Potentiometric Methods A.) Introduction:

Name: Potentiometric Methods A.) Introduction:
Uploaded: 2017-07-10T07:22:51+00:00
Duration: PTM21S28
Channel: Devin Bardon
Description: Potentiometric Methods A.) Introduction:

http:\\asadipour.kmu.ac.ir 24 slides
Potentiometric Methods A.) Introduction: 1.) Potentiometric Methods: based on measurements of the potential of electrochemical cells in the absence of appreciable currents (I →0) 2.) Basic Components: a) reference electrode: gives reference for potential measurement b) indicator electrode: where species of interest is measured c) potential measuring device 920310 24 slides

Electrodes and Potentiometry
Potential change only dependent on one ½ cell concentrations Reference electrode is fixed or saturated  doesn’t change! Ecell=Ecathod-Eanod Anod is conventionally reference electode Fe3+ +e- Fe2+ AgCl(s) + e- → Ag + Cl- Reference electrode, [Cl-] is constant Potential of the cell only depends on [Fe2+] & [Fe3+] Unknown solution of [Fe2+] & [Fe3+] Pt wire is indicator electrode whose potential responds to [Fe2+]/[Fe3+] 920310 24 slides

B.) Reference Electrodes: (Instead of SHE) Need one electrode of system to act as a reference against which potential measurements can be made  relative comparison. Standard hydrogen electrodes are cumbersome Requires H2 gas and freshly prepared Pt surface Desired Characteristics: a) known or fixed potential b) constant response c) insensitive to composition of solution under study d) obeys Nernest Equation e) reversible > > > 920310 24 slides

Electrodes and Potentiometry Reference Electrodes 1.) Silver-Silver Chloride Reference Electrode Eo = V Activity of Cl- not 1E(sat,KCl) = V 920310 24 slides

Electrodes and Potentiometry Reference Electrodes 2.) Saturated Calomel Reference Electrode (S.C.E) Saturated KCl maintains constant [Cl-] even with some evaporation Eo = V Activity of Cl- not 1E(sat,KCl) = V 920310 24 slides

Electrodes and Potentiometry Indicator Electrodes 1.) three Broad Classes of Indicator Electrodes 1) Metal Electrodes Develop an electric potential in response to a redox reaction at the metal surface 2) Ion-selective (Membrane) Electrodes Selectively bind one type of ion to a membrane to generate an electric potential 3) Molecular Selective Electrode 920310 24 slides Remember an electric potential is generated by a separation of charge

1) Metallic Indicator Electrode (3 Main Types) a) Metallic Electrodes of the First Kind b) Metallic Electrodes of the Second Kind c) Metallic Redox Indicators i. Involves single reaction ii. Detection of catione derived from the metal used in the electrode iii. Example: use of copper electrode to detect Cu2+ in solution ½ reaction: Cu2+ + 2e-  Cu (s) Eind gives direct measure of Cu2+: Eind = EoCu – (0.0592/2) log aCu(s)/aCu2+ since aCu(s) = 1: Eind = EoCu – (0.0592/2) log 1/aCu2+ or using pCu = -log aCu2+: Eind = EoCu – (0.0592/2) pCu 920310 24 slides

b) Metallic Electrodes of the Second Kind i. Detection of anion derived from the interaction with metal ion (Mn+) from the electrode ii. Anion forms precipitate or stable complex with metal ion (Mn+) iii. Example: Detection of Cl- with Ag electrode ½ reaction: AgCl(s) + e-  Ag(s) + Cl- EO = V Eind gives direct measure of Cl-: Eind = Eo – (0.0592/1) log aAg(s) aCl-/aAgCl(s) since aAg(s) and aAgCl(s)= 1 & Eo = V: Eind = – (0.0592/1) log aCl- 920310 24 slides

c) Metallic Redox Indicators i. Electrodes made from inert metals (Pt, Au, Pd) ii. Used to detect oxidation/reduction in solution iii. Electrode acts as e- source/sink iv. Example: Detection of Ce3+ with Pt electrode ½ reaction: Ce4+ + e-  Ce3+ Eind responds to Ce4+: Eind = Eo – (0.0592/1) log aCe3+/aCe4+ 920310 24 slides

2) Membrane Indicator Electrodes a) General i. electrodes based on determination of cations or anions by the selective adsorption of these ions to a membrane surface. ii. Often called Ion Selective Electrodes (ISE) or pIon Electrodes iii. Desired properties of ISE’s 1) minimal solubility – membrane will not dissolve in solution during measurement. – silica, polymers, low solubility inorganic compounds , (AgX) can be used 2) Need some electrical conductivity 3) Selectively binds ion of interest 920310 24 slides

