Sensing electrode diffusion

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... 

Gas-sensing electrodes differ from ion-selective electrodes in that no species in solution can interfere with the electrode response as only gases can diffuse through the membrane. However, it should be noted that any gas which causes a pH change in the internal electrolyte solution will affect electrode response. 

Still with an enzyme monolayer, the synthesis and current responses of a system that involves simultaneous attachment of the cosubstrate to the electrode coating are then described. The next step consists in constructing a multilayered coating constituted by successive layers of enzyme built thanks to antigen-antibody interactions. Sensing the diffusion of the cosubstrate through the film thus constructed provides evidence for spatial order and an estimate of the distances between layers. 

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. 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

The reactant gas must diffuse through the electrode structure which contains air (02, N2) and any products of reaction (CO2, N02, NO, H2O vapor, etc.). Response characteristics are dependent on electrode material, Teflon content, electrode porosity, thickness and diffusion/reaction kinetics of the reactant gas on the catalytic surface. By optimizing catalytic activity for a given reaction and controlling the potentiostatic voltage on the sensing electrode, the concentration of reactant gas can be maintained at essentially zero at the electrode/electrolyte interface. All reactant species arriving at the electrode/electrolyte interface will be readily reacted. Under these conditions, the rate of diffusion is proportional to C, where... 

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. 

Measured Species Diffusing Species Equilibria in Electrolyte Sensing Electrode... 

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... 

Apart from these, the detailed physical form of the sensor can also have an important influence on selectivity. The target gas has to diffuse through the microporous sensor material toward the sensing electrodes. The diffusion rate will depend on the mean free path of the gas molecules in relation to the diameter of the channels in the solid. 

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