Phosphate and arsenate electro-insertion processes into a N,N,N′,N′-tetraoctylphenylenediamine redox liquid

The electro-insertion of ions is a well-known phenomenon, which allows the transfer of anions or cations across phase boundaries to be monitored and driven electro-chemically. Extremely hydrophilic anions, such as phosphate and arsenate, are not usually observed to undergo electro-insertion. It is shown here that at organic redox liquid|water|electrode triple interfaces these anions can be forced electro-chemically to transfer into organic media. The transfer process of phosphate anions from aqueous buffer solutions into organic microdroplets of the redox liquid N,N,N′,N′-tetraoctylphenylenediamine (TOPD) is pH and concentration sensitive. It is shown that phosphate is transferred in the form of PO₄HK^- in the presence of phosphate buffer. Two distinct potential regions are identified and attributed to (i) interfacial redox processes at the liquid|liquid interface associated with deprotonation and (ii) bulk redox processes associated with anion transfer from the aqueous to the organic phase. The comparison of phosphate and arsenate electro-insertion processes suggests that arsenate is less hydrophilic and transferred into the organic phase preferentially.