Electrochemical filter

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

We will begin with a description of electrochemical sensors or more specifically composition sensors based on electrochemical principles (i.e., we refer to an electrochemical detection of composition). Another group of applications refers to devices in which the transference of mass and charge is used primarily to change composition or produce chemicals (electrochemical pumps and electrochemical reactors, or electrochemical filters) we will term such devices composition actors. At the end we will discuss energy conversion and storage devices (which we do not subsume under the term composition actors as here the energy aspect is to the fore). 

A transfer of this definition of the cross-flow term to the chemical reactor field implies that the so-called cell reactor [26], consisting of thin, porous catalyst plates mounted in a rack like a filter press, should also be of interest to describe here. The same may apply to the great number of different electrochemical filter-press cell reactors [27]. It may be noted that the cell reactor principle is, however, not valid for the so-called parallel-passage reactor [28,29]. In this case the same fluid flows on both sides of the catalyst plates without any need for communication and exchange between the fluids through the plates. The advantage of this reactor is its being dust-proof, since dust present... 

The three kinds of reactors already described in this section are all traditional cross-flow reactors with permeable plates or membranes. The electrochemical filter-press cell reactors used, e.g., for electrosynthesis, are equipped with cation-selective membranes to prevent mixing of the anolyte and the catholyte. These cell reactors are therefore good examples of the extended type of cross-flow reactors according to the definition transferred from the filtration field. The application of the electrochemical filter-press cell reactor technique... 

It should also be observed that the catalytic cell reactor (described in Section II.D), which is intended to be an alternative reactor to trickle beds for liquid-phase hydrogenations, is a further-developed electrochemical filter-press cell based on the firm Electro Cell AB s concept with respect to the preparation of thin, porous electrodes. 

Lakshmanan, B., W. Huang, and W.J. Weidner. 2002. Electrochemical filtering of CO from fuel-cell reformate. Electrochem. Solid State Lett. 5 A267-A270. 

For the measurement of lower levels of CO2 it is necessary to remove the much larger current from the reduction of ambient oxygen. We have shown that one can use a metallised membrane as an efficient electrochemical filter [8]. The principle of the method is shown in Figure 14. Our metallized membranes were homemade by the method of Bruckenstein[12] rather than by the earlier technique pioneered by Bergmann [13]. In the Bruckenstein method gold resinate solution is painted into a porous teflon membrane and fired with a hot air gun. 

Fig. 14. The use of a metallized membrane electrode as an electrochemical filter to remove O2 so that smaller concentrations of CO2 can be determined.

Okawa, Y., H. Kobayashi, and T. Ohno. 1998. Direct and simultaneous determination of ascorbic acid and glucose in soft drinks with electrochemical filter/biosensor FIA system. Bunseki Kagaku 47 443-445. 

Filters have a time constant r = R x C which increases the damping of the measuring instrument. The time constant depends on the required attenuation and the interfering frequency, but not on the internal resistance of the measuring instrument. The time constants of the shielding filter are in the same range as those of the electrochemical polarization, so that errors in the off potential are increased. Since the time constants of attenuation filters connected in tandem are added, but the attenuation factors are multiplied, it is better to have several small filters connected in series rather than one large filter. 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

A significant step for the commercialization of bipolar electrochemical promotion units has been made recently by Christensen, Larsen and coworkers at Dinex Filter Technology A/S in Denmark.18 20 The goal is the development of an efficient catalyst system for the aftertreatment of Diesel exhausts. This is one of the most challenging problems of current catalytic research. 

J. Dinesen, S.S. Nissen, and H. Christensen, Electrochemical Diesel Particulate Filter, SAE paper 980547, Diesel Exhaust Aftertreatment (SP-1313) 197-201 (1998). 

Dinex, electrochemically promoted filter, 525 Dipole moment... 

Verheijen, J., Bongaerts, P., and Verspui, G., Low Pressure Chemical Vapour Deposition of Temperature Resistance Colour Filters, Proc. 10th. Int. Conf. on CVD, (G. Cullen, ed.), pp. 977-981, Electrochem. Soc., Pennington, NJ 08534 (1987)... 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

T.F. Brown and S.D. Brown, Resolution of overlapped electrochemical peaks with the use of the Kalman filter. Anal. Chem., 53 (1981) 1410-1417. 

Separation and detection methods The common methods used to separate the Cr(III)/(VI) species are solvent extraction, chromatography and coprecipitation. In case of Cr(VI) from welding fumes trapped on a filter, a suitable leaching of the Cr(VI) from the sample matrix is needed, without reducing the Cr(VI) species. The most used detection methods for chromium are graphite furnace AAS, chemiluminescence, electrochemical methods, ICP-MS, thermal ionization isotope dilution mass spectrometry and spectrophotometry (Vercoutere and Cornelis 1995)- The separation of the two species is the most delicate part of the procedure. 

Thompson, M. Lennox, R. B. McClelland, R. A., Structure and electrochemical properties of microfiltration filter-lipid membrane systems, Anal.Chem. 54, 76-81 (1982). 

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