The lipophilicity of the anionic forms of drugs and model compounds was assessed by their transfer across (i) the water-2-nitrophenyloctyl ether (NPOE), (ii) the water-nitrobenzene (NB), and (iii) the water-noctanol interfaces by using the three-phase electrode technique. The lipophilicities, expressed in terms of logarithm of partition coefficients, range for the studied anions from -3.46 to 0.68 (log P A -,aq QNPOE ) for NPOE, from -3.81 to 2.62 (log P A -,aq QNB ) for NB, and from -6.20 to -3.20 (log P A -,aq Qn-oct ) for n-octanol. Although NPOE shares with nitrobenzene the aromatic part and with n-octanol the hydrophobic carbon chain, only very weak correlation was observed between the NPOE-based data with the n-octanol-based data, and the same is true for the correlation of the NB-based and n-octanol-based data. However, there is a strong and even linear correlation between the NPOE-based and the NB-based data.

A three-phase electrode consisting of a droplet of a nitrobenzene solution of iron(III) tetraphenyl porphyrine chloride (Fe(III)-TPP-Cl) attached to a graphite electrode and immersed in an aqueous electrolyte solution was applied to determine the standard Gibbs energies of transfer of cations between water and nitrobenzene. The reduction of Fe(III)-TPP-Cl prompts the transfer of the cations from the aqueous to the organic phase. The system is chemically and electrochemically reversible.

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