Development of a low-cost semi-automated robotic orthophosphate system for batch analysis

Monitoring the level of nutrients in soil and their availability for crops can be time-consuming or require expensive instrumentation. This work describes a low-cost (<€500) portable, semi-automated colourimetric orthophosphate (PO₄^3-) analyser supplemented with 3D printed parts. Colour development was based on the phosphomolybdenum blue formation coupled with spectrophotometric detection using a low-cost LED-photodiode assembly. The batch analysis technique required only minimal autonomous additions of reagents to the reaction vessel. In addition, the reaction time was reduced with vigorous automated stirring of the small quantity of reactants. Continuous monitoring of the absorbance throughout the reaction also decreased contact time, eliminating the prerequisite of a blank and warm-up time, customarily associated with colourimetric measurements. The semi-automated Robotic Orthophosphate System (saROS) has a linear dynamic range between 10-750 μg L^-1 P-PO₄^3-, and a limit of detection of 3 μg L^-1 P-PO₄^3- with good repeatability (RSD of 2.4%). In addition to portability and low cost, the prototype is an accurate and reproducible device for measuring phosphorus in aquatic ecosystems and soil extracts.