2.2. Preparation of the Ion-Selective Electrodes

Six membranes of different compositions were prepared by using several plasticizers (NPOE, TCP or DOS) and ion exchangers (cation-exchanger KTClPB or anion–exchanger TDMACl). The membranes were prepared by dissolving approx. 100 mg of PVC, 200 mg of the corresponding plasticizer and 2 mg of the ion exchanger in 3 mL of THF. The exact composition of each membrane is presented in Table 1. Each solution was poured into a Fluka glass ring (inner diameter 28 mm, height 30 mm) placed on a Fluka glass plate and allowed to settle overnight until total evaporation of THF had occurred, thus obtaining the corresponding plasticized PVC membrane. A 6 mm diameter piece was cut out with a punch (Fluka, Spain) and incorporated into an ISE electrode body (Fluka) containing 1.0 × 10⁻³ M KCl as the internal filling solution. The electrodes were conditioned in KCl 1 × 10⁻³ M solution until they reached a constant potential baseline (±0.5 mV/min). Normally, overnight conditioning ensures such a response. When not in use, the electrodes were kept in KCl 1 × 10⁻³ M solution. Notably, the utilized ISEs were already reported in the literature, and the figures of merit related to calibration graphs of individual cations/anions were assessed [17,18]. Accordingly, such experiments have not been herein performed, and so, the electrodes were directly utilized to measure in the water samples.

Detailed membrane compositions and ion-selective electrodes (ISEs) labels.