Analytical Chemistry II Spring 2017 Dr Violeta Jevtovic

Introduction to Electrochemical Methods of Analysis

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Oxidation-Reduction (Redox) reactions involve the transfer of electrons (e-) Results in electric current (amps) and potential (volts) generated in an electrochemical cell OIL RIG – Oxidation Is Loss e-, Reduction Is Gain e- Oxidation occurs at the anode (-ve terminal) Electrons flow from anode to cathode (+ve terminal)

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells e.g. lead-acid battery used in cars: Pb(s) + SO42-(aq) ® PbSO4(s) + 2e- [OX] PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- ® PbSO4(s) + 2H2O(l) [RED] Net: Pb(s) + PbO2(s) + 2H2SO4 ⇌ 2PbSO4(aq) + 2H2O(l) Chemical energy is converted into electrical energy, each cell produces 2 V, six cells will give familiar 12 V battery

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells What are the oxidation numbers of lead in the net reaction? Pb(s) + SO42-(aq) ® PbSO4(s) + 2e- [OX] PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- ® PbSO4(s) + 2H2O(l) [RED] Pb(s) + PbO2(s) + 2H2SO4 ⇌ 2PbSO4(aq) + 2H2O(l)

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells e.g the Daniell cell Zn(s) ⇌ Zn2+(aq) + 2e- (anode) Cu2+(aq) + 2e- ⇌ Cu(s) (cathode) Cu2+(aq) + Zn(s) ⇌ Cu(s) + Zn2+(aq) 2 half cells: Zinc electrode in ZnSO4 solution Copper electrode in CuSO4 solution Generates potential of 1.1 V http://video.google.com/videoplay?docid=566682988710852020# http://www.youtube.com/watch?v=0oSqPDD2rMA http://www.youtube.com/watch?v=nNG5PMlHSoA&feature=related http://www.youtube.com/watch?v=A0VUsoeT9aM&feature=related

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Circuit must be completed with external metal wire and a salt bridge e- flow through the metal wire from one electrode to the other (anode to cathode) Salt bridge allows charge (ions) to transfer through the solutions (completes circuit), prevents the complete mixing of the solutions, maintains electro-neutrality Sodium sulfate, sodium chloride, potassium nitrate or similar

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells As the oxidation-reduction reaction occurs, the anode becomes more positive as Zn2+ is produced whilst the cathode becomes more negative as Cu2+ is removed from solution Salt bridge allows for the migration of ions in both directions to maintain neutrality

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells → → Spontaneous (used in potentiometric analysis) Non-spontaneous Used in voltammetric analysis

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V) emf depends on concentration of REDOX species, described by the Nerst equation: E = Eo – RT x ln [OX] substituting for R, T, F and nF [RED] converting to log10 (x 2.303) E = Eo – 0.059 log [OX] n [RED] Where E = electrode potential (V), E0 = standard potential for the reaction under standard conditions, R = gas constant (8.3145 J/K.mol), T = temperature (K), n = no. moles of charge transferred, F = Faraday constant (96,485 C/mol)

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells For the Daniell cell: E = Eo – 0.059 log ([Zn2+]/ [Cu2+]) E0 can be calculated from tables e.g the Daniell cell Zn(s) ⇌ Zn2+(aq) + 2e- E0 = 0.763 V Cu2+(aq) + 2e- ⇌ Cu(s) E0 = 0.337 V Cu2+(aq) + Zn(s) ⇌ Cu(s) + Zn2+(aq) E0overall = E0OX + E0RED = +1.10 V +ve E value indicates a spontaneous reaction

Tro, Chemistry: A Molecular Approach

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V) E = Eo – 0.059 log [OX] n [RED] What is the emf of a Daniel cell when [OX] = [RED]? log (1) = 0 E = Eo – 0.059 x log (1) = E0 - 0 2 E = E0 = 1.10 V

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Notation: shorthand description of Voltaic cell electrode | electrolyte || electrolyte | electrode oxidation half-cell on left, reduction half-cell on the right single | = phase barrier if multiple electrolytes in same phase, a comma is used rather than | double line || = salt bridge

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells What would be the notation for the following cell? Zn | Zn2+ || Cu | Cu2+

