Detection of Iron (Fe) Concentration in Water Using Light Absorbance-Based OPT101 Photodiode Sensor

¹ Department of Physics, Faculty of Mathematics and Sciences, Universitas Negeri Jakarta, Jakarta 13220, Indonesia
² Faculty of Science and Engineering, University of Groningen, Groningen 9747 AG, The Netherlands

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 2 July 2026

Abstract

Water contaminated with heavy metals, especially iron (Fe), can have adverse effects on health and the environment. This research developed an optical sensor-based Fe concentration detection system in water with the principle of light absorbance. The system is designed using an ESP32 microcontroller, OPT101 photodiode sensor, and OLED display as a data viewer, and features a WebSocket connection for real-time data transmission to local websites. The light source uses a green HPL LED (±515 nm) that corresponds to the peak absorption of the Fe-fenantrolin complex. The measurement mechanism refers to the Lambert-Beer Law by comparing the intensity of the blank light (I₀) and the sample (I₁). The test results showed that the system was able to detect Fe in the range of 0.01–10 ppm with an average measurement error of 6.36%. Testing on samples of PAM water, groundwater, and drinking water showed that Fe levels were below the threshold of 0.3 ppm in accordance with the Minister of Health Regulation No. 32 of 2017. The developed system has the potential to be an IoT-integrated water quality analysis tool, thus supporting efficient and real-time monitoring of water pollution, especially in areas with limited laboratory access.