Evaluation of chloride and sodium ion selective electrodes (ISEs) for use in biological regenerative life support systems

Interest in longer range and duration manned extraterrestrial missions are on the rise, and have been made possible by technological leaps in recent decades. Recirculating hydroponic systems are researched as a method of providing regenerative sources of food, oxygen and clean water. For these purposes accurate and low-maintenance monitoring methods for water quality and nutrient solution levels in hydroponic systems are necessitated. One possible method of monitoring individual nutrients is the use of potentiometric sensors, like ion-selective electrodes (ISEs). In this thesis ion selective electrodes for chloride (HACH ISECl181) and sodium (HACH ISENa381) have been evaluated for use in hydroponic systems. The electrodes have been tested with regards to interferences, drift and accuracy. An at-line sampling and analysis method with the electrodes has also been tested, with regards to electrode drift and biofouling. Most ions present in nutrient solutions were found to impact the electrodes. Nitrate and sulphate had the largest impact on the chloride electrode, while potassium and magnesium was found to have the largest impact on the sodium electrode. Interfering ions were determined by the preparation of calibration curves in solutions with analyte and interfering ions, and by an experiment with spiking of interferences into a sodium chloride solution. The chloride electrode was found to drift significantly over a period of 12 days in a synthetic nutrient solution, while the sodium electrode was more stable. Both electrodes were used to analyze the chloride and sodium contents in samples collected from a hydroponic experiment. Neither electrode provided sufficient accuracy when compared to the alternative methods, ion chromatography for chloride and ICP-MS for sodium. Two flow cells for the electrodes were designed and implemented at-line in a hydroponic system, and an experiment was conducted over a period of 4 weeks and 5 days. When compared to alternative methods the electrodes still had subpar accuracy, and biofouling of the chloride electrode was observed after 4 weeks. No biofouling was observed on the sodium electrode over the duration of the experiment. More research into the influence of interference on specific ion selective electrodes is recommended before implementation into hydroponic systems. Flow cell design should be improved, the volume used minimized, and anti-biofouling methods that do not negatively impact plant health should be investigated.