You are right. Platinized platinum is used in 4-electrode measurements, for example in myocardial impedance studies, because it is a simple way to increase the true surface area of the electrodes and to reduce the electrode polarization compared to shiny platinum. Also, if I remember correctly, when combined with iridium, the coating increases the stiffness of the electrode, which is useful for penetrating tissue using needle electrodes. Another reason is biocompatibility. Anyway, the electrode impedance will be different from zero (especially noticeable at low frequencies) and, therefore, the word "mitigation" might be more appropriate. If the impedance electrode obtained using the 4-electrode method is still unacceptable, a more sophisticated approach can be implemented with a 5-electrode setup, where the impedance under test is obtained from a two-step 4-electrode measurement. In this case, it would be equivalent to performing a calibration in situ. 

Take a look at the following work, it may be helpful.

McAdams et al., Effect of surface topography on the electrode-electrolye interface impedance. The high frequency, small signal, interface impedance - A review. Surface topography, 2, 107-122, 1989.

McAdams, E T and Jossinet J, The detection of the onset of electrode-electrolyte interface impedance nonlinearity: a theoretical study, IEEE Trans Biomed Eng, 41(5), 498-500, 1994.

Regards

Benjamin

In my opinion, platinized electrodes are used to minimize the polarization effect that is more probable at lower frequency measurements.

Thank you for the insights.

@Benjamin: thank you for the additional references that I carefully read and appreciated.

So far, my conclusions are the following:

1) The 4-electrode (4E) method can mitigate, but (strictly speaking) not eliminate the EP effects because a non-zero current (although typically very small) still passes through the Potential electrodes/probes; I am working on an empirical setup to measure the EP effects when 4E is used with tiny stainless steel probes.

2) The 4E method introduces additional artifacts to the empirical results, particularly at the higher-end portion of the spectra under consideration; these are caused by the fact that metallic objects are introduced at the "middle of the capacitor", thus disturbing the homogeneity of electric field lines through the dielectric cell, a typical assumption in such kind of dielectric measurements. H.P. Schwan highlighted this fact in his works and he proposed a kind of dielectric cell that can mitigate this issue. Unfortunately, it can only be applied for liquid specimen.

Agnelo

Additional conclusions:

3) Regarding EP effects, it is usually not possible to completely void these effects for certain set of (low) frequency and electrical properties of the specimen and this applies to both 2E and 4E systems, although the latter can strongly reduces the EP effects by many orders of magnitude in some cases.

4) A reliable way - the only one that I am really convinced - to qualitatively evaluate if the EP effects were strongly reduced is the electrode distance separation, which I call the DISTANCE EP-TEST.   There is a theoretical/mathematical foundation for this statement presented by H. P. Schwan in one of his papers. Unfortunately, I could not find a single 4E-method study which simply repeats the same experiment(s) for 2 different distances between the potential probes, such as for distances "d" and "2d".  I was able to find 2E-related papers discussing this distance test, but the main point in this discussion is actually to find way to check the reliability of 4E systems regarding the EP effects. When employing the DISTANCE EP-TEST, if the conductivity and dielectric measurements for distances "d" and "2d" lead to roughly the same results, it seems for me that qualitatively this 4E system can be "practically" considered EP-free. Otherwise, the instrumentation's artifacts are potentially present at the empirical data.

5) Excluding "standard" materials, such as KCl solutions, the quantitative error analysis due to the EP effects is still an open research aspect and this missing topic applies for both 2E and 4E systems.

Please, be free to criticize the above statements or to suggest literature that agrees/disagrees with the above conclusions.