3D-Printable RGB LED Photometer Controlled by an Arduino-Python Interface for Molecular Absorption Applications in Chemistry Laboratories

Among quantitative analysis techniques, molecular absorption methods in the ultraviolet/visible region are the most commonly employed in chemical laboratories worldwide. In general, the absorbance measurements are performed with a benchtop ultraviolet-visible spectrophotometer or photometer that has the characteristic of being a “black box”; that is, they cannot be opened to show to the students their basic components and functioning principles. Therefore, for a better understanding of the molecular absorption fundamentals, it is beneficial to use a device that allows showing its basic components. Herein, we report a low-cost photometer developed by compactly assembling a RGB LED, a light sensor, and an Arduino microcontroller in a 3D-printed housing. To manage the RGB LED and the light sensor and to compute and visualize absorbance values, an interface between Arduino and Python was created. The photometer was connected via USB port to a computer. We demonstrated the functionality of the photometer by studying Lambert-Beer’s law in colored aqueous standard solutions such as chromium(III) nitrate, potassium permanganate, and copper sulfate. The plots of absorbance versus concentration have an excellent adjusted coefficient of determination: R_adj² = {0.99977, 0.99952, 0.99983} for chromium(III) nitrate, potassium permanganate, and copper sulfate, respectively, suggesting that the photometer can be used to validate Lambert-Beer’s law. In addition, a Didactic Guide was designed with learning objectives and a step-by-step process to guide the assembly of the photometer. Moreover, the photometer was used with students in an Instrumental Analysis course. The students made the electrical connections, uploaded the code to the Arduino, assembled the photometers, measured the absorbances, constructed the calibration curves, and determined the concentration of an unknown solution. Their results proved to be both precise and accurate. Therefore, the present work provides an affordable photometer for absorbance measurements in colored solutions, contributes to the democratization of expensive instruments for students, and is valuable for the teaching of basic concepts of molecular absorption spectroscopy in Analytical Chemistry and Instrumental Analysis courses.