Alkaline Error: At high pH, glass electrode indicates pH less than true value Low [H+] means membrane exchanges with alkali meCHM 320 - Lecture 26 Chapt 15Chapter 15 Electrodes and PotentiometryHomework: Due Wednesday, April 12Problems – 15-1, 15-6, 15-13, 15-21, 15-24, 15-35CHM 320 - Lecture 26 Chapt 15Potentiometry• Potentiometry is the field of electroanalytical chemistry in which potential is measured under the conditions of no current flow. • The measured potential may be used to determine the the concentration of some component of the analyte solution. • The potential that develops in the electrochemical cell is the result of the free energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition has been satisfied. •The potential differencebetween the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell.CHM 320 - Lecture 26 Chapt 15Reference electrodes• Used for half of the cell to determine the potential of the analyte of interest.• Maintains a fixed potential (i.e. reference, stable over time) –in contrast, the indicator electrode responds to the analyteactivity• Follows Nernst equationExamples of reference electrodesSaturated Calomel Electrode (SCE):Hg2Cl2(s)+ 2e− ↔ 2Hg(l)+ 2Cl−or Hg|Hg2Cl2(sat'd, KCl) ||...CHM 320 - Lecture 26 Chapt 15Silver/Silver Chloride Electrode:Similar construction to calomel reference electrode• Ag wire coated with AgCl• Solution of KCl sat'd with AgClAgCl(s)+ e−↔Ag(s)+ Cl−Or Ag| AgCl(sat'd, KCl) ||...Which one should be used– Ag|AgCl or SCE?• Ag/AgCl better for uncontrolled temperature (lower T coefficient)• However, Ag reacts with more ionsPrecautions 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)CHM 320 - Lecture 26 Chapt 15CHM 320 - Lecture 26 Chapt 15Indicator Electrodes• Two types: metal and membrane indicator electodesMetal Indicator Electrodes• These electrodes develop an electric potential in response to a redoxreaction at the metal surface.• These measurements are made against a reference electrode (whichhas a known, stable potential).• Typically, Pt or Au are used for the metal indicator electrode as they are inert (do not contribute to the reaction).• Metal electrodes of the “first kind” respond directly to changing activity of electrode ion – this type of electrode is not selective for a specific analyte.Ecell= Eindicator− EreferenceCHM 320 - Lecture 26 Chapt 15Example of Metal Indicator ElectrodeAg++ e-⇔ Ag0(s)E° = 0.799 V(SCE ref.)The same electrode can be used to measure [Cl-] since AgCl ⇔ Ag++ Cl-CHM 320 - Lecture 26 Chapt 15In general, metal indicator electrodes of “first kind”:• Simple• Not very selective• Some metals easily oxidized (deaerated solutions)• Some metals (Zn, Cd) dissolve in acidic solutions• Electrodes of the “second kind” - respond to changes in ion activity through formation of complex (as seen in example).CHM 320 - Lecture 26 Chapt 15Junction Potentials• Arise because of the differing mobility of different ions in solution• A charge (potential) develops when one ion moves to an electrode more rapidly than the other ion• Counteracted by the use of strong electrolytes in solution and/or in the salt-bridge– The strong electrolyte helps overcome mobility-induced differences in charge between two cellsCHM 320 - Lecture 26 Chapt 15Ion Selective Electrodes• Physical phenomena which do not involve explicit redoxreactions, but whose initial conditions have a non-zero free energy, also will generate a potential. • An example of this would be ion concentration gradients across a semi-permeable membrane. • This can also be a potentiometric phenomena, and is the basis of measurements that use ion-selective electrodes.CHM 320 - Lecture 26 Chapt 15Membrane• Low solubility - solids, semi-solids and polymers• Some electrical conductivity - often by doping• Selectivity - part of membrane binds/reacts with analyteTwo general types - crystalline and non-crystalline membranes• Non-crystalline membranes:¾Glass - silicate glasses for H+, Na+¾Liquid - liquid ion exchanger for Ca2+¾Immobilized liquid - liquid/PVC matrix for Ca2+and NO3-• Crystalline membranes:¾Single crystal - LaF3for F-Polycrystalline¾Mixed crystal - AgS for S2-and Ag+CHM 320 - Lecture 26 Chapt 15CHM 320 - Lecture 26 Chapt 15Potential difference across the membrane:a1, a2 – activities of the ion on both sides of the membrane.Common ion selective electrode: pH electrodeContains two (reference) electrodes - glass membrane is pH sensitiveCHM 320 - Lecture 26 Chapt 15Glass Membrane Structure:SiO44- framework with charge balancing cations-SiO272 %, Na2O 22 %, CaO 6 %CHM 320 - Lecture 26 Chapt 15CHM 320 - Lecture 26 Chapt 15Alkaline Error:At high pH, glass electrode indicates pH less than true valueLow [H+] means membrane exchanges with alkali metal ionsin solution