![2.INDICATOR ELECTRODE
2. Membrane or Ion Selective Electrode
-Response of Ion selective electrode
𝐸"#$ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 ±
W.WXYZ
#
log𝑎[#O]^0(outer)
Constant: Eref, Ej, Easy (Eb)
𝜷 = 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒎𝒐𝒕𝒊𝒗𝒆 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 ∼ 𝟏](https://image.slidesharecdn.com/lec3-4-electroanalyticalmethodspart1-200222101851/85/Lec-3-4-electroanalytical-methods-part-1-18-320.jpg)
![A fluoride ion-selective electrode has a selectivity coefficient kF ,OH=0.1.
What will be the change in electrode potential when 1x 10-4 M F at pH
5.5 is raised to pH 10.5?
E = constant - 0.0592log(1x10-4)
At pH 5.5
The change is -7.1 mV which is quite significant
Billones PowerNotes
E = constant + 236.8 mV
E = constant - 0.0592log[(1x10-4 + (0.1)(3.2x10-4)]
At pH 10.5
E = constant + 229.7 mV](https://image.slidesharecdn.com/lec3-4-electroanalyticalmethodspart1-200222101851/85/Lec-3-4-electroanalytical-methods-part-1-22-320.jpg)
![2.INDICATOR ELECTRODE
2. Response of F-1 Selective Electrode
-Direct Potentiometry: Requires a comparison of the potential
developed in a cell containing the indicator electrode in the analyte
solution with its potential when immersed in one or more standard
solutions of known analyte concentration.
𝐸"#$ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 +
0.0592
𝑛
𝑝𝐹
𝑝𝐹 =
𝑛(𝐸o0OO − 𝐾)
0.0592
K is made up of several constants that must be evaluated experimentally.
Determined by measuring Ecell for one or more standard solutions of known p[ ]](https://image.slidesharecdn.com/lec3-4-electroanalyticalmethodspart1-200222101851/85/Lec-3-4-electroanalytical-methods-part-1-23-320.jpg)
Lec 3 4-electroanalytical methods part 1
- 3. Electroanalytical methods Interfacial Methods Static Methods (I=0) Dynamic Methods (I > 0) Bulk Method Conductometry TYPES OF ELECTROANALYTICAL METHODS
- 4. Static Method ( I=0) Potentiometry pH-metry Ion-selective electrode Potentiometric Titration TYPES OF ELECTROANALYTICAL METHODS
- 5. Dynamic Method ( I > 0 ) Controlled Potential Voltametry Amperometry Constant current Electrogravimetry Coulometry TYPES OF ELECTROANALYTICAL METHODS
- 6. POTENTIOMETRIC Methods Based upon measurements of the potential of electrochemical cells in the absence of appreciable currents. Equipment for potentiometric methods •reference electrode (anode by convention) •indicator electrode (cathode by convention) •potential measuring device
- 7. Typical Potentiometric cell Ecell = Ecath- Eano Ecell = Eind - Eref + Ej Therefore, in a potentiometric cell:
- 8. 1.REFERENCE ELECTRODE “Electrode that has known/constant potential.” Characteristics of Ideal Reference Electrode Insensitive to the composition of the analyte solution Rugged and easy to assemble Reversible and obeys Nernst Equation
- 9. Electrode Potential (V), vs. SHE Temperature °C 0.1 M Calomel 3.5 M Calomel Saturated Calomel 3.5 M Ag/AgCl Sat’d Ag/AgCl 10 0.256 0.215 0.214 12 0.3362 0.2528 15 0.3362 0.254 0.2511 0.212 0.209 20 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 35 0.3344 0.246 0.2376 0.197 0.189 38 0.3338 0.2355 40 0.244 0.193 0.184 Potentials of Reference Electrodes in Aqueous Solutions Billones PowerNotes
- 10. 1.REFERENCE ELECTRODE 1. Saturated Calomel Electrode Consist of Hg in contact with a solution that is saturated with Hg2Cl2 (calomel) and also contains a known concentration of KCl Dependent upon KCl concentration Half-cell representation Hg | Hg2Cl2 (sat’d), KCl (x M) || The electrode potential is determined by the reaction: Hg2Cl2(s) + 2e ⇔ 2Hg(l) + 2Cl- At 25 °C, E of SCE = 0.2444 V
- 11. Typical Calomel Electrode At 25 °C, E of SCE = 0.2444 V Disadvantages - Large T coefficient - cannot be used with temperature>60C
- 12. 1.REFERENCE ELECTRODE 2. Silver/Silver Chloride Electrode Consists of silver electrode immersed in a solution of KCl that has been saturated with AgCl Most widely marketed RE Half-cell representation Ag | AgCl (sat’d), KCl (x M) || The electrode potential is determined by the reaction AgCl(s) + e ⇔ Ag(s) + Cl- The AgCl-coated Ag wire is immersed in KCl solution that is saturated with AgCl.
