G. FREDERICK SMITH CHEMICAL COMPANY
1
λ <*«*£& $»*£&» G . F R E D E R I C K SMITH CHEMICAL COMPANY 867 McKinley Ave. Columbus, Ohio 43223 rOt PtOMPT SHIPMENT 614 / 224-5343 GENERAL CATALOG FEATURING OVER 445 CHEMICALS NOW AVAILABLE FREE CIRCLE 188 ON READER SERVICE CARD while the ensemble averaged PFT data are more than equal to the task (10). Finally, results of excellent preci- sion have been realized in membrane admittance spectrum measurements by de Levie et al. {13). FFT Measurements of Faradic Nonlinearity Glover and Smith (9) and Fujiwara et al. (Il, 12) have described proce- dures whereby FFT analysis of the cell current response to a sinusoidal input is used to simultaneously measure the dc, fundamental harmonic, and sec- ond harmonic components as a func- tion of dc potential. The procedure has been referred to as "harmonic multiplexing" (9) and has found con- siderable use in studies where kinetics of coupled homogeneous chemical re- actions and the heterogeneous charge transfer step have been determined si- multaneously (mixed rate control) and for evaluation of thermodynamic po- tentials (28-30). The availability of second or higher harmonic data (non- linear components) greatly enhances the information content of one's data set, making more certain conclusions regarding electrode reaction mecha- nisms, rate parameter magnitudes, 3.00 I 2 · 00 I 1.00 0.00 etc. Thus, we anticipate that FFT characterization of faradic nonlineari- ty will find continued application. Conclusions We have outlined in this article in- strumentation, data acquisition, and processing strategies appropriate for implementing an FFT approach to measuring the electrochemical cell ad- mittance spectrum with unprecedent- ed speed and precision. For the nor- mal frequency range, standard labora- tory equipment (potentiostat and minicomputer) suffices to implement the optimized strategies which involve specially tailored test signal wave- forms and careful synchronization of the data record and perturbation waveform periods. In a subsequent ar- ticle we will consider the role of FFT- implemented convolution and decon- volution operations in ERM. When one considers the consequences of combining these latter data enhance- ment strategies with the admittance spectral measurement approach de- scribed in this article, one begins to envision a new era for ERM in both analysis and kinetic-mechanistic ap- plications. ' t 996.8 Hi 10.07 Hz • (FREQUENCY)" 2 996.9 Ήϊν 1.40 1.30· 1.20· 1.10 (FREQUENCY)" 2 Figure 5. Frequency spectrum of peak faradic admittance magnitude and phase angle contangent obtained by ensemble averaging 64 replicate measurements System, etc., same as Figure 4, except results depict 64-pass ensemble average. Data correspond to ks rate parameter uncertainty of ±0.31 % (relative standard deviation) Taken from ref. 7 by courtesy of Elsevier Publishing Co. 238 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Transcript of G. FREDERICK SMITH CHEMICAL COMPANY
λ
<*«*£& $»*£&»
G . F R E D E R I C K S M I T H C H E M I C A L C O M P A N Y 867 McKinley Ave. Columbus, Ohio 43223
r O t P t O M P T S H I P M E N T
614 / 2 2 4 - 5 3 4 3
GENERAL CATALOG FEATURING OVER 445 CHEMICALS
NOW AVAILABLE FREE
CIRCLE 188 ON READER SERVICE CARD
while the ensemble averaged PFT data are more than equal to the task (10). Finally, results of excellent precision have been realized in membrane admittance spectrum measurements by de Levie et al. {13).
FFT Measurements of Faradic Nonlineari ty
Glover and Smith (9) and Fujiwara et al. (Il, 12) have described procedures whereby FFT analysis of the cell current response to a sinusoidal input is used to simultaneously measure the dc, fundamental harmonic, and second harmonic components as a function of dc potential. The procedure has been referred to as "harmonic multiplexing" (9) and has found considerable use in studies where kinetics of coupled homogeneous chemical reactions and the heterogeneous charge transfer step have been determined simultaneously (mixed rate control) and for evaluation of thermodynamic potentials (28-30). The availability of second or higher harmonic data (nonlinear components) greatly enhances the information content of one's data set, making more certain conclusions regarding electrode reaction mechanisms, rate parameter magnitudes,
3.00
I 2 · 0 0
I 1.00
0.00
etc. Thus, we anticipate that FFT characterization of faradic nonlinearity will find continued application.
Conclusions
We have outlined in this article instrumentation, data acquisition, and processing strategies appropriate for implementing an FFT approach to measuring the electrochemical cell admittance spectrum with unprecedented speed and precision. For the normal frequency range, standard laboratory equipment (potentiostat and minicomputer) suffices to implement the optimized strategies which involve specially tailored test signal waveforms and careful synchronization of the data record and perturbation waveform periods. In a subsequent article we will consider the role of FFT-implemented convolution and decon-volution operations in ERM. When one considers the consequences of combining these latter data enhancement strategies with the admittance spectral measurement approach described in this article, one begins to envision a new era for ERM in both analysis and kinetic-mechanistic applications.
' t 996.8 Hi
10.07 Hz •
(FREQUENCY)"2
996.9 Ήϊν
1.40
1.30·
1.20·
1.10
(FREQUENCY)"2
Figure 5. Frequency spectrum of peak faradic admittance magnitude and phase angle contangent obtained by ensemble averaging 64 replicate measurements System, etc., same as Figure 4, except results depict 64-pass ensemble average. Data correspond to ks rate parameter uncertainty of ± 0 . 3 1 % (relative standard deviation) Taken from ref. 7 by courtesy of Elsevier Publishing Co.
238 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
