Selectivity of conventional gas sensors, con t:

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Adele Maxwell
5 years ago
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1 Selectivity of conventional gas sensors, con t: for ammonia sensor---volatile amines that are strong bases; methylamine, ethylamine, etc.--- R-NH 2 H 2 O ----> R-NH 3 OH - if species are stronger bases than NH 3 --then you can observe larger EMF response to these amines-- Get conversion of internal thin film layer of electrolyte from ammonium chloride, to alkylammonium chloride --usually observe super-nernstian region during this conversion process --if you put alkylammonium chloride electrolyte as inner solution to start, then Nernstian response to volatile amine! E cell R-NH 2 NH 3 log [ ]

2 For sensors---so 2 and NO 2 are significant interferences (stronger acids than ) Also, neutral organic acids can interfere---when outer gas permeable membrane is a silicone rubber type material. Response times---slower than ISEs, and other potentiometric sensors---must reach equlibrium between thin film of electrolyte and sample--- 1 min. at high concentration of dissolved gas min at very low concentraitons Polymer Membrane Electrode Based Gas Sensors -use of inner ISEs that are polymer membrane electrodes to measure ionic form of analyte gas -- e.g., NH 4, HCO 3-, HSO 3-, NO best example is NH 3 sensor using neutral carrier based NH 4 electrode as inner transducer -also novel differential designs---two polymer membrane ISEs---best example: sensor

3 inner electrolyte of ISE buffer electrolyte NH 3 H <-----> NH 4 NH 4 - selective membrane--made with nonactin as neutral carrier NH 3 ph = pk a log ([NH 3 ]/NH 4 ]) fix ph of inner solution with strong buffer; therefore, thin film of electrolyte effectively traps ammonia gas as ammonium ions! always have favorable flux (J = (dc/dx)) (dx = gas permeable membrane thickness)

4 Nonactin-based ammonium sensor has K pot NH4, K =0.1 --not very selective over K --but this does not matter since K cannot permeate the outer gas permeable membrane; Therefore, to optimize detection limits always use inner buffer the does no have K and/or other cations for which the inner membrane electrode does not display high selectivity over!---otherwise, detection limits will be less than optimal! -Because of the buffer trap mechanism---the sensors can have lower detection limits compared to sensors with inner ph electrode as transducer! (e.g., at ph 7.3 inner---for 10-7 M NH 3(g) in sample phase---inner buffer film at equilibrium will have 10-5 M NH 4 ) However--because of trapping effect---gas sensor will only display Nernstian response toward gas provided that ph of inner film buffer does not change---once buffer capacity exceded, response becomes sub-nernstian!

5 0.1 M Tris-HCl, ph 7.5 E cell 0.01 M Tris-HCl, ph 7.5 log [NH 3 ] NH 3 BH <-----> NH 4 B ph film = pka (buffer) log ([B]/[BH]) What about selectivity? only positive interferences from volatile amines if these species --in protonated form--yield significant response from the inner nonactin-based membrane -If binding constant of protonated amine were high with ionophore--then such gases would interfere; fortunately, this is not the case---

6 For polymer membrane electrode with nonactin (and tetrphenylborate counteranion sites), K pot NH4, RNH3 = 10-4 (what must this mean?) However, at high concentration of volatile amines-- you can get negative interference---since such levels will also change ph in thin film within sensor decreasing the equil.level of NH 4 ions formed for a given level of NH 3 gas in the sample phase. Therefore, must be concerned about cumulative effect of both volatile amine and analyte ammonia levels on the ph of the inner buffer electrolyte!! -- Sensor of this type also developed for based on inner carbonate electrode, with improved select. over volatile acidic gases Differential design: Ag/AgCl, NaCl, buffer /polymer ph mem./ sample/ polymer ph mem. / NaHCO 3, NaCl, AgCl/Ag

7 sample buffered internal soln. layer indentical polymer ph sensitive membranes (using TDDA as ionophore) bicarbonate internal --is capable of diffusing through both ph sensing membranes, but only changes the ph behind the membrane in the sensor that has thin film of bicarbonate electrolyte as inner electrolyte. The outer surface of the membranes both respond to the ph of the sample, but this potentiometric response cancels! Note--equation for E cell and Vetter diagram-- --Will derive on blackboard!!

8 DIFFERENTIAL PCO 2 SENSOR Ecell = K log P mv Ag/AgCl bicarbonate soln. H H H HCO HCO 3 3 H polymer ph sensitive membranes CO 2 H Ag/AgCl buffered soln. H H

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