PVC membrane-based portable ion analyzer for hydroponic and water monitoring

Rapid on-site measurement of chemical ions in hydroponic and water solutions would allow efficient management of nutrients used for plant production as well as early monitoring of chemical pollutants. This paper reports the development of an embedded portable analyzer incorporated with a sensor array of polyvinyl chloride (PVC) membrane-based ion-selective electrodes (ISEs) to directly measure the concentrations of NO3, K, and H ions in hydroponic solutions and water. The developed ion analyzer consisted of an AVR microcontroller, five channels of sensor inputs, a 16-bit analog-to-digital converter, a 7-inch LCD touch panel, and SD memory card-based data storage. The use of a two-point normalization method consisting of sensitivity adjustment followed by offset compensation was effective in minimizing signal drifts resulting from a series of sample measurements while reducing variability in response among multiple ISEs during replicate measurements in practical manners. The sensitivity and predictive capability of the PVC membrane-based H ISEs, in conjunction with a three-point calibration method, were satisfactory, showing sub-Nernstian slopes of 51.9-56.1 mV/decade over a pH range of 4-10 (R2 > 0.96) and providing results in close agreement with the results of a conventional pH meter (a nearly 1:1 regression slope and a y-intercept near 0). A response time of 50 s and a wide sensitivity range of 11-884 mg L−1 measured with the prototype portable ion analyzer equipped with PVC membrane-based NO3 and K ISEs, each with a lifetime of 60 days, enabled direct analysis of hydroponic and water samples without the need to dilute samples. Strong linear relationships between the prototype ion analyzer and the standard instrument methods (R2 > 0.97, slopes ranging from 0.89 to 1.24) were exhibited.