Variations on Polarography - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Variations on Polarography - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Variations on Polarography • Can apply different types of electrical signals to electrochemical cells to modify and improve output signal 1) Alternating Current (AC) Polarography time E AC component on a linear voltage scan docsity.com • Surface concentrations change in response to AC potential • Expanded view of 1 cycle in vicinity of E½ time Ecathodic Reduction occurs above the E½ Ox + ne-
Red E½ Nothing happens below E½ Oxidation occurs when E goes below E½ again Red - ne-
Ox Reduction begins again above E½ docsity.com • Frequency of AC voltage component typically 60 – 100 Hz • The higher the frequency
the faster the electron transfer rate must be to keep up with potential changes and give an appreciable AC current
related to time- scale of the measurement • Irreversible processes generate no AC wave • Magnitude of the applied AC voltage is typically 10 mV peak to peak docsity.com Advantages of AC Polorography • Detection limits slightly better than DC polarography, but still limited by capacitive current • Capacitive current produces an AC component associated with charging and discharging of the electrode surface • AC polarography produces peaks which solves problem of small wave on top of big wave • Multiple components gives multiple peaks • Can use solid electrodes instead of DME with AC applied potential waveform docsity.com Phase Selective AC Polarography • Capacitive current lags voltage by 90o • Faradaic current & applied potential are in-phase E I time IF IC measure current when IC = min docsity.com During pulse sequence E time IF IC measure current when IC = min exponential decay (e-kt) Cottrell equation docsity.com Pulsed Polarography • Only measure current during the later part of the pulse • Take advantage of the fact that IC decays more rapidly than IF • Improves detection limits to 10-7 M or slightly lower • Easy to accomplish with modern electronic instrumentation docsity.com Normal Pulse Polarography or Integral Pulse Polarography Normal or Integral Pulse waveform Typical pulsed waveform A very widely used form of polarography docsity.com • Pulse amplitude increases with time eventually reaching the E½ value and exceeding it • At the end of the pulse the applied signal returns to the baseline level (zero) • Resulting output signal is a wave docsity.com Potential @) ¢ Normal Pulse Polarography (NPP) Time +120.0 Current/uA -1.080 +0.35 Potential/V -0.25 docsity.com DC Polarography hs a 0.25 HA ii ‘A . i NPP showing steps in wave l l wt deg de 0.2 0.1 0 0.2 0.1 0 Potential, V vs. Ag/AgCl ® docsity.com DPP for low concentration sample showing a hint of the steps in the peak docsity.com Applied potential waveform for DPP Linear potential ramp Pulses on ramp showing pulse duration and sampling times before & at the end of pulse Current behavior during pulse docsity.com Advantages of DPP • DPP gives a well resolved peak allowing the determination of species that have E½ values as close as 40 mV to be measured • Detection limits to approx. 10-8 M • Relatively fast with modern DME’s and scan rates in the 10 – 50 mV/sec range • Instrumentation costs are comparatively low in the $5K to $10K range docsity.com