Planar Microstrip Antenna Sensor for pH Measurements

This paper presents a planar microstrip antenna sensor for monitoring the pH of different solutions. The proposed sensor uses radio frequency (RF) signals to identify different pH levels in millilitre drops of liquid solutions. The sensor was tested using a drop of 0.2 mL of five pH standard solutions applied on a small sensing area of one cm
²
. The sensor showed a good frequency shift of 180 MHz between our lowest acidic solution having a pH of 3 and the highest basic solution having a pH of 10. For probing different fluids, the sensor has 3.1 % resonant frequency shift between air and the solution having pH 3 and 0.26 % between air and the solution having pH 10. The measured frequency shift as a function of pH values showed good second order polynomial fit with a fitting factor of 92.16 %. The reusability, sensitivity to small pH changes, and simple operation of the sensor, make it a good candidate for a new microwave based sensor dedicated to online pH monitoring applications.