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

Download Membrane - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Electrical connection seal 0.1 M HCl Filling solution Ag wire coated with AgCl Thin glass membrane Membrane Electrodes Several types - Glass membrane electrode - Liquid membrane electrode - Solid State membrane electrode - Permeable membrane electrode 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 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 Enzyme Electrode e.g., Urea Electrode pH electrode Glass membrane (blue)Hydrophobic membrane - gas permeable (yellow) Filling solution (gray) An electrode sensitive to urea can be prepared by immobilizing a thin layer of the enzyme urease on the surface of the NH3 electrode O H2N-C-NH2 + H2O 2 NH3 + CO2 - Urea comes in contact with urease immobilized on the surface - Urea is broken down to NH3 & CO2 in this enzyme layer -NH3 diffuses through membrane to give response Enzyme layer docsity.com Potentiometry - Conclusion • Electrochemical (galvanic) cell with essentially no current flow • Requires a solution that is conductive i.e., contains a “supporting electrolyte” • Laboratory pH/millivolt meters should be capable of measuring + 0.1 mV • This corresponds to 0.4 x n % uncertainty • Electrodes measure activity not concentration • Measure “free” or uncomplexed ions not total docsity.com Voltammetry • Methods based on an electrolytic cell • Apply potential or current to electrochemical cell & concentrations change at electrode surface due to oxidation & reduction reactions • Can have 2 or 3 electrodes • Stirred or unstirred solution • Measure current or voltage docsity.com Mass Transport or Mass Transfer 2) Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution) docsity.com Voltammetry • Methods based on an electrolytic cell • Apply potential or current to electrochemical cell & concentrations change at electrode surface due to oxidation & reduction reactions • Can have 2 or 3 electrodes • Stirred or unstirred solution • Measure current or voltage docsity.com In all electrochemical methods, the rate of oxidation & reduction depend on: 1) rate & means by which soluble species reach electrode surface (mass transport) 2) kinetics of the electron transfer process at electrode surface (electrode kinetics), which depend on: a) nature of the reaction b) nature of electrode surface c) temperature (we don’t have much control over #2) docsity.com