Anodic Stripping Voltammetry - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Anodic Stripping Voltammetry - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Anodic Stripping Voltammetry (ASV) Combines two techniques discussed previously 1) The first step is Electrodeposition at constant potential for the purpose of preconcentrating species of interest - Done in stirred solution to enhance mass transfer - Either plate 100% of material (usually a metal) onto or into the electrode - Or plate with reproducible stirring for a known time interval to plate a constant fraction each run (most common approach) 2) The second step is a linear potential sweep in the anodic direction to reoxidize or strip out the plated material - Unstirred solution during stripping step docsity.com Linear Scan ASV time Ecathodic A cathodic or reducing potential is applied for a fixed time interval reducing Mn+ Eanodic Ianodic Mn+ + ne- ↔ Mo M o- ne -↔ M n+ Potential is scanned in anodic or oxidizing direction to strip out Mo Convention requires plotting upside down peaks with backward scan docsity.com • Can improve by use differential pulse waveform during anodic scan (i.e., stripping step) • Measure current at points indicated time Ecathodic docsity.com 2.0 DPASV peaks 15 , showing $ steps 5 1-0 caused 5 by pulses 0.5 Caen Gee | ean ey, Potential (V vs. $.C.E.) docsity.com • A thin film mercury electrode (TFME or MFE) can also be prepared by plating mercury onto the surface of a carbon electrode • This is usually accomplished by adding a solution of mercury ions to the sample and plating out the mercury simultaneously with the analyte ions • The other metals strip out first and mercury last docsity.com • Can also do cathodic stripping voltammetry (CSV) or adsorptive stripping voltammetry using an anodic potential to adsorb anionic species on the surface of the electrode • Ions like MoO42- and CrO42- • Stripping step is a cathodic scan docsity.com Square Wave Voltammetry • One of the most modern electroanalytical techniques (approximately 20 years old) • Very sensitive • Very fast (1 Volt scan in 0.5 sec) • Can be used with all types of electrodes • Can be used with a preconcentration step as in ASV (square wave stripping) • Uses large amplitude square wave • I = n F A D½ C π½ τ½ ψ(Es,Ep) docsity.com Square wave voltammetry combines a square wave and a staircase wave Es = step height (10 mV) Ep = pulse height (50 mV) τ = length of step (5 ms) Pulse frequency (200 Hz) Measure current at 1 and 2 docsity.com Analytical Applications of Electrochemistry • Quantitative analysis – Bulk analysis of samples in lab – Sensors for in situ measurements • Environmental
Clinical
Process – Flow through systems • HPLC
FIA
Pipelines • Metal speciation • Study mechanisms of reactions • Reagent cleanup • Preconcentration docsity.com FIA/HPLC flow cells – use low volume cell with working electrode upstream, reference and counter electrode downstream Several possible types of measurements 1) Conductance – measure conductivity of flow stream as in ion chromatography (IC) 2) Amperometry – like chronoamperometry with fixed potential measuring current as solutes reach electrode 3) Voltammetry – need fast method like SWV 4) Coulometry – fix E (or I) and count coulombs as solutes go by (not efficient) docsity.com Frontiers in Electrochemistry 1) Hydrodynamic electrochemistry e.g. using Rotating Disk Electrode (RDE) - improved mass transport - reproducible convection Rotating Ring Disk Electrode (RRDE) - reduce something at the disk - reoxidize it as it passes the ring - similar to cyclic voltammetry disk ring docsity.com 2) Membrane covered electrodes - put dialysis membrane or other membrane over the surface of the electrode for selectivity - mass transport is hindered 3) Chemically modified electrodes - modify surface of electrode by attaching functional groups that are either oxidized or reduced themselves or that bind other species that are electroactive 4) Conducting polymers as electrodes docsity.com 5) Ultramicroelectrodes (or microelectrodes) - smaller than 20 um - hemispherical shape (usually Hg or Ir) - several unique characteristics - Short diffusion path - Faster rate of mass transport - Steady state conditions established in < 1 µs - No need for convection - IR drop low - Capacitive current low - Can use low ionic strength docsity.com