Electrochemical permeable membrane

Craig, J.B. Meares, P. Webster, J. "Physico-chemical studies on semi-permeable membranes" Final report on EMR 1799, United Kingdom Atomic Energy Authority Harwell, 1968. Demisch, H.-U. Pusch, W. J. Electrochem. Soc., 1976, 123, 370. 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

Both cells are equipped with gas-permeable membranes that allow for nearly specific gas transfer from the sample into a thin indicator layer (buffer) of a cell that is in contact with the electrochemical sensing electrode. For the C02-GSS, the indicator layer is a flat pH glass membrane, while for the 02-CSS it is a cathode made of platinum or gold. 

For long term operation other approaches have to be used. To protect the Pt working electrodes against fouling and to prevent erroneous reading due to electrochemical interference, an electropolymerised semi-permeable membrane can be utilized [76]. 

Gas sensors usually incorporate a conventional ion-selective electrode surrounded by a thin film of an intermediate electrolyte solution and enclosed by a gas-permeable membrane. An internal reference electrode is usually included, so that the sensor represents a complete electrochemical cell. The gas (of interest) in the sample solution diffuses through the membrane and comes to equilibrium with the internal electrolyte solution. In the internal compartment, between the membrane and the ion-selective electrode, the gas undergoes a chemical reaction, consuming or forming an ion to be detected by the ion-selective electrode. (Protonation equilibria in conjunction with a pH electrode are most common.) Since the local activity of this ion is proportional to the amount of gas dissolved in the sample, the electrode response is directly related to the concentration of the gas in the sample. The response is usually linear over a range of typically four orders of magnitude the upper limit is determined by the concentration of the inner electrolyte solution. The permeable membrane is the key to the electrode s gas selectivity. Two types of polymeric material, microporous and homogeneous, are used to form the... 

When two electrolyte solutions at different concentrations are separated by an ion--permeable membrane, a potential difference is generally established between the two solutions. This potential difference, known as membrane potential, plays an important role in electrochemical phenomena observed in various biomembrane systems. In the stationary state, the membrane potential arises from both the diffusion potential [1,2] and the membrane boundary potential [3-6]. To calculate the membrane potential, one must simultaneously solve the Nernst-Planck equation and the Poisson equation. Analytic formulas for the membrane potential can be derived only if the electric held within the membrane is assumed to be constant [1,2]. In this chapter, we remove this constant held assumption and numerically solve the above-mentioned nonlinear equations to calculate the membrane potential [7]. 

All of the various types of membrane used in ISEs operate by incorporating the ion to be analysed into the membrane, with the accompanying estabhsh-ment of a membrane potential. The scope of electrochemical analysis has been extended to measuring gases and non-ionic compounds by combining ISEs with gas-permeable membranes, enzymes, and even immobilized bacteria or tissues. 

Davies AT, Genders JD, and Fletcher D. Ion Permeable Membranes. The Electrochemical Consultancy, Romsey, England, 1997. 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

Membranes, selective to either cation or anion transport, are employed in many of the electrochemical treatment systems already discussed in this chapter. For example, in the cathodic recovery of metals from waste streams, a cation-permeable membrane can prevent the migration of anions such as Cl- from the catholyte to anolyte, where oxidation to CI2 can occur with... 

Figure 21-17 illustrates the essential features of a potentiometric gas-sensing probe, which consists of a tube containing a reference electrode, a selective ion electrode, and an electrolyte solution. A thin, replaceable, gas-permeable membrane attached to one end of the tube serves as a barrier between the internal and analyte solutions. As can be seen from Figure 21-17, this device is a complete electrochemical cell and is more properly referred to as a probe rather than an electrode, a term that is frequently encountered in advertisements by instrument manufacturers. Gas-sensing probes have found widespread use in the determination of dissolved gases in water and other solvents. 

A solution of silver nitrate in 8-molar nitric acid is electrolyzed to produce the Ag(II) cations at the anode of a commercially available electrochemical cell. A semi-permeable membrane separates the anode and the cathode compartments of the cell to prevent mixing of the anolyte and catholyte solutions but allowing the passage of cations and water across the membrane. 

Because water is ubiquitous both in the sensing environment and inside many sensors (especially electrochemical sensors), the hydrophobic or hydrophilic nature of a polymer used in a sensor is often crucial. For example, a polymer that is to be used as a hydrogel is by definition hydrophilic. On the other hand, gas-permeable membranes are often made of hydrophobic polymers to prevent passage of water through the membrane. These conventions are not always the case, however. An electrolyte for a sensor operating with non-aqueous electrochemistry may be less hydrophilic. Similarly, an in situ sensor to analyse polar degradation products in motor oil may use a hydrophilic membrane to allow passage of the analyte into the aqueous electrolyte from the non-polar hydrocarbon sample [14]. 

The principle of operation of a multicompartmented electrodialysis unit (11) is shown in Fig. 1.11. The cation and anion permeable membranes carry a fixed charge thus they prevent the migration of species of like charge. In a commercial version of Fig. 1.11, there would be several hundred rather than three compartments, multicompartmentalization being required to achieve electric power economics, since electrochemical reactions take place at the electrodes. 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

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