A solvent-free method for preparing improved quality ion-selective electrode membranes

Abstract

A solvent-free method for making hot-pressed ion-selective membranes has been developed. The method not only overcomes the expensive use and recovery of organic solvents but also gives rise to superior quality tougher and longer-lived membranes. As described the method has been applied to an established nitrate selective ion exchange electrode membrane. The technique however should have a much wider general applicability in membrane technology. The method used a combination of milling technology with a clay-based filler (PoleStar^TM 200R). N,N,N-triallyl leucine betaine (nitrate sensor), polystyrene-block-polybutadiene-block-polystyrene (SBS polymer), 2-nitrophenyl octyl ether (solvent mediator), dicumyl peroxide (free radical initiator) and PoleStar^TM 200R (filler) which were mixed using a rolling mill without solvent. After cross-linking and appropriate conditioning, the resulting rubbery membrane exhibited a linear response from 1400 to 0.07 ppm nitrate-N with a slope of −58.5 mV per decade. The limit of detection was 0.01 ppm nitrate-N and the selectivity coefficient for nitrate against chloride, K^pot_{NO3⁻, Cl⁻}, was 4.0 × 10⁻³. A field evaluation in agricultural drainage waters of the resulting clay-based membrane was performed for 15 weeks. The nitrate levels determined with the novel nitrate-ISE compared favourably with the standard method using a nitrate-ISE previously developed by our group (R² = 0.96, y = 1.1x + 0.55). Clay based membranes had an approximately threefold improvement in both tensile strength and resistance to penetration relative to the clay free membranes. Field performance was also improved by at least a 20% increase in lifetime.