Elusive pKa’ of Aminoferrocene (cas 1273-82-1) determined with voltammetric methods in buffered and unbuffered systems and practical aspects of such experiments

Add time:08/09/2019    Source:sciencedirect.com

Redox behaviour of Aminoferrocene (cas 1273-82-1) (FcNH2) in buffered and unbuffered systems has been studied. In unbuffered solutions with background electrolyte only, two discrete redox pairs of unprotonated and protonated forms were evident. On the other hand, in buffered systems only one redox centre was visible continuously shifting with pH.If one wants to determine the apparent acid dissociation constant pKa’ of ferrocene with protonatable side group, buffered environment will be the first choice. Throughout the electrochemical experiments at such conditions, the most limiting factor was the type of the electrode – gold, platinum, glassy carbon and boron-doped diamond electrodes were probed. We found that the influence of the surface of the electrode was so significant that either the determination of pKa’ was not possible at aqueous conditions or observed differences among the determined values were up to five units. In that case, elusive constants were determined. On the other hand, titrations performed in unbuffered systems appeared to be useable despite inconsistences and deficiencies that can be expected. Determined pKa's agreed well with that quantified using spectrophotometry. Although, experimental work at unbuffered conditions can be considered as incorrect or misleading at the first sight, interesting facts and unexpected mechanisms can be revealed. Especially, when one is aware of the possible risks, and the measurement in different (even unbuffered) media and with different tools are done simultaneously. Finally, a method for estimation of pKa’ of unstable compounds without their separation and isolation was introduced.

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