent to adhere on its surface, thereby the fouling of the electrode was avoided. Notably, the stability of the proposed PVA-SbQ/GOD planar electrode is superior to the most commercially available membrane-covered electrodes which have a use life of about ten days only. Compared to the conventional three-dimensional electrodes the proposed planar electrode exhibits a similar... 

Another method used for nitrate determination on dried and milled herbage employs the nitrate selective electrode. One of the first published methods was that of Paul and Carlson (1968). Other anions, especially chloride, can interfere. These authors removed chloride with silver resin, but Barker ef al. (1971) omitted the resin because it tended to foul the electrode and cause excessive drift. Normally the Cl N03 ratio is so low as not to interfere, but saline precipitation from coastal plots could affect this. The method was further modified to allow storage of extracts for up to 64 h by adding a preservative of phenyl-mercuric acetate and dioxane, both very toxic (Baker and Smith, 1969). This paper mentions the need to change the electrode s membrane, filling solution and liquid ion exchanger every 2 months to minimize chloride interference. It is easy to overlook electrode maintenance between batches of nitrate analyses, and this can lead to errors and sluggish performance. 

However, the oxidation of NAD(P)H to NAD(P)+ at practical rates on most electrode materials proceeds only at high overpotentials [182] and often fouling of the electrode surface has been observed. This situation induces the search for suitable mediators or electrode modification processes to accelerate the highly irreversible oxidation of NAD(P)H. The stability of the reaction products in the presence of each other is also a necessary condition for usefulness. 

Sources of Error. Several common causes nl measurement problems are electrode interferences and/or fouling of the pH sensor, sample matrix effects, reference electrode instability, and improper calibration of the measurement system. 

In the RNPV technique (see Ref. 68 and references therein), which is a derivative of LAPV, the initial potential is set to a value where the electrode reaction of interest occurs, causing an accumulation of the reaction products. Next, an analysis pulse similar to that used in LAPV is applied so as to oxidize or reduce the reaction products as appropriate [68]. The RNPV technique is well suited for studying reactions complicated by adsorption or those producing products that foul the electrode surface, because the potential program can be adjusted so as to elec-... 

Most aliphatic amines and alcohols are considered to be nonelectroactive. The reason for this is that the product of the oxidation adsorbs to the electrode surface, fouling the electrode. Therefore, most reactions of these compounds at noble metal electrodes have been transient and not amenable to direct amperometric detection. In voltammetry experiments, electrodes are cleaned between experiments by electrochemical or chemical treatment to restore the electrode response. 

Electrodes which are selective to ions other than H are commonly available. These are called specific ion or plon sensors and are listed in Table 6.12. Major difficulties with pH and plon sensors are non-linearity, noise and sensitivity. Fouling of the probe can also be troublesome and many industrial instruments employing these principles are provided with the means of cleaning electrode surfaces ultrasonicaily. 

Commonly, the transduction mechanisms characteristic for electrochemical MIP chemosensors can fall into two categories, as shown in Scheme 4. For some transductions, like conductometric, impedimetric or potentiometric, sole presence of the target analyte in the MIP film is sufficient to produce an appreciable detection signal. However, this presence is insufficient for application of other techniques like chronoamperometry or voltammetry. As mentioned above in Sects. 4.4.4 and 4.4.5, proper electrode reaction is necessary in the latter techniques to generate the detection signal. Moreover, in the case of chronoamperometry or voltammetry, electrogenerated products may often foul the electrode surface. That is, these... 

However, problems associated with the reproducibility between electrodes derived from the screen-printing process and the partial electrode fouling have compromised the sensitivity of the biosensors. Work is in progress to improve both the reproducibility and the limits of detection by the use of new types of electrodes. The toxin overestimation observed with the amperometric biosensor, in the case of the microcystin analysis, suggests the use in parallel to other analytical techniques in order to minimise the risk of false-positive results. Nevertheless, the electrochemical strategy is appropriate to discriminate between toxic/non-toxic samples. 

It should also be pointed out that in the case of an in vivo measurement, the microdialysis probe will be able to recover not only glucose but also many other biological compounds with low molecular weight from the subcutaneous tissue. The electrochemical interferents are greatly reduced by the use of PB at a low applied potential. However, other biological compounds could negatively affect the stability of the enzymatic membrane. Also, it is possible to have a sort of passivation or fouling of the electrode surface due to the absorption of... 

M. Kyrolainen, H. Hakanson, B. Mattiasson and P. Vadgama, Minimal fouling enzyme electrode for continuous flow measurement of whole blood lactate, Biosens. Bioelectron., 11 (1997) 1073-1081. 

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