Comparison of trihexadecylalkylammonium iodides as ion-exchangers for polyacrylate and poly(vinyl chloride) based iodide-selective electrodes

Potentiometric properties of iodide-selective electrodes were tested on the example of sensors with either poly(vinyl chloride) (PVC) or polyacrylate based membranes (containing n-butyl acrylate, acrylonitrile and/or isodecyl acrylate units). Sensitivity to iodide ions was achieved by the presence of ion-exchanger: trihexadecylalkylammonium ions with different number of carbon atoms in the alkyl chain (3, 7 or 12).Potentiometric characteristics were linear, usually within iodide concentration range from 0.1 M to 10−6 M, with slope close to Nernstian. In the case of PVC and polyacrylate (copolymer of isodecyl acrylate and acrylonitrile) membranes containing acrylonitrile units in the polymer, increase in the length of alkyl chain in the ion-exchanger results in improvement of selectivity and lowering the detection limit of the sensor (down to around 10−7 M). A different picture was obtained for poly(n-butyl acrylate) membrane sensor, in this case the analytical parameters for ion-exchanger with 12 carbon atoms substituent are less promising than methyl- or especially heptyl-substituted trihexadecylammonium iodide. This can result from steric hindrances in reaching anion exchanging site in the membrane. The optimized membrane composition (pBA membrane and 7 carbon atoms in the alkyl chain of ion-exchanger) was used to detect I− changes in the range 10−5 to 10−4 M in the presence of 10−2 M KCl.