Low-Cost Automatic Sensor for in Situ Colorimetric Detection of Phosphate and Nitrite in Agricultural Water

This study proposed a low-cost sensor for in situ automatic monitoring of phosphate and nitrite in agricultural water environments, involving a series of "Fish-Bite" reservoirs, multiple reagent capsules, and a colorimetric sensor. The Fish-Bite reservoir is an alternative to the pumps, valves, and filters that are widely used for water sample collection and also offers a closed cell for chromogenic reactions afterward. Up to two capsules can be embedded in each reservoir to support chromogenic reactions that use two different reagents in sequence. From the results of calibration tests in the laboratory, the limit of detection was found to be approximately 0.01 mg/L for both phosphate and nitrite, with a linear range of 0.01-1.00 mg/L for phosphate and 0.01-0.20 mg/L for nitrite. Furthermore, an in situ experiment was successfully carried out in an irrigation canal beside farmland to demonstrate the practicability and robustness of the device. The averaged concentrations of phosphate and nitrite were 0.0113 mg/L and 0.0383 mg/L, respectively. The relative deviations were 20.2% and 11.7%, respectively, referred to results obtained by using the standard spectrophotometric methods. With the advantages of being robust, fast, and low cost, this in situ device is promising for the formation of agricultural sensor networks.