Sensing electrode permeability

Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way. 

Gas-sensing electrodes. A gas-sensing electrode consists of a combination electrode that is normally used to detect a gas in its solution by immersion. The sensor contains the inner sensing element, usually a glass electrode or another ISE, and around this a layer of a 0.1 Af electrolyte, surrounded by a gas-permeable membrane. On immersion of the sensor this membrane contacts the solution of the gas which diffuses through it until an overall equilibrium is established, i.e., the partial pressure of the gas attains an equilibrium between sample solution and membrane and between membrane and sensor electrolyte. For a better understanding of the interaction between this electrolyte and the... 

The schematic diagram of a gas-sensing electrode is illustrated in Figure 16.8, that comprises of essentially a reference electrode (E), a specific-ion electrode (B), and an internal electrolyte solution (F) contained in a cylindrical plastic tube (G). One end of the plastic tubing is provided with a thin, replaceable, gas-permeable membrane that separates the internal electrolyte solution from the external solution containing gaseous analyte. However, the exact composition and specifications of this gas-permeable membrane is usually described by its respective manufacturers. It is normally made up of a thin microporous film fabricated from a hydrophobic plastic material. 

Membrane electrodes used to measure species such as NHj that are in equilibrium with the gaseous form (i.e., NH-,) in solution are known as gas-sensing electrodes. In this case, the solution to be analyzed is separated from the analyzing solution by a gas-permeable membrane. The gas in the solution to be analyzed diffuses through the membrane and changes the pH of the internal solution, which is monitored using a standard glass electrode. 

Step 2 is usually limited by the permeability of the membrane. In certain sensor designs, the membrane is eliminated to avoid this step. Step 4 refers to the diffusion of the solvated gas in the electrolyte to the electrode-electrolyte interface. Diffusion in liquids is often considerably slower than diffusion across a membrane. If the sensing electrode is flooded with electrolyte, the response is slow because the gas must diffuse through the electrolyte before reaching the reaction surface. 

CO sensor allows detection of CO in the presence of hydrocarbons and other adsorbable contaminants. The membrane Is usually chosen for Its ability to protect the sensing electrode. However, If It has low permeability to air, the sensor will have a slower response time. The electrolyte and counter electrode have also been reported as Influencing selectivity and device performance In the determination of hydrazines (5) and NO2 (9), respectively. Finally, materials of construction are typically Teflon and high-density plastics like polypropylene because such materials must be compatible with reactive gases and corrosive electrolytes. 

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. 

Figure 5.41 Selective-ion electrodes (a) glass membrane (b) liquid ion exchange (c) homogeneous solid membrane (d) heterogeneous solid membrane (e) solid membrane without reference electrode (/) gas-permeable membrane 1, sensing electrode 2, electrolyte, 2(e) ohmic contact, 2(f) gas-permeable membrane 3, membrane sur-port 4, reference electrode, 4(f) outer electrode body, 5(b) liquid ion exchanger 5(f) electrode body 6(b) reference electrode body, 6(f) electrolyte 7, liquid junction.

Gas-sensing electrodes are examples of multiple membrane sensors these contain a gas-permeable membrane separating the test solution from an internal thin electrolyte film in which an ion-selective electrode is immersed. For example, for the ammonia sensor, the pH of the recipient layer is determined by the Henderson-Hasselbach equation [Eq. (18)], derived from the chemical equilibrium between solvated ammonia and ammonium ions ... 

Ion-selective membranes attain their permselectivity from ion-exchange, dissolution, or complexation phenomena. Different types of membranes are available for the construction of ion-selective electrodes glass and other solid state rods (crystals), liquid or polymer ion ecchangers, or dissolved ionophores. Many electrodes are commercially available with selec-tivities for different ions, mainly H, alkali metal cations, heavy metal ions, and halides or pseudohalides. Also gas-sensing electrodes may be constructed from an ion-selective electrode and a gas-permeable membrane [182]. Ion selective electrodes and gas-selective electrodes... 

During the past three decades, several gas-sensing electrochemical devices have become available from commercial sources. In ihe manufacturer literature, I hose devices are often called gas-sensing "electrodes. figure 2.1-12 shows that these devices arc not, in fact, electrodes but instead are electrochemical cells made upof a specific-ion electrode and a reference electrode immersed in an internal solution retained by a thin gas-permeable membrane. Thus, gas-sensing probes is a more suitable name for ihese gas. sensor.s. 

The operation of gas-sensing electrodes can be illustrated by considering the siilfur-dioxide electrode, which responds directly to dissolved SO2. Sulfite (SOs ) and bisulfite (HSOa") are measured by acidifying the sample to convert these species to SO2. Dissolved SO2 diffuses through the gas-permeable membrane until an equilibrium is established in the internal filling solution by the reaction of SO2 with water... 

To increase the utility and selectivity of gas-sensing electrodes, another membrane (e.g., pig intestine or collagen serving as a support) with an immobilized enzyme can be placed over a hydrophobic gas-diffusion membrane. A sample is usually injected into a small mixing chamber with the electrode. The immobilized enzyme is then exposed to an analyte and the enzymatic reaction produces a pH-changing gas, which diffuses both back to the sample and towards the pH sensor (through the enzyme support and the gas-permeable membrane). 

Another interesting immobilization technique based on the adsorption of suitability modified biomolecules on to fluorocarbon surfaces has been described by Kobos et al. [179]. This method minimizes nonspecific binding which affects the detection limit of biosensors unfavourably, because the transducer surface with immobilized enzyme can be treated with neutral fluorosurfactant. An enzyme can be immobilized directly onto the gas-permeable membrane of a gas-sensor. An urea electrode where perfluoroalkylated urease was immobilized onto an ammonia gas-sensing electrode may serve as an example [180],... 

Smit and Rechnitz [299] introduced plant leaves as biocatalysts in combination with potentiometric gas-sensing electrodes. They detached the cuticle from either the upper or low er epidermal layer and fixed the remaining leaf structure at the surface of a gas-sensing potentiometric electrode where the epidermal layer contacted the sample and the gas-permeable waxy cuticle was attached to the sensor. In the case described, the substrate L-cysteine is degraded by L-cysteine desulphhy-drase, present in cucumber leaf discs, to pyruvate and ammonia which is monitored by an ammonia electrode [302]. 

Guest-induced changes in permeability through intermolecular voids between membrane molecules has also been applied by other groups for chemical sensing of various analytes. Sensing of Ca " or urea ° by electrodes coated with... 

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