- 13. Responds rapidly and reproducibly to changes in the concentration of an analyte ion Potential proportional to ion activity specific (one ion) or selective (several ions) 2.INDICATOR ELECTRODE Electrode used with reference electrode to measure potential of unknown solution. ü Metallic ü Membrane ü Ion-sensitive Field Effect transistors (ISFET)
- 14. 2.INDICATOR ELECTRODE 1. Metallic Indicator Electrode A. electrode of the first kind B. electrode of the second kind C. electrode of the third kind A. Electrode of First Kind -Responds to its ion Example: Copper indicator electrode Cu2+ + 2e- = Cu(s) 𝐸"#$ = 𝐸& − 𝑅𝑇 𝑛𝐹 log𝐾01 𝐸"#$ = 0.337 𝑉 − 0.0592 2 log 1 𝑎𝐶𝑢2 + 𝑬𝒊𝒏𝒅 = 𝟎. 𝟑𝟑𝟕 𝑽 − 𝟎. 𝟎𝟐𝟗𝟔𝒑𝑪𝒖 Other ions can be reduced at Cu surface (those with higher +ve Eo Ex: Ag, Hg, Pd
- 15. 2.INDICATOR ELECTRODE 1. Metallic Indicator Electrode A. electrode of the first kind B. electrode of the second kind C. electrode of the third kind A. Electrode of Second Kind -Responds to changes in activity of the ion that forms a complex with the metal electrode ion 𝐸"#$ = 0.222 𝑉 − 0.0592 1 log𝑎NO Example: Silver works as halide indicator electrode if coated with silver halide. AgCl(s) + e− ↔ Ag(s) + Cl− Eo = +0.222V 𝑬𝒊𝒏𝒅 = 𝟎. 𝟐𝟐𝟐 𝑽 + 𝟎. 𝟎𝟓𝟗𝟐𝒑𝑪𝒍
- 16. 2.INDICATOR ELECTRODE 1. Metallic Indicator Electrode A. electrode of the first kind B. electrode of the second kind C. electrode of the third kind A. Electrode of Third Kind -respond to the activity of the ion different than that of metal electrode (inert metallic electrode that respond to REDOX system) Example: Pt, Au, Pd, and C electrodes can be used to monitor redox systems. (use Pt electrode)
- 17. 2.INDICATOR ELECTRODE 2. Membrane or Ion Selective Electrode Do not involve REDOX processes -Presence of thin membrane capable of binding only the intended ion which generates electrode potential Low solubility Selectivity v Crystalline v Non-Crystalline Membrane Ecell = Eind - Eref + Ej
- 18. 2.INDICATOR ELECTRODE 2. Membrane or Ion Selective Electrode -Response of Ion selective electrode 𝐸"#$ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 ± W.WXYZ # log𝑎[#O]^0(outer) Constant: Eref, Ej, Easy (Eb) 𝜷 = 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒎𝒐𝒕𝒊𝒗𝒆 𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 ∼ 𝟏
- 19. 2.INDICATOR ELECTRODE 2. Membrane or Ion Selective Electrode A. Crystalline (solid-state : inorganic salt) II. Polycrystalline or mixed crystal – Ag2S for S2- and Ag+ I. Single crystal - Fluoride electrode - LaF3 doped with Eu2+ (create anion vacancy so F- migrates across )
- 20. Because Eu2+ has less charge than La3+, an anion vacancy occurs for every Eu2+ A neighboring F can jump into the vacancy, thereby moving the vacancy to another site. Repetition of this process moves F through the lattice.
- 21. 2.INDICATOR ELECTRODE 2. Response of F-1 Selective Electrode -Response of Ion selective electrode 𝐸"#$= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 − W.WXYZ # log𝑎kl(outer) 𝐸"#$ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 + 0.0592 𝑛 𝑝𝐹 Nernstian: 10-6M to 1M (diluted) pH 5.5 Only interfering species: OH- (selectivity coefficient, kF,OH= 0.1 At low pH, F is converted to HF, electrode becomes insensitive
- 22. A fluoride ion-selective electrode has a selectivity coefficient kF ,OH=0.1. What will be the change in electrode potential when 1x 10-4 M F at pH 5.5 is raised to pH 10.5? E = constant - 0.0592log(1x10-4) At pH 5.5 The change is -7.1 mV which is quite significant Billones PowerNotes E = constant + 236.8 mV E = constant - 0.0592log[(1x10-4 + (0.1)(3.2x10-4)] At pH 10.5 E = constant + 229.7 mV
- 23. 2.INDICATOR ELECTRODE 2. Response of F-1 Selective Electrode -Direct Potentiometry: Requires a comparison of the potential developed in a cell containing the indicator electrode in the analyte solution with its potential when immersed in one or more standard solutions of known analyte concentration. 𝐸"#$ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 + 0.0592 𝑛 𝑝𝐹 𝑝𝐹 = 𝑛(𝐸o0OO − 𝐾) 0.0592 K is made up of several constants that must be evaluated experimentally. Determined by measuring Ecell for one or more standard solutions of known p[ ]
- 24. 2.INDICATOR ELECTRODE 2. Membrane or Ion Selective Electrode A. Non-Crystalline Liquid - liquid ion exchanger for Ca2+ Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3 - Glass - silicate glasses for H+, Na+
- 25. Glass Membrane Electrode (Combination Probe: RE+IE) -contains two (reference) electrodes - glass membrane is pH sensitive Billones PowerNotes
- 26. Glass Membrane Structure SiO4 4- framework with charge balancing cations - SiO2 (72 %) - Na2O (22 %) - CaO (6 %)
- 27. Ion exchange Equilibria on Glass Membrane H+ replaces metal ions bound to the negatively charged oxygen atoms. The pH of the internal solution is fixed. As the pH of the external solution (the sample) changes, the electric potential difference across the glass membrane changes