Electrochemical Methods Introduction to Electrochemical Theories Review of Redox Chemistry and Electrochemical Cells Daniell Cell Demo

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Potentiometry Coulometry Voltammetry Conductometry

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Potentiometry Two types: Indirect potentiometry - titration Direct potentiometry Indirect: In a potentiometric titration an abrupt change in potential (E) is used for the end-point Potentiometric titrations can be automated and used where color-changing indicators are difficult to see Direct: Measurement of analyte concentration according to Nernst equation using potential (E) Potential of indicator electrode w.r.t. a reference electrode e.g. pH meter

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Coulometry Measures quantity of electricity or the charge generated by a reaction at an electrode Voltage is applied to drive the nonspontaneous redox reaction (electrolytic cell) Charge (Q) is related to current (I) according to: Q = It (constant current) Q = ∫ I dt (controlled potential) Where I = current (A) and t = time (s).

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Coulometry Constant current techniques: ‘Colulometric titrations’ Reagent is generated an one electrode, reacts stoichiometrically with analyte (titrant) Controlled potential techniques: Current resulting from reaction of analyte is monitored w.r.t. time Equation relating Q to quantity of analyte (m) is Faraday’s law of electrolysis: m = Q M F n Where F = Faraday constant (96,485 C/mol e-), M = molec. mass, and n = no. moles e- per mole analyte

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Coulometry Constant current techniques: Controlled potential techniques: e.g. 0.5 A was used to deposit Cu2+ in 10 mins in the Daniell Cell, the amount of copper in solution m = Q M F n m = Q M = (0.5 A x 10 mins x 60s/min) x 63.5 g/mol F n 96,485 C/mol e- x 2 mol e-/mol Cu m = = 9.87 x 10-2 g C

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Voltammetry Current (I) is measured as a function of changing potential (E) Electrolytic cell consisting of 3 electrodes: Micro indicator electrode Reference electrode Auxillary counter electrode

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Voltammetry Changing potential (E) is applied on the indicator electrode (working electrode) to drive a nonspontaneous redox reaction Counter electrode serves to conduct electricity between the two electrodes Reference electrode has a constant potential throughout

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Conductometry Measures conductivity – ability of a solution to carry electric current Conductivity cell has two electrode plates placed in a solution Ohm’s law: R = V/I Where R = resistance (ohms), V = voltage (volts), I = current (amps) e.g. Chloride determination using conductivity meter Mohr method uses potassium chromate as indicator with a difficult to see end-point, conductometric method is faster and more accurate

Electrochemical Methods Introduction to Electrochemical Theories General Principles of Electroanalytical Methods Conductometry Conductance (G), ohms-1: G = 1/R (units ohms-1 or mho or Siemens) Conductivity (k): k = GK = GL/A = L/AR (units ohms-1 cm-1, or S cm-1) Where K (cm-1) = cell constant (L/A)

Electrochemical Methods Introduction to Electrochemical Theories Types of Electrodes and Notations for Electrochemical Cells Electrodes are important for electrochemical analysis! Reference electrodes Indicator electrodes

Electrochemical Methods Introduction to Electrochemical Theories Types of Electrodes and Notations for Electrochemical Cells Reference electrodes Provide constant potential – not affected by sample composition Follows Nernst equation Two common reference electrodes: 1. Saturated calomel electrode (SCE) Mercury in contact with a solution saturated with mercury chloride || KCl (saturated), Hg2Cl2(s)|Hg(s) (E0 = +0.214 vs. SHE at 25 ºC) 2. Silver wire coated with silver chloride in a saturated KCl solution || KCl (saturated), AgCl(s)|Ag(s) (E0 = +0.197 vs. SHE at 25 ºC) E0 above is calculated based on reference to the standard hydrogen electrode (SHE)

Electrochemical Methods Introduction to Electrochemical Theories Types of Electrodes and Notations for Electrochemical Cells Indicator electrodes: Respond to changing analyte concentrations Two types of indicator electrodes: 1. Metallic electrodes – metal/metal ion, metal/metal salt/anion no longer used 2. Membrane electrodes (AKA ion-selective electrodes) – glass electrodes, polymer membrane electrodes (liquid-phase), crystalline membrane electrodes (solid-state) ISE has both reference and indicator electrodes