actually the frequency adjusting depends on solution that impedance measurement will be performed. these solutions are non-faradaik or faradaik solutions. if you use non-faradaik then you can choose higher frequencies, however if you use faradaik solutions, then you can choose lower frequencies for example, 0.05Hz. I am using an electrolyte solution ferri/ferro couple and KCl i chose 10000 to 0.05 Hz. In higher frequencies Warburg impedance is ignored, shows no domination or signal.

Thank you Zihni

I'm using NaCl 0.01M and the frecuency range from 10KHz to 10mHz

The range of frequency more suitable for any system depends strictly on the nature of the system itself.

In particular you can relate the frequency to the time constant of the circuital element you are studying. Any dielectric layer is characterized by a time constant that depends on its resistance, R, as well as on its capacitance, C. For example if you imagine to describe any dielectric layer in contact with an electrode surface with an RC mesh you can associate to each mesh a time constant equal to the product RxC. So if you perform impedance measurements in the range of frequencies around the value of the reciprocal of RxC product you will be sure to obtain information about the element you are interested in. At high frequency the impedance spectrum is dominated by low impedance elements, the opposite happens at low frequency.

To conclude before choosing the range of frequency to detect on your system you have to know the system itself and a preliminary screening on the whole range of frequency is advisable, usually the range is 100000-0.01 Hz. It is fundamental to record the spectrum beginning from the higher frequencies.

Actually i dont know your system but you should start higher frequency to lower.

The adoption of frequency range is depend on the system, as wll as the element you like to investigate.

I work with gold microelectrodes and I immobilize DNA self assembeld layer and then I measure impedance in 2 mM Ferro/Ferri cyanide solution. Frequency range I use 100 KHz - 0.1 Hz. The problem I face is that at low frequency around 1-0.1 Hz, I see a flat tale rather than any warburg impedance. What does that mean? Do I need to narrow down my range?

I am not so confident in the use of electrochemical impedance spectroscopy when faradaic reactions are occurring at the electrode surface. In any case I have been working with functionalized mercury electrodes even of micrometric dimension.

From what you are writing I imagine you are describing a Nyquist plot. Well, which kind of instrument and particularly which software are you using for data acquisition? Do you have the possibility to see plots different from the Nyquist, for example Bode plot or any other? Moreover are you performing EIS measurements at a particular applied potential or are you scanning a range of potentials? Finally can you compare the results obtained with a DNA covered microelectrode in contact with the electrolytic solution in the absence of electroactive species with the results obtained with the redox couple?

I think that a comparison between the results observed at different applied potentials as well as the differences between a functionalized electrode in contact with the electroactive couple or not may help you to understand your problem. Also the profile of the Bode plot may help you if you are confident with this kind of plot.

the Warburg impedance usually exhibits certain shapes, such as semicircles, depressed arcs, and straight lines, which are depended on the characteristic frequency and its difference to the adjacent impedance element [J. Chen, et al. Electrochimica Acta, 2011, 56, 4624-4630].

In order to separate the impedance sign for each element, adjust the reaction rate and/or change the diffusion coefficient are feasible approaches. You can easiily to change the density of the electrolyte, or change the solvent.

mohtashim try this; Potential: 0.18mV 5mM ferri/ferro + 100mM KCl in pH=7 and 10000 Hz to 0.05 Hz, i mean that the reduction the resistance of the solution. then we will discuss it later.

There are other things that should be taken into account, For instance the potentiostat used in the measurement. Most of the potentiostat includes a measurement artefact at frecuencies above of 50 kHz

I'm using NaCl 3.5%, pH 6.2 and the Frequency range I use 0.1 Hz to 100 KHz -Usually this is the most frequency range more used.

I know I am late to this discussion, but I wanted to throw one or two more considerations in. You also have to keep in mind that your system itself is stable over the time scale of the measurement. If you system itself is changing while you are reading impedance data you will get irregular shapes/data and you will have problems with reproducibility. You see this most when trying to measure at the low end of frequency where a slow drift in your system is more likely to cause problems. I also agree with Ms. Larios-Duran that you can easily run into artifacts and instrumental responses at high frequency ranges if your instrument/software can't keep up with the data acquisition rate. This might be something you can determine from using a dummy cell (A cell composed of actual electronic components instead of a solution based chemical system) and making measurements to see. Best of luck!

The frequency follows the electro chemical process in system ,therefore we must take wide range of frequencies to follow all the slow and the speed polarization

Starting frequencies depends on the machine itself being fitted to do that. It could be that the frequency modulator will stop this action; it also depends on what you are looking for on the surface electrolyte interface or coating interfacial layer interface.

At high frequencies, fast electrochemical processes are detected, which include the migration of charges, ions and electrons in the electrolyte bulk. At low frequencies, the electrochemical processes of the electrode-electrolyte interface are involved, which are slow, and these diffusion and charge transfer processes are limiting. Of course, at low frequencies, both of these processes are involved. Investigations deliberately start at high frequencies to include the bulk electrolyte response.