

RESOURCES > EIS > CPE > C from Omega(max) What is the TRUE Capacitance?If the nvalue of a CPE is a little less than 1.0, then the CPE behavior is close to that of a capacitance. Many researchers wish to calculate the "correct" capacitance corresponding to the Q° value of the CPE. The similarity of the equations makes it inviting.
where Q° is numerically equal to the admittance (1/ Z) at =1 rad/s. It is tempting to simply associate the value of Q° for a CPE with the capacitance value, C, for an equivalent capacitor. Alas, an examination of the units of C (farad or Ss ) and Q° ( Ss^{n} ) shows that they can not be the same! See ref 1 for unit abbreviations. For the case of a 'classical' depressed semicircle (CPE in parallel with a resistance) Hsu and Mansfeld (ref 2) have given this equation for calculating the 'true' capacitance, C: C = Q° ( _{MAX} )^{n1} In this equation, _{MAX} represents the frequency at which the imaginary component reaches a maximum. It is the frequency at the top of the depressed semicircle, and it is also the frequency at which the real part ( Z' ) is midway between the low and high frequency xaxis intercepts. An online calculator implementing this equation
is available on this web site. Another calculator uses the R and Q parameters to calculate
the "true" capacitance. 


Comments from V. D. Jovic, (Univ. of Belgrade) can be viewed or downloaded as a PDF file. I am indebted to him for bringing the Orazem (ref 3) article to my attention.
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