Potentiometric Systems - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Potentiometric Systems - 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 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 Indicator Electrodes – Metallic Indicator Electrodes 2) Active metal in contact with slightly soluble precipitate involving the metal cation –responds to anion concentration This is an Electrode of the Second Kind e.g., Silver/Silver Chloride Electrode (AgCl) AgCl + e-
 Ag + Cl- Eo = 0.222 v docsity.com Ag wire AgCl coating on Ag wire Silver/Silver Chloride Electrode to measure Cl- ions AgCl + e-
 Ag + Cl- docsity.com 0.0591 [Cl-] E = 0.222 - ----------- log --------- 1 1 For the Silver/Silver Chloride system AgCl + e-
 Ag + Cl- Eo = 0.222 v The Nernst equation becomes Electrode potential is directly proportional to the concentration (activity )of the chloride ions for this Electrode of the Second Kind docsity.com Membrane Electrodes • Several types – Glass membrane electrode - Solid State “ “ - Liquid Junction “ “ -Permeable “ “ • Most important is glass electrode for pH [H+] = a1 [H+] = a2 solution 1 solution 2 (test soln.) (internal soln.) potential develops across membrane thin glass membrane docsity.com Glass is SiO2 docsity.com docsity.com Glass pH Electrode • E = K’ – 0.0591 pH • Combine with reference electrode and meter • Half cell voltage proportional to pH • Nernstian slope • Intercept is K’, no Eo • Calibrate with buffers Electrical connection seal 0.1 M HCl Filling solution Ag wire coated with AgCl Thin glass membrane docsity.com - awl er’, / ¥ , 3 a a ORION RESEARCH or RE mara Fir) eee ci wif a ea A! ~ y ia docsity.com Proper pH Calibration • E = K’ – 0.0591 pH • Meter measures E vs pH – must calibrate both slope & intercept on meter with buffers • Meter has two controls – calibrate & slope • 1st use pH 7.00 buffer to adjust calibrate knob mV pH 4 7 Calibrate knob raises and lowers the line without changing slope docsity.com Errors in pH Measurement 1 • pH measurements are only as good as the buffers used to calibrate – Accuracy good to +0.01 units* – Precision may be good to +0.001 units • Junction potential dependent on ionic strength of solution – Ej may be a significant error if test solution has different ionic strength than buffers * Unless using special buffers, temp. control & a Faraday cage docsity.com Errors in pH Measurement 2 • Asymmetry potential is another non-ideal potential that arises possibly from strain in the glass. When both internal & external H+ solutions are the same activity, potential should be 0 but it’s not Ecell = Eind – Eref + Ej +Ea • Temperature of electrodes, calibration buffers and sample solutions must be the same primarily because of T in Nernst Eq. ATC probes are available for many meters docsity.com Errors in pH Measurement 3 • Alkaline Error or Sodium Error occurs when pH is very high (e.g., 12) because Na+ concentration is high (from NaOH used to raise pH) and H+ is very low. Electrode responds slightly to Na+ & gives a lower reading than actual pH. This is related to the concept of selectivity coefficients where the electrode responds to many ions but is most selective for H+. Problem occurs because Na+ is 10 orders of magnitude higher than H+ in the solution. docsity.com