Novo método voltametrico para cálculo de pK em compostos fracamente solúveis

It is well known that the voltammetric pKs can be determined from the potential peak versus pH curve dependence. Obviously, this curve is easily obtained for water soluble compounds but in the case wherein the compounds are not sufficiently soluble the Ep vs pH curve is not possible to be obtained. In our laboratory, we are working with the following coumarin derivatives that are weakly soluble. Recently we have described a simple way to modify multiwalled carbon nanotubes (MWCNT) with nitroaromatics [1] that involve the encapsulation of the nitroaromatic compounds on the MWCNT three-dimensional network. Now we extended that procedure to the above coumarin derivatives. With this procedure, we change the normal way to carry out voltammetry because the electroactive specie is encapsulated in the electrode phase and not dissolved in the solution phase. In such way, we oxidize the coumarin derivatives (OH oxidation) adsorbed at the electrode with aqueous solutions at different pHs in solution phase obtaining the Ep vs pH curve and consequently the voltammetric pKs values. We have obtained pK values of 10.37, 6.85 and 9.07 for 6-cum, 7-cum and 8-cum respectively. In a separate study a pK value of 6.6 is informed for the 7-cum derivative [2] given validity to our results. The procedure is very easy and no time consuming.