Ch. 14. ELECTRODES AND POTENTIOMETRY 14.1 Analytical chemists design electrodes (voltage sensitive to conc. change) galvanic cells ion-selective electrodes ion-sensing field effect transistors potentiometry --- measurement of cell voltage indicator electrode : responds to activity of ANALYTE reference electrode : maintains a fixed potential = cell voltage : difference in two potentials 14-1. Reference electrodes 14.2 Right e : Fe +3 + e - Fe +2 E +0 = 0.771 V Left e : AgCl (s) + e - Ag(s) + Cl - E -0 = 0.222 V since E +0 = 0.771 V is more positive, reaction goes as written Fe +3 + e - Fe +2 the other is proceed to backward. E = E + - E - = [Cl - ] is constant (saturated KCl sol.) half-cell on the left --- reference thus, potential changes by
14-1. Reference electrodes 14.3 1) Ag-AgCl reference electrode, dashed area in Fig 14-1 Fig 14-2 simplifies to Fig 14-3, 14-4 utilized as typical ref electrode Ag AgCl electrode : AgCl(s) + e - Ag(s) + Cl - E 0 = 0.222 V since A clis not 1, E(sat. KCl) = 0.197 V 14-1. Reference electrodes 14.4 2) Calomel electrode Hg Hg 2 Cl 2 electrode Metal Metal salt Hg 2 Cl 2 (s) + e - Hg (l) + Cl - E 0 = 0.268 V E(sat. KCl) = 0.241 V saturated calomel electrode (SCE) 3) Voltage conversion
14-2. Indicator electrode 14.5 the most common --- Pt (inert) metal electrode is best provided with large & clean surface for ref : use calomel electrode In case of silver, measure conc. of silver Ag + + e - Ag (s) E +0 = 0.799 V for ref, SCE E - = 0.241 V E = E + - E - = voltage measurement Since Ag + is related to AgCl (s) Ag + + Cl - K sp = 14-2. Indicator electrode 14.6 ex) A 100.0-mL solution containing 0.1000 M NaCl was titrated with 0.1000 M AgNO 3, and the voltage of the cell shown in Fig 15-6 was monitored. Calculate the 10 voltage after the addition of 65.0, 100.0, 103.0 ml of AgNO 3. 1. 8x10 K sp Silver electrode = halide electrode [Ag + ] = K K E = 0.558 + 0.05916 log sp sp [ Cl ] [ Cl ] voltage --- [Cl - ] metal types : Ag, Cu, Zn, Cd, Hg can be used as indicator for their aqueous ions.
14-3. What is junction potential? 14.7 In case of two dissimilar electrolyte solutions basically, E measured = E cell + E junction (E j ) fundamental limitation in accuracy Cl - diffuses faster (higher mobility) than Na + excess negative charge at front potential difference build up KCl has the smallest E j since K + and Cl - have similar mobility thus, KCl is used in salt-bridge 14-4. How ion-selective electrodes work? 14.8 ion-selective electrode - responds to only one species How ion-selective electrode works? membrane separates two C + solutions contains a ligand that can bind and transport C + but not R - (insoluble in water) C + diffuse to lower activity (conc.) region finally positive charge build up on low act. side. steady state is preventing further migration of C + liquid junction (constant pot. diff) Free energy difference L: ligand having high affinity for C + A membrane G = G solvation - RT ln = - n F E A outler 0.059 E = const + log A outer n n=1 for C +
14-5. ph measurement with a glass electrode 14.9 most widely used ion-selective electrode : glass electrode for H + (ph) Ag(s) AgCl(s) Cl - (aq) H + (aq, outside) : H + (aq, inside), Cl - (aq) AgCl(s) Ag(s) ------------------- ----------------- -------------- ------------- outer ref. ele analyte inner H + inner ref. ele 14.10
14-5. ph measurement with a glass electrode 14.11 Surface of a glass electrode? silicate lattice in glass Cross section of glass membrane of a ph electrode Si O Cations such as Li +, Na + 14-5. ph measurement with a glass electrode 14.12 both sides - will be swollen by water (hydrated gel) : most of metal cations diffuse out H+ in sol. diffuses into the membr. The more H + in sol, the more H + will be bound to the glass surface. potential difference between inner & outer Response of glass electrode A H ( out ) : E = const + (0.05916) log A H ( in) ~ 1.00 (>0.98) electromotive efficiency const : asymmetry potential (two sides, inner and outer, are not identical) with calibration, all const is covered ph electrode must be calibrated before it is used
14-5. ph measurement with a glass electrode 14.13 Errors in ph measurement 1) accuracy 0.01 ph unit 2) junction potential at porous plug near the bottom ~ 0.01 ph unit 3) when H + is very low (alkaline), electrode responds to Na + with H + apparent ph is lower than the true. alkaline error 4) when H + is very high, (strong acidic) measured ph is higher than 5) allowance time 6) dry electrode needs soaking 7) calibrated at same temp 14-6. Types of ion-selective electrodes 14.14 1) glass membranes for H + & monovalent cation 2) solid-state electrodes (salt crystal) 3) liquid-based : hydrophobic polymer membrane 4) compound electrodes 1) solid-state electrodes
14-6. Types of ion-selective electrodes 14.15 inorganic crystal LaF 3 doped with Eu +2 in 0.1M NaF & 0.1M NaCl principle : F - ion in sol migrates to LaF 3 crystal anion vacancy from EuF 2 dopping in response : E = const - b (0.05916) log A F-(outside) ~ 1 [F - ] = 10-6 ~ 1 M 14-6. Types of ion-selective electrodes 14.16 Ag 2 S for membrane responds to Ag + & S -2 doping with CuS, CdS, PbS : sensitive to Cu +2, Cd +2, Pb +2 respectively
14-6. Types of ion-selective electrodes 14.17 2) Liquid-based ion-selective electrodes membrane saturated with a hydrophobic liquid ion Exchanger a calcium chelator Ca +2 transport across the membrane to establish voltage difference 0.05916 E = const + log A =1 2 Ca ( outside) 2 ion exchanger : calcium dodecylphosphate in dioctylphenylphosphate 14-6. Types of ion-selective electrodes 14.18 3) compound electrodes CO 2 gas-sensing electrode conventional glass ph electrode surrounded by an electrolyte solution enclosed in a semipermeable membrane made of rubber, Teflon, PE When CO 2 diffuses through membrane, it lowers the ph - detected NH 3, SO 2, H 2 S, No x
14-7. Using ion-selective electrodes 14.19 Advantage 1. wide range of linear response 2. non-destructive 3. non-contaminating 4. short response time 5. unaffected by color or turbidity cares must be required 1. precision (no better than 1 %) 2. electrode contamination by proteins, organic solutes - sluggish response, interference 3. fragile, limited shelf life concentration measurements? standard addition method