Solvatochromic Dyes as pH-Independent Indicators for Ionophore Nanosphere-Based Complexometric Titrations

For half a century, complexometric titrations of metal ions have been performed with water-soluble chelators and indicators that typically require careful pH control. Very recently, ion-selective nanosphere emulsions were introduced that exhibit ion-exchange properties and are doped with lipophilic ionophores originally developed for chemical ion sensors. They may serve as novel, highly selective and pH independent complexometric reagents. While ion optode emulsions have been demonstrated as useful indicators for such titrations, they exhibit a pH cross-response that unfortunately complicates the identification of the end point. Here, we present pH-independent optode nanospheres as indicators for complexometric titrations, with calcium as an initial example. The nanospheres incorporate an ionic solvatochromic dye (SD), ion exchanger and ionophore. The solvatochromic dye will be only expelled from the core of the nanosphere into the aqueous solution at the end point at which point it results in an optical signal change. The titration curves are demonstrated to be pH-independent and with sharper end points than with previously reported chromoionophore-based optical nanospheres as indicator. The calcium concentration in mineral water was successfully determined using this method.