《仪器分析 Instrumental Analysis》课程教学资源（教材讲义）11 Potentiometry

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 aCl￾Most 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) + Cl￾Again 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

Electrodes of the second kind-respond to changes in ion activity through formation of complex Example:Silver works as halide indicator electrode if coated with silver halide Silver wire in KCl (sat'd)forms AgCl layer on surface AgCl(s)+e>Ag(s)+CI-E=+0.222 V Eind=+0.222-0.0592 n log aci- =+0.222+0.0592pCl Electrodes of the third kind-respond to changes of different ion than metal electrode CEM 333 page 11.6

• Electrodes of the second kind - respond to changes in ion activity through formation of complex Example: Silver works as halide indicator electrode if coated with silver halide Silver wire in KCl (sat'd) forms AgCl layer on surface AgCl(s) + e - « Ag(s) + Cl- E 0 = +0.222 V Eind = +0.222 - 0.0592 n log a Cl - = +0.222 + 0.0592pCl • Electrodes of the third kind - respond to changes of different ion than metal electrode CEM 333 page 11.6

Membrane (or Ion Selective)Electrodes: Membrane: Low solubility -solids,semi-solids and polymers Some electrical conductivity -often by doping Selectivity-part of membrane binds/reacts with analyte Two general types-crystalline and non-crystalline membranes Non-crystalline membranes: Glass-silicate glasses for H+,Na+ Liquid-liquid ion exchanger for Ca2+ Immobilized liquid-liquid/PVC matrix for Ca2+and NO3 Crystalline membranes: Single crystal-LaF3 for F- Polycrystalline or mixed crystal-AgS for S2-and Ag+ CEM333 page 11.7

Membrane (or Ion Selective) Electrodes: Membrane: • Low solubility - solids, semi-solids and polymers • Some electrical conductivity - often by doping • Selectivity - part of membrane binds/reacts with analyte Two general types - crystalline and non-crystalline membranes • Non-crystalline membranes: Glass - silicate glasses for H+, Na+ Liquid - liquid ion exchanger for Ca2+ Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3 - • Crystalline membranes: Single crystal - LaF3 for F￾Polycrystalline or mixed crystal - AgS for S2- and Ag+ CEM 333 page 11.7

Glass Membrane Electrodes: Fig 23-3 To pH meter Saturated calomel electrode Glass electrode Wax insulation Solution of unknown pH Ag wire Heavy-walled glass Ag wire 0.1 M HCI saturated with AgCI Thin glass Thin glass membrane membrane Magnetic stirrer Ref 1 Analyte Glass Electrode SCEH3O(a=a)Membrane|H3(a=a2),CI (a=1 M),AgCI(sat'd)Ag Ref 2 Combination plon electrode(ref+ind) Contains two(reference)electrodes-glass membrane is pH sensitive CEM 333 page 11.8

Glass Membrane Electrodes: Fig 23-3 SCE Ref } 1 ||H3O + (a = a1 ) 6 4 Analyte 4 7 4 4 8 |Membrane|H3O + (a = a2 ),Cl - (a = 1 M),AgCl(sat'd)|Ag Ref 2 1 4 4 4 4 4 2 4 4 4 4 4 3 6 4 4 4 4 4 4 4 4 4 Glass 4 4 Electrode 7 4 4 4 4 4 4 4 4 4 4 4 8 Combination pIon electrode (ref + ind) Contains two (reference) electrodes - glass membrane is pH sensitive CEM 333 page 11.8

Glass Membrane Structure: SiO44-framework with charge balancing cations -Si0272%,Na2022%,Ca06% Fig 23-5 ● 0 Q -1 O 0 ● ●si ○0 o○Cations In aqueous solution,ion exchange reaction at surface Ht+Na"Glass←→Glass"+Na H+carries current near surface Na+carries current in interior Ca2+carries no current (immobile) CEM 333 page 11.9

Glass Membrane Structure: SiO4 4- framework with charge balancing cations - SiO2 72 %, Na2O 22 %, CaO 6 % Fig 23-5 In aqueous solution, ion exchange reaction at surface H + + Na+Glass- ¾ ¬ ¾ ® H +Glass- + Na+ • H+ carries current near surface • Na+ carries current in interior • Ca2+ carries no current (immobile) CEM 333 page 11.9

Membrane al a a2 a2 Analyte Ag/AgCl Solution NaG Reference Electrode H++GHG H+G2->H++G2- E E2 Surface where more dissociation occurs becomes negatively charge with respect to other surface Boundary potential Eb=E1-E2 Potential difference determined by .Eref1-SCE (constant) Eref 2-Ag/AgCl (constant) ·Eb CEM 333 page 11.10

H +G 2 - « H + + G 2 - H + + G 1 - « H +G 1 - Na +G Analyte - Solution Membrane Ag/AgCl Reference Electrode a 1 a'1 a'2 a 2 E E 1 2 Surface where more dissociation occurs becomes negatively charge with respect to other surface Boundary potential Eb = E1 - E2 Potential difference determined by • Eref 1 - SCE (constant) • Eref 2 - Ag/AgCl (constant) • Eb CEM 333 page 11.10