Characterization of a Ag + -Selective Electrode Based on Naphthalimide Derivative as Ionophore

A naphthalimide derivative has been explored as neutral ionophore for Ag + -selective electrode. Potentiometric response revealed that electrode based on the proposed ionophore with 2-nitrophenyl octyl ether as solvent in a poly (vinyl chloride) membrane matrix shows a measuring range of 1.0 × 10 -6 -1.0 × 10 -2 M with a slope of 50.4 ± 0.3 mV/decade. This electrode has high selectivity to Ag + with respect to alkaline, alkaline earth and other heavy metal ions.


Introduction
The favorite donor binding site atom based on the hard-soft acid base (HSAB) concep t is still the primary factor, [1] so the most of the reported heavy metal ionophores for use in ion selective electrodes (ISEs) possess sulfur and/or nitrogen ato ms. [2] Among the heavy metal io ns, Ag + has attracted the interest of researchers, and many compo unds have been used to construct Ag + -ISEs. [2][3][4] Among these compounds, crown ethers show strong ability to form remarkably stable and selective complexes with heavy metal ions and have been found to exhibit interesting host-g uest complexatio n characteristics. [3] Especially, crown ethers containing nitrogen and sulfur donor atoms are of interest as they exhibit high affinities towards Ag + . [5] During the last decade, considerable attentio n has been devoted to the constructio n of Ag + -selective electrodes with mixed-do nor crown ethers as ionophores. [2][3][4] In this work, a thiaazacrown ether has been characterized as Ag + -ISE ionophore and the response of polymeric membrane Ag + -ISE has been studied.

Membrane preparation
The membranes were prepared as reported method. [6] The membrane components were dissolved in 3.0 mL of THF and shaken vigorously for at least 2 h, and then poured into a glass ring (30 mm i.d.) fixed on a glass plate and covered with a big beaker. The solvent was allowed to evaporate overnight a t room temperature to give a transparent membrane (thickness of 0.18 mm), and the resulting membrane was cut to size (6 mm i.d.) and glued with a PVC/THF slurry on the top of PVC tube mechanically fixed onto a pipette tip.

Conditioning of membranes and potential measurements
The membranes were conditioned in 0.01 M AgNO3 solution with 0.1 M AgNO3 as inner solution for at least one day. Activity coefficients were calculated according to the Debye-Hückel approximatio n and EMF values were corrected for liquid-junctio n potentials with the Henderson equa tion. The reference electrode Hg/Hg2Cl2 with double junctio n was used with 1.

Selectivity measurements
For selectivity measurements, the electrodes were conditio ned in 0.01 M NaNO3 solutio n overnig ht with 0.001 M NaCl as an inner filling solutio n, and the electrode was firstly measured against the interfering metal ions and then used to measure the response of silver nitrate solutions. The measurement was done in triplicate (using new membrane for one replicate). The interfering ions stud ied here were H + , Li + , Na + , K + , Zn 2+ , Mg 2+ , Ca 2+ , Cu 2+ , Cd 2+ , Pb 2+ and Hg 2+ .

Optimization of membrane compositions
The composition of PVC membrane affects the selectivity, linearity and sensitivity of the electrode and optimization was done by varying the ratio of ionophore and ion-exchanger (NaTFPB). [7] Ag + -ISEs were prepared with different mole ratio of ionophore to NaTFPB (4:1, 2:1, 1:1).

Effect of plasticizers and working concentration range and slopes
The nature of plasticizers such as lipophilicity, high molecular weight, lo w vapor pressure and high capacity to dissolve the substrate and other additives present in the polymeric membrane has large effect on the sensitivity and stability of sensors. [8] We have tested several membranes of different plasticizers DOP, DBP, DOS and o-NPOE in PVC matrix and the results obtained are shown in Table 2. It can be seen from Table 2 that the best results were obtained with the ISEs prepared by using o-NPOE as plasticizer, this indicates the solvent medium of o-NPOE can probably provide the best complexation environment between Ag + and their respective carriers. Electrode based on ionophore with optimal compositions show the best responses over a wide concentration range of 1.0×10 -6 -1.0×10 -2 M with a detection limit 3.3×10 -7 M, the measurement sequence of high-to-low was from 1.0×10 −2 to 3.0×10 −8 M, and the dynamic response time is less than 10 s (Fig. 1).

Lifetime of proposed Ag + -ISE
The high lipophilicity of ionophore and plasticizer ensure stable potentials and longer lifetime for the membranes. [9] The sensor based on o-NPOE and proposed ionophore can be used for two months. During this period, changes in potential were within the standard deviation (± 5 mV). It is important to emphasize that the membrane was kept in dark when not in use in order to avoid the silver nitrate photolysis. Table 3 Potentio metric selectivity coefficients, logKAgL pot , obtained with membrane based on ionophore (ISE No. b).

Potentiometric selectivity of Ag + -ISE
Potentiometric responses suggested that electrode based on the proposed ionophore has high selectivity toward Ag + . The selectivity coefficients of Ag + -ISE were explored based on the so-called "unbiased selectivity coefficient" proposed by Bakker and co-workers. [10] The characteristics of membranes examined based on the proposed ionophore are summarized in Table 3.

Conclusions
In this study, an azathiacrown ether has been characterized as Ag + -ionophore. The potentiometric properties reveal that the proposed compound has good selectivity to Ag + .