Distance-Based Paper Device for the Determination of Salicylic acid Using the Reaction of Fe³⁺ and Polyethyleneimine to Form Complex Compounds

This research focuses on the analysis of salicylic acid using a distance-based measurement method on a paper-based analytical device (PAD). The objective is to determine the amount of salicylic acid in compliance with the regulations of the Food and Drug Administration (FDA), which allows the use of salicylic acid at concentrations not exceeding 2.00% in skincare products to prevent skin irritation and damage. In this research, a distance-based measurement method was selected due to its affordability, the lack of need for specialized equipment, portability, simple procedure, and improved accuracy in results through precise distance measurements. The method relies on the complexation reaction of Fe³⁺- polyethyleneimine (PEI) coated onto the paper, which initially appears light yellow. Upon reaction with salicylic acