a Ag but activity of silver ion in the membrane phase (σ)=1 Therefore, overall ISE cells containing such a membrane has and

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1 Ag 2 S -based membranes behave as ion-exchangers in which Ag + conduct charge through membranes Membrane has no diffusion potential within---only two phase boundary potentials at each side of the pressed pellet. Therefore E mem = E mem = z i log a sam + a Ag Ag + int. a Ag + a Ag + but activity of silver ion in the membrane phase (σ)=1 Therefore, overall ISE cells containing such a membrane has and E cell = K log (a Ag +) sam However, such an electrode will also respond to sulfide ion activity--since: : Ag 2 S (s) -----> 2 Ag + + S = Ksp = (a Ag ) 2 a S

2 a Ag + = Ksp a S 2 = Ksp a S 2 1 / 2 E cell = K log Ksp a S 2 E cell = K ' log(a S 2 ) 1 / 2 To measure sulfide---must operate electrode in solution with high ph---ph or so; so S -2 exists at reasonable concentrations-- (S -2 + H + ---> HS - + H > H 2 S ) Detection limits determined by Ksp of Ag 2 S (s) 1.6 x therefore, at equilibrium, there is approx M Ag + in solution in equilibrium with membrane

3 Selectivity: Depends on Ksp of interfering ions with Ag + and S Ag 2 S (s) <-----> Ag + + S -2 S -2 + Hg +2 <------> HgS (s) increasing activity of Hg +2, increases activity of Ag + at surface of electrode due to lower Ksp of HgS! pot K Ag +, M + = Ksp Ag 2 S Ksp MS If Ksp MS << Ksp Ag2S ; then M will be significant interferent species for measurement of Ag + MIXED CRYSTAL Type ISEs e.g., membrane made of pressed pellet of Ag 2 S/CuS ---> electrode that can measure Cu +2 Ag 2 S (s) < > 2 Ag + + S -2 S -2 + Cu +2 <------> CuS (s)

4 -sulfide ion activity at interface of membrane controlled by solubility product of CuS-- -in turn, Ag + activity controlled by this S -2 activity. therefore: increasing Cu +2 in sample, decreases S -2 activity at electrode surface, thereby increasing Ag + activity---which is what membrane detects! For a less soluble M 2 S z, then M z+ will be interferent! pot K i, j = Ksp IS Ksp MS for low level calibrations of these M 2 S z /Ag 2 S based membrane electrodes for metal ions, must use metal ion buffers----since at conc. < µm, loss of metal ions due to adsorption on walls of vessel, becomes problem. Can also make mixed crystal type membrane elect. for measurement of given anions: Ag 2 S/AgX X = SCN -, Cl -, Br -, I -, CN - same principal----silver ion activity will be controlled by greater solubility of AgX; thus, silver ion levels will change when X is added to sample phase!

5 All of these Ag 2 S -based ISEs suffer severe limits when used in biological samples---owing to the very strong interaction of Ag + with thiol groups of cysteine residues in proteins (e.g., indeed, such sensors respond to protein levels when they are present in samples!) LaF 3 -single crystal membranes for Fluoride electrode NaF, NaCl soln LaF 3 crystal doped with Europium to improve conductivity Fluoride anion is mobile in crystal lattice--due to defects---ala the Ag 2 S crystalline membranes! Fluoride ion mobility was proven by Durst and Rosscoulometricallly generated F - through mem. with 99% efficiency.

