Membrane Electrodes - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

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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 potential develops across membrane thin glass membrane 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 • Slope comes from RT/nF in Nernst Equation • Slope is temperature sensitive • Other factors influence slope including – Impurities in glass membrane – Overall quality of electrode construction • Many electrodes exhibit “full Nernstian response” while others may give only 90% Cell for pH measurement (shorthand notation) Ag(s) AgCl(s) Cl-(aq) H+unk HCl(0.1M) AgCl(sat’d) Ag(s) reference electrode test soln glass electrode 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 4 • Acid Error – electrode reads slightly higher than the actual pH in very acidic solutions (not well understood) • Response Time – related to activity for all potentiometric electrodes & is fast at high activity (concentration) & slow at low conc. • Hydration of Glass Surface – glass electrodes must be kept hydrated for good measurement & must be rehydrated for 24 hrs if it dries out – will cause noisy readings docsity.com Glass Electrode Summary • Glass membrane electrodes are very good indicator electrodes in potentiometry • Must exercise care in calibration and in maintaining integrity of glass membrane • Some errors exist & are unavoidable • Glass electrodes available for Na+, K+, NH4+, Rb+, Cs+, Li+, Ag+ (cations only) by varying glass composition • Combination electrodes combine pH & ref. docsity.com Combination pH Electrode +) Leads to pH Meter (+) 4 Air Inlet Liquid level of outer reference solution —- Aqueous outer solution saturated with AgCl and KCl AgCl paste with Ag wire +— Porous plug to allow slow drainage of electrolyte out of electrode Glass Membrane Inner solution 0.1 M HCl, saturated with KCI docsity.com Response of calcium ion liquid membrane electrode Potential vs. activity ~ > = a ne +20 e L 29.58 mV 3 Potential vs. 2 0 E concentration = 8 m -20 10+ 1073 102 1071 Activity or concentration of Ca*, mol/L docsity.com Table of liquid membrane electrodes Analyte Concentration Ton Range, M Interferences Ca?* 10° to 5 x 1077 1075 Pb?*; 4 x 10-3 Hg?+, H*, 6 X 1073 Sr?+; 2 X 107? Fe**; 4 & 107? Cu2*; 5 X 107? Ni?*; 0.2 NH3; 0.2 Na*; 0.3 Tris*; 0.3 Lit; 0.4 Kt; 0.7 Ba?*; 1.0 Zn?*; 1,0 Mg?* BF; 10° to 7 x 1076 5 X 10-7 C104; 5 X 10-6 I-; 5 x 10-3 ClOg; 5 x 10-4 CN; 10-7 Br; 10-3 NO3; 5 X 10-3 NOg; 3 X 10-3 HCO3; 5 X 10-2 Cl-; 8 x 10-2 H2POS, HPO?-, PO}”; 0.2 OAc™; 0.6 F~; 1.0 S037 NO} 10°to7X10-§ = 10-7 CIOF; 5 X 10-6 I-; 5 & 10-5 CIOZ; 10-4 CN~; 7 X 1074 Br-; 1073 HS-; 10-2 HCO3; 2 X 10-2 CO}-: 3 X 10-2 Cl“; 5 x 10-2 HpPO4, HPOZ, PO}; 0.2 OAc™; 0.6 F~; 1.0 SO2- clo; 10° to 7 x 1076 2 107317; 2 X 10-2 ClO}; 4 X 10-2 CN-, Br7; 5 X 1072 NO, NO3; 2 HCO;, CO}, Cl-, HyPO4, HPO}, PO}, OAc™, F-, SOF” Kt 10° to 1076 3 X 1074 Cst; 6 X 1073 NHj, TI*; 10-2 H*; 1.0 Ag*, Tris*; 2.0 Lit, Nat Water Hardness 1073to6X10-& 3% 1075 Cu2+, Zn2+; 1074 Ni2+; 4 & 1074 Sr2+; (Cat+ + Mg?*) 6 X 1075 Fe?*: 6 & 10-4 Ba?+: 3 & 10-2 Nat; 0.1 Kt ® docsity.com Solid State Membrane Electrodes Ag wire Filling solution with fixed [Cl-] and cation that electrode responds to Ag/AgCl Solid state membrane (must be ionic conductor) Pb2+Ag2S + PbS Cd2+Ag2S + CdS Cu2+Ag2S + CuS Ag+, S2-Ag2S Ag+, I-AgI Ag+, Br-AgBr Ag+, Cl-AgCl F-, La3+LaF3 Ion DeterminedMembrane Solid State Membrane Chemistry docsity.com Permeable Membrane Electrodes Gas Permeable Membrane Electrodes Gas Sensing Electrodes • Membrane that is permeable to a gas (e.g., NH3) is the key component of electrode • Membrane is part of a small chamber which encloses a filling solution with a pH electrode housed inside • Filling solution has “fixed” [NH4+] which responds to changes in [NH3] passing membrane according to NH3 + H2O NH4+ + OH- docsity.com pH Electrode Bulb Styles 1 ye docsity.com Gas Permeable Membrane Electrodes pH electrode Glass membrane (blue) Hydrophobic membrane - gas permeable (yellow) Filling solution (gray) - Electrode immersed in test solution - NH3 diffuses through membrane - NH3 in test solution equilibrates with NH3 in filling solution NH3 + H2O NH4+ + OH- [NH4+][OH-] Kb = ----------------------[NH3] Kb[OH-] = --------- [NH3][NH4+] pH = 14 – pOH = pNH3 docsity.com