Potentiometry (Chapter 23) Reference electrodes: ·reversible ·little hysteresis follows Nernst equation stable potential with time Saturated Calomel Electrode (SCE) HglHg2CI2(sat'd),KCI(a=x M)I. Inner tube containing a Saturated KCl Small hole Fritted disk Ground (a) (b) (Fig23-1) CEM 333 page 11.1
Potentiometry (Chapter 23) Reference electrodes: • reversible • little hysteresis • follows Nernst equation • stable potential with time Saturated Calomel Electrode (SCE): Hg|Hg2Cl 2 (sat'd), KCl(a = x M)||. (Fig 23-1) CEM 333 page 11.1
Half-cell for Calomel Electrode: Hg2Cl2(s)+2e>2Hg(1)+2CI Position of equilibrium affected by ac-from KCI so Eo depends on aci- Most common saturated calomel electrode SCE ([CI-1~4.5 M) Silver/Silver Chloride Electrode: Similar construction to calomel Ag wire coated with AgCl solution of KCl sat'd with AgCl Ag|AgCl(sat'd).KCl(a =x M). AgCI(s)+e←→Ag(s)+CI Again depends on aci-,but commonly sat'd (~3.5 M) CEM 333 page 11.2
Half-cell for Calomel Electrode: Hg2Cl2 (s) + 2e- « 2Hg(l) + 2Cl - Position of equilibrium affected by aCl- from KCl so E0 depends on aClMost common saturated calomel electrode SCE ([Cl-]~4.5 M) Silver/Silver Chloride Electrode: Similar construction to calomel • Ag wire coated with AgCl • solution of KCl sat'd with AgCl Ag|AgCl(sat'd),KCl(a = x M)||. AgCl(s) + e - « Ag(s) + ClAgain depends on aCl-, but commonly sat'd (~3.5 M) CEM 333 page 11.2
Potential vs.SHE↓ TABLE 23-1 Potentials of Reference Electrodes in Aqueous Solutions Electrode Potential (V),vs.SHE 0,1M 3.5M Saturated 3.5Mb Saturated Calomela Calomel Calomel Ag/AgCl Ag/AgCl 10 0.256 0.215 0.214 12 0.3362 0.2528 15 0,362 0,254 0.2511 0.212 0.209 汤 0.3359 0.252 0.2479 0.208 0.204 25 0.3356 0.250 0.2444 0.205 0.199 30 0.3351 0,248 0.2411 0.201 0.194 36 0.3344 0.246 0.2376 0.197 0.189 38 Q.3338 0.2355 40 0.244 0.193 0.184 0 ded I.M.Kole Which one? Ag/AgCl better for uncontrolled temperature (lower T coefficient) Ag reacts with more ions Precautions in Use: Level of liquid inside reference electrode above analyte level to minimize contamination Plugging problematic if ion reacts with solution to make solid (e.g.AgCl in Cl-determination) CEM 333 page 11.3
Potential vs. SHE ¯ Which one? • Ag/AgCl better for uncontrolled temperature (lower T coefficient) • Ag reacts with more ions Precautions in Use: • Level of liquid inside reference electrode above analyte level to minimize contamination • Plugging problematic if ion reacts with solution to make solid (e.g. AgCl in Cl- determination) CEM 333 page 11.3
Measuring Concentration using Electrodes: Indicator Electrodes for Ions: Electrode used with reference electrode to measure potential of unknown solution potential proportional to ion activity specific (one ion)or selective (several ions) Ecell =Eindicator-Ereference Two general types-metallic and membrane electrodes CEM 333 page 11.4
Measuring Concentration using Electrodes: Indicator Electrodes for Ions: Electrode used with reference electrode to measure potential of unknown solution • potential proportional to ion activity • specific (one ion) or selective (several ions) Ecell = Eindicator - Ereference Two general types - metallic and membrane electrodes CEM 333 page 11.4
Metallic Indicator Electrodes: Electrodes of the first kind -respond directly to changing activity of electrode ion Example:Copper indicator electrode Cu2++2e->Cu(s) acu(s)=- 1 aCu2+aCu2+ Eind=E0_RT nF -log Keq Eind =E0 Cu/Cu2+ 0.05921og,1 2 aCu2+ =0.337V-0.296pCu BUT other ions can be reduced at Cu surface -those with higher +ve Eo(better oxidizing agents than Cu) Ag,Hg,Pd. In general,electrodes of first kind: ·simple ·not very selective some metals easily oxidized (deaerated solutions) some metals(Zn,Cd)dissolve in acidic solutions CEM 333 page 11.5
Metallic Indicator Electrodes: Electrodes of the first kind - respond directly to changing activity of electrode ion Example: Copper indicator electrode Cu2 + + 2e - « Cu(s) Keq = aCu(s) a Cu2+ = 1 a Cu2+ Eind = E 0 - RT nF log Keq Eind = E Cu/ Cu2+ 0 - 0.0592 2 log 1 a Cu2+ = 0.337 V - 0.296pCu BUT other ions can be reduced at Cu surface - those with higher +ve E0 (better oxidizing agents than Cu) Ag, Hg, Pd. In general, electrodes of first kind: • simple • not very selective • some metals easily oxidized (deaerated solutions) • some metals (Zn, Cd) dissolve in acidic solutions CEM 333 page 11.5