6 equilibria at interface of crystal: LaF 3(m) < > La +3 (sam) + 3 F - (sam) Ksp = (a La )(a F ) 3 LaF 3(m) < > LaF 2 + (m) + F - equilibrium that generates phase boundary potential acts as fixed site anion exchanger with selectivity for fluoride anion Selectivity----very high for Fluoride--- K pot F,Cl = 10-4 K pot F,Br = 10-3 Again---based on Ksp of various LaA salts! OH - is major interference---la(oh) 3 --very insoluble---at high ph--looks like lots of F -

7 K pot F,OH = 10-1 in constant 10-3 M F - H + + F > HF E cell La OH - ---> La(OH) 3(s) 4 7 ph Potentiometric Gas Sensors: -use membrane electrodes behind gas permeable membrane -measure equilibrium activities of ions formed (by E cell ) within thin film of electrolyte between ISE mem. and gas permeable membrane as a function key point: called sensor because in conventional designs, the entire electrochemical cell is in one soln. behind a gas permeable membrane (only mem in contact with sample)---see Fig. on next page: 10

8 E cell reference electrodes-- (Ag/AgCl) internal electrolyte of ISE internal electrolyte of gas sensor Ion-selective membrane--(glass polymeric, etc.) gas permeable membrane (teflon,silicone rubber) thin film of electrolyte in which change in activity of ion occurs as gas reaches equilibrium across gas permeable membrane If internal ISE is ph electrode---then sensors for following gases can be made--- NH 3(g) + H 2 O <------> NH 4 + OH - SO 2(g) + H 2 O < > HSO H + CO 2(g) + H 2 O < > HCO H + 2NO 2(g) + H 2 O <------> NO NO H +

9 Theory of gas sensors---must make following assumptions: -composition of internal electrolyte is not affected by sample (concentration of ions in internal does not change due to osmotic effects, etc.) -Thin electrolyte film at the sensor s tip is an essentially isolated region from bulk internal electrolyte---only in electrolytic contact on edges. You don t want ph to change within bulk of internal electrolyte of gas sensor. - Henry s constant for gas is same for sample and internal solution and equilibrium, and hence constant E cell is achieved when the partial pressure of analyte gas is equal on both sides of gas perm. membrane. (P g = H[gas]) First potentiometric gas sensor---(for solution measurements)---severinghaus and Bradley-1957 Used only NaCl as internal electrolyte for CO 2 sensor- for blood P CO2 measurements

10 CO 2 + H 2 O <------> HCO H + K a = (a H+ ) (a HCO3 -) / P CO2 and a H + = K a P CO2 / a - HCO3 but E cell = K log (a H + (film) ) When sensor at equilibrium---p CO2(sam) = P CO2(film) therefore: E cell = K log (K a P CO2 / a HCO3 -) When no bicarbonate salt is incorporated into the internal solution---then a HCO3 - changes as the P CO2 increases---so numerator and denominator change- ====yields measured slope of < 59 mv/decade change in P CO2 However---if we add NaHCO 3 at 0.1 M in internal solution (along with NaCl---so we have Cl - for Ag/AgCl reference) ---Then both K a and a HCO3 - are constant---then E cell = K log P CO2(film)

11 To make ammonia gas sensor----use NH 4 Cl as internal electrolyte---along with same ph sensor NH 3 + H 2 O <------> NH OH - pk b = 4.7 NH 4 + < > H + + NH 3 pka = 9.3 therefore --K a = P NH3 a H +/ a NH4 + solve for a H + and substitute into E cell equation for response of inner ph electrode: E cell = K log (K a a NH4 +/ P NH3 ) therefore if NH 4 Cl is used as internal solution and NH 4 + concentration is high enough---then E cell = K log P NH3 since P NH3 = H[NH 3 ] then : E cell = K log [NH 3 ] sam

12 Detection limits of such gas sensors are generally determined by the equilibrium concentration of gas that originates from the internal solution electrolyte-- The higher the concentration of electrolyte--(bicarbonate for CO2 sensor, ammonium for NH3 sensor) the higher the detection limits----however, to achieve linear response of E cell with log [gas] to high levels of gas---then you need high electrolyte concentration. Also depends exchange between bulk internal electrolyte solution and the solution within the film-- Hence, detection limits will also depend on geometry- (if no exchange---then loss of gas from thin film can occur when soaking in solutions without the analyte gas---and this will improve the detection limits). Selectivity: only species which can permeate the outer gas permeable membrane and change the ph within the internal electrolyte film, are interferences.