Gas sensitive electrodes

This is achieved in the electrochemical C02 sensor to be described now. The gas sensitive electrode in the emf sensors for C02 detection is based on an alkaline or alkaline earth carbonate.41"43 If Na2C03 is to be used, Na- (5" -alumina or Nasicon (Part I2) are suitable electrolytes. A thermodynamically well-defined sensor is obtained by using a mixture of a binary and a ternary oxide such as42 Na2C03, C02 Na+ -conductorj Na2M03, M02... 

Potentiometric gas sensors for the reaction products, NH3 and CO2, have also been employed. Since these measurements are based on gas diffusion through a hydrophobic membrane, no direct disturbances by sample constituents occur. As early as 1969, Guilbault et al. coupled immobilized urease with a carbon dioxide sensor. Anfalt et al. (1973) applied an ammonia gas sensitive electrode to urea assay. A major drawback of these sensors is their long response time which is due to the slow diffusion of the gases. Since it takes several additional minutes to reach a new baseline after each measurement, only a few samples can be processed per hour. Guilbault et al. (1985) therefore tried an NH3 electrode, the interned buffer of which was exchanged after each measurement (double injection electrode). This approach led to a substantial decrease of the washing time. 

Z. Gao and J. Chen, Determination of Total Nitrogen in Soils with an Ammonia Gas-Sensitive Electrode by the Flow Injection Merging Zone Method [in Chinese]. Fenxi Huaxue, 12 0984) 1096. 

This is achieved in the electrochemical CO2 sensor to be described now. The gas sensitive electrode in the emf sensors for CO2... 

Ion-selective electrodes (ISEs) can be designed using various ion-selective elements, such as glass membranes (for pH electrodes), solid membranes (for ions such as F , Cl , Br , I, SCN , or S ), liquid ion-exchange membranes (for ions such as N03, Cl , BF4, or K+), or gas-sensitive electrodes on the basis of pH electrodes, containing a hydrophobic, gas-permeable membrane (for acidic or basic gases such as NH3 or CO2). An example of the latter is depicted in Fig. 3 [40-42]. 

A strong limitation of these biosensors is their dependence on the pH conditions in the analyte solution. Thus, gas-sensitive electrodes were developed in which the pH electrode is separated from the analyte... 

Multimembrane systems are another type of potentiometric sensor based on ISEs, wherein the ISE response is modified by an additional membrane of a different function. To this group of sensors belong gas-sensitive electrodes, with a hydrophobic membrane isolating the analyte firom the complex sample medium. The chemical reaction of the gaseous analyte with a component of the solution surrounding the ISE membrane proper directly produces the species to be sensed by the electrode. In food analysis, electrodes used for sulfur dioxide or ammonia belong to this group of sensors. 

Ammonium Indophenol blue Increase pH and measure NH3 with a gas-sensitive electrode... 

Another class of bioselective membranes uses leaves or vegetable tissue in conjunction with gas-sensitive electrodes. Fbr example, cucumber leaves are fixed to a pNHa electrode for the potentiometric determination of cysteine [263]. The construction of this sensor entails... 

Fig. 30. Construction of gas-sensitive electrodes. A Membrane type, B Airgap type (A = air space, S = stirrer)...

Table 6 summarizes the most important features of the commercially available gas-sensitive electrodes. In general, the gas transport through the membrane foil (as through the air-gap) is diffusion controlled. The driving force for this diffusion is the partial pressure gradient. Thus when a gas-sensitive electrode is dipped into a solution it indicates the different partial pressures of the gases in solution. The relationship between the partial pressure of a gas Pg and the concentration of physically dissolved material [C] is desciibed by Henry s Law ... 

Isotonic solutions can be attained if the ionic strengths of the sample solution and the reaction solution are the same. Many gas-sensitive electrodes closely follow the Nemst equation between 0.1 and 10 M ... 

Golovanov, V., J. L. Solis, V. [.antto, and S. Leppavnori. Different Thick-Film Methods in Prinr-mg ot one-Electrode. Semiconductor Gas Sensors. Sensors Actuators B34 (1996), pp. 410-416. Lantto, V., and V. Golovanov. A Comparison of Conductance Behaviour between SnO> and CdS Gas-Sensitive Films, Sensors Actuators B24-25 (1995), pp. 614-618. 

Selectivity of Gas-sensing Electrode The selectivity of the gas-sensing electrode may be enhanced by making use of such an internal electrode which is particularly sensitive enough to certain species other than the H+ ion. 

The design of sensitive elements (transducers) is based on the conductive principle of response registration the output signal appears to depend on the conductive properties of the TCNQ compound as a result of its interaction with gas components. The gas-sensitive substance of the transducer has been manufactured from complex TCNQ salts with N-alkylisoquinoUnium cations ([N-C H -iso-Qn] (TCNQ)Q [26]. The substance of the gas sensitive layer is put in contact with copper or silver current feeding electrodes, put on the dielectric substrate manufactured from glass-cloth laminate. 

Gas sensing membrane Not true membrane electrodes as no current passes across the membrane. The ion being determined diffuses through the membrane into an electrochemical cell. The consequent change in the chemistry of the cell is monitored by an ion sensitive electrode. nh3, so2, co2... 

It should be noted that some gas-sensitive sensors are amperometric, as in the Clark electrode for oxygen (Section 14.3). 

It should be emphasized that a noble gas environment containing atmospheric contaminants at the ppm level cannot be considered inert for sensitive electrodes such as active metals. The inertness of a glove box atmosphere can be sufficient for preparation and work with nonaqueous solutions at low H20, 02, C02 and N2 contamination levels. Work with active metal electrodes, especially when spectroelectrochemical studies are important, requires special electrode surface preparation, as described later. As a rule, a glove box may be sufficient for maintaining a low contamination level in bulk materials, but not on active surfaces. 

Bovine serum albumin (BSA) and cyclic AMP (cAMP) are determined by a competitive binding enzyme immunoassay (315). With urease as label, an ammonia gas-sensing electrode is used to measure the amount of urease-labeled antigen bound to a double-antibody solid phase by continuously measuring the rate of ammonia produced from urea as substrate. The method yields accurate and sensitive assays for proteins (BSA less than 10 ng/mL) and antigens (cAMP less than 10 nM), with fairly good selectivity over cGMP, AMP, and GMP. 

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