Characteristics of Gas-Sensitive Electrodes

The proportionality constant k reflects the physical solubility of the corresponding material in the solution. It is dependent upon the composition of the solution. Because k should be the same for the calibration solution and the sample solution, the calibration solution should resemble the sample solution as closely as possible. In cases where the composition of the sample solution is in doubt, it should be diluted and remeasured. According to the same principle, water vapor transport can also occur if the osmotic pressures of the sample and reaction solutions differ. This process can dilute the concentrations in the thin electrolyte film and cause the measured values to drift. 

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  

Caution is required when working in strongly basic solutions (pH 13), because upon long exposure to such media the membrane may swell. This can decrease the size of the pores necessary for gas transport and influence the diffusion. The same applies to working in non-aqueous solvents. In order to achieve the full sensitivity of these gas sensors, it is recommended that one work with sample solutions within the optimal pH ranges listed in Table 6. In the absence of ammonia, the NHa-sensor can be used to indicate volatile amines (for example, to monitor the air in the vicinity of a plant manufacturing such a substance). To free NH3 bound in metal complexes (copper or zinc tetrammine complexes, for example) the addition of 15 g/1 EDTA is recommended [168]. 

The optimal pH value for SO2 and NO2 is below 0.7, and can be established with H2SO4 or HCIO4. If one is forced to use pH s at which the equilibrium does not lie 

Indicated Gas Indicator- electrode Membrane Reaction solution [M] ) Detection limit [M] optimum pH-value Interferences Manufacturer see also Appendix A.8 

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