A new liquid membrane surfactant sensor based on a 1,3-didecyl-2-methylimidazolium-tetraphenylborate ion-exchange complex was tested indifferent ethoxylated nonionic surfactants (EONS) and polyethylene glycols. The sensor also responded to tetraphenylborate (slope = -56.1 +/- 0.8 mV/decade of activity) in the 1 x 10(-6) to 2 x 10(-3) M concentration range. The selectivity performance of the sensor toward alkaline, alkaline earth, and heavy metal cations was investigated. The sensor exhibited a fast response (within a few seconds), when in solution with some metal cations, for a 10-fold concentration change of Triton X-100. Its main application is in indicating the end-point in a potentiometric titration of EONS using sodium tetraphenylborate as the titrant. Several pure and technical grade EONS and polyethylene glycols were successfully titrated. The sensor can be used for titrations within a pH range of 3-11. The resulting potentiometric titration curves revealed an analytically usable inflection for all investigations, enabling reliable equivalence point detection using the first derivative method. The sensor was also applied to the study of barium ion reaction stoichiometry and Triton X-100 pseudoionic complex formation, as well as its reaction with tetraphenylborate. (C) 2009 Elsevier Ltd. All rights reserved.