Electrodes and Potentiometry Indicator Electrodes Ion-Selective Electrodes Responds Selectively to one ion Contains a thin membrane capable of only binding the desired ion Does not involve a redox process Membrane contains a ligand (L) that specifically and tightly binds analyte of interest (C+) The counter-ions (R-,A-) can’t cross the membrane and/or have low solubility in membrane or analyte solution C+ diffuses across the membrane due to concentration gradient resulting in charge difference across membrane Potential across outer membrane depends on [C+] in analyte solution A difference in the concentration of C+ exists across the outer membrane. 920310 24 slides Remember an electric potential is generated by a separation of charge

Electrodes and Potentiometry Indicator Electrodes Ion-Selective Electrodes Responds Selectively to one ion Contains a thin membrane capable of only binding the desired ion Does not involve a redox process C+ diffuses across the membrane due to concentration gradient resulting in charge difference across membrane A difference in the concentration of C+ exists across the inner membrane. Potential across inner membrane depends on [C+] in filling solution, which is a known constant Electrode potential is determined by the potential difference between the inner and outer membranes: where Einner is a constant and Eouter depends on the concentration of C+ in analyte solution where [C+] is actually the activity of the analyte and n is the charge of the analyte 920310 24 slides Remember an electric potential is generated by a separation of charge

pH Electrode i. most common example of an ISE based on use of glass membrane that preferentially binds H+ ii. Typical pH electrode system is shown pH sensing element is glass tip of Ag/AgCl electrode Two reference electrodes here one SCE outside of membrane //// one Ag/AgCl inside membrane Combined electrod 920310 24 slides

Electrodes and Potentiometry pH Electrodes 1.) pH Measurement with a Glass Electrode Ag(s)|AgCl(s)|Cl-(aq) || H+(aq,outside) H+(aq,inside) , Cl-(aq)|AgCl(s)|Ag(s) Outer reference electrode [H+] outside (analyte solution) [H+] inside Inner reference electrode Eref1 Ej E1 E2 Eref2 Boundary potential difference (Eb) = E1 - E2 Glass membrane Selectively binds H+ Eb = c log[H+] Eb = c – pH 920310 24 slides Electric potential is generated by [H+] difference across glass membrane

iii. pH is determined by formation of boundary potential across glass membrane Boundary potential difference (Eb) = E1 - E2 where from Nernst Equation: Eb = c – 0.059pH Eb = c log[H+] -log aH+ (on exterior of probe or in analyte solution) constant Selective binding of cation (H+) to glass membrane 920310 24 slides

pH Electrodes Glass Membrane Irregular structure of silicate lattice Cations (Na+) bind oxygen in SiO4 structure 920310 24 slides

Electrodes and Potentiometry pH Electrodes Glass Membrane Two surfaces of glass “swell” as they absorb water Surfaces are in contact with [H+] 920310 24 slides

pH Electrodes Glass Membrane H+ diffuse into glass membrane and replace Na+ in hydrated gel region Ion-exchange equilibrium Selective for H+ because H+ is only ion that binds significantly to the hydrated gel layer Charge is slowly carried by migration of Na+ across glass membrane Potential is determined by external [H+] 920310 24 slides Constant and b are measured when electrode is calibrated with solution of known pH

iii. pH is determined by formation of boundary potential across glass membrane At each membrane-solvent interface, a small local potential develops due to the preferential adsorption of H+ onto the glass surface. Glass Surface 920310 24 slides

Electrodes and Potentiometry Junction Potential 1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact Develops at solution interface (salt bridge) Small potential (few millivolts) Junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements Don’t know contribution to the measured voltage Different ion mobility results in separation in charge Again, an electric potential is generated by a separation of charge 920310 24 slides

iv. Alkali Error ‚ H+ not only cation that can bind to glass surface - H+ generally has the strongest binding ‚ Get weak binding of Na+, K+, etc ‚ Most significant when [H+] or aH+ is low (high pH) - usually pH $11-12 At low aH+ (high pH), amount of Na+ or K+ binding is significant  increases the “apparent” amount of bound H+ 920310 24 slides

v. Acid Error ‚ Errors at low pH (Acid error) can give readings that are too high ‚ Exact cause not known - usually occurs at pH # 0.5 c) Glass Electrodes for Other Cations i. change composition of glass membrane ‚ putting Al2O3 or B2O3 in glass ‚ enhances binding for ions other than H+ ii. Used to make ISE’s for Na+, Li+, NH4+ 920310 24 slides

Example 18: The following cell was used for the determination of pCrO4: SCE||CrO42- (xM), Ag2CrO4 (sat’d)|Ag Calculate pCrO4 if the cell potential is 920310 24 slides

Titration curve.xlsx 920310 24 slides

Potentiometric Methods A.) Introduction:

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