Electroanalytical Methods - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Electroanalytical Methods - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Electroanalytical Methods Two general categories: 1) Potentiometric Systems – measure voltage (i.e., potential) of a galvanic cell (produces electricity spontaneously) 2) Voltammetric Systems – control potential & usually measure current in an electrolytic cell (consumes power to cause an electrochemical reaction to occur) docsity.com Potentiometry • Determine concentrations by measuring the potential (i.e., voltage) of an electrochemical cell (galvanic cell) • Two electrodes are required 1) Indicator Electrode – potential responds to activity of species of interest 2) Reference Electrode – chosen so that its potential is independent of solution composition. docsity.com Ecell = Eind – Eref (+ EJ) EJ = junction potential, a non-ideal potential which develops across the interface between two dissimilar solutions Cell Potential (Voltage) docsity.com Ecell = Eind – Eref (+ EJ) Nernst Equation RT [Red] E = Eo - ------ ln --------- nF [Ox] Where R = gas constant T = absolute temperature n = number of electrons in reaction F = Faraday’s constant E = potential Eo = standard potential [Red] = molar concentration of reduced form of species [Ox] = molar concentration of oxidized form of species docsity.com Reference Electrodes • The Normal Hydrogen Electrode (NHE) is important historically and could serve as a reference electrode today, however, it is impractical, requiring a source of H2 gas at constant pressure, and is highly flammable. docsity.com Calomel Reference Electrode ~a— electrical connection bung -q— glass tube +——— platinum wire ~—4— glass tube -;——— mereury 1———mercury I chloride porous plug saturated KCI “li potassium chlonde crystals t porous plug SS hereurcus Chie nee =) Whxed with Hg Saturated EC] docsity.com Calomel Reference Electrode • Can use 1 M or 0.1 M KCl rather than a saturated solution • E for reference changes slightly with any change in concentration from the Nernst Eq • Temperature coefficient of reference electrode is less with 1 M or 0.1 M than for SCE • SCE often gets clogged if solution dries out docsity.com Silver/Silver Chloride Reference Electrode • Reaction is AgCl + e-
 Ag + Cl- • Eo = 0.222 v • E = 0.228 for 1 M KCl • Easily made in the lab • Reliable • Non-toxic vs Calomel glass or plastic body silver chloride coating on silver wire docsity.com Indicator Electrodes • potential “indicates” activity of species • terms Working Electrode or Sensing Electrode are sometimes used • Coupled to reference and meter as usual docsity.com Indicator Electrodes Electrode of the First Kind – Metallic Indicator Electrodes 1) Active metals (e.g., Ag, Cu, Hg, Pb, Cd) can serve as indicators for their own ions Ag+ + e-
 Ag Eo = 0.799 v docsity.com At 25 oC this becomes RT [Red] E = Eo - ------ ln --------- nF [Ox] 0.0591 [Red] E = Eo - ----------- log --------- n [Ox] Nernst Equation docsity.com