Ammonium-Selective Neutral Carrier Membrane Electrodes

Abstract

Ammonium-selective PVC membrane electrodes based on a macrotetrolide antibiotics mixture (tetranactin 65%, trinactin30%, dinaction 5%) as neutral carrier were prepared. The preparation of membranes mainly followed the conventional method. Two series of membrane compositions were tested. First, carrier content was fixed to 5wt% and the plasticizer content was varied. Secondly, the plasticizer content was fixed to 70wt% and the carrier content was varied. The electrode membranes summarized in Table 1 uniformly showed the Nernstian response over 10^-5-10^-1mol·dm^-3 NH₄Cl (Fig.2). The lifetime of the electrodes which were stored in 0.1 mol·dm^-3NH₄Cl was 3^-5 months or longer. However, observed that the electrode membrane which contained a small amount of neutral carrier tended to have a short lifetime. The selectivity coefficients of univalent cations were measured by the separate solution method; K⁺ 3.3 ×10^-1, Rb⁺ 7.0 × 10-2, Na ⁺ 4.4 × 10-3, Cs⁺ 3.3 × 10-3, Li⁺4.4 × 10^-5. Interferences of bivalent cations (Ca2⁺, Mg2⁺, Ba2⁺) were less than that of lithium ion. The practiCally important selectivity coefficients were approximately O.23 for potassium ion and 0.0024 for sodium ion according to the fixed interference method (5.0 × 10^-8mol. dm^-3KCl, 0.15 mol. dm^-3 NaCl, respectively), and were independent of membrane composition (Table 2). Anion interferences were marked for thiocyanate, iodide, and nitrate ion (Fig.6). In the range of pH 4-8, the electrode response was normal (Fig.7). The response time of electrodes was measured by a manual set-up which comprises two nozzles located toward the electrode membrane. As summarized in Table 3, the measured response times depended upon composition of solution, concentration ratio, direction of concentration change. Response time t9, for the typical concentration change 10-3 to ca.10^-2mol dm^-3 (potential change is 50 mV) was 70 ms, and that for the change of opposite direction was 110 ms. Adding 5.0 × 10^-3mol. dm^-3 KCl and/or 0.15 mol. dm^-3 NaCl to NH₄Clsolution increased the response time. However, these response times areshortenough, so that. these electrodes may be useful for medical analysis.