A portable and low-cost parallel-plate capacitor sensor for alkali and heavy metal ions detection

A portable and low-cost design of parallel-plate capacitor sensor is proposed and investigated for detection of heavy and alkali metal ions concentration in liquid solution. The application of a thin and long-sided parallel PCB plate allows one to measure the dielectric response of each sample through capacitive sensing technique. The measurements were based on the peak output voltage corresponding to the capacitance of the output measured by an oscilloscope with metal ions concentration of the solution varying from 0 to 10ppm and input frequency ranging from 1kHz to 10MHz. The ICF of Li, Na and K in chloride solution was 2.4, 2.1 and 1.9MHz, respectively. The ICF of Na and K in hydroxide solution was 1.5MHz and 1.1MHz, respectively, while the ICF of Pb(NO3)2 was 490kHz. The detected ICF was applied to further construct the portable sensor system, which is simple in design, low-cost in fabrication and easy to operate. The finding of ICF of each chemical element is necessary to solve the selectivity challenge of the capacitance-based sensor, which will be appropriate for chemical, environmental and engineering applications.