A novel, fast responding, low noise potentiometric sensor containing a carbon-based polymeric membrane for measuring surfactants in industrial and environmental applications

A new high-sensitivity potentiometric sensor for anionic surfactants was fabricated using the dimethyldiocta-decylammonium-tetraphenylborate (DDA-TPB) ion associate as an ionophore that was incorporated into a liquid PVC membrane. Carbon powder was used for immobilization of the ionophore in the membrane, thus significantly reducing its ohmic resistance and reducing its signal drift. The sensor exhibits a sub-Nernstian response for both dodecylbenzenesulfonate (DBS) and dodecyl sulfate (DS) in H2O (55.3 and 58.5 mV/decade of activity, respectively) in a range between 3.2x10(-7) and 4.6x10(-3) M for DS and 2.5x10(-7) and 1.2x10(-3)M for DBS. The sensor also exhibited a sub-Nernstian response for DS and DBS in 10 mM Na2SO4 (55.4 and 57.7 mV/decade of activity, respectively) between 2.5x10(-7) and 4.6x10(-3)M for DS and 1.5x10(-7) and 8.8x10 M for DBS. The detection limits for DS and DBS in H2O were 2.5x10(-7) and 2.0x10(-7) M and in 10 mM Na2SO4 the detection limits were 2.5x10(-7) and 1.2x10(-7) M, respectively. The response time of the sensor was less than 5 s for changes at higher concentration levels (above 1x10(-4) M) in both water and 10 mM Na2SO4. At lower concentrations (below 1x10(-5) M) the response times were 8 and 6 s in water and 10 mM Na2SO4, respectively. The signal drift of the sensor was 1.2 mV/hour. The new carbon-based sensor exhibited excellent selectivity performance for DS over ahnost all of the anions commonly present in commercial formulations and it was successfully employed as an end-point detector in potentiometric titrations of anionic surfactants in a pH range from 3 to 12. Three-component mixtures containing, sodium alkanesulfonate (C-10, C-12 and C-14) were successfully differentially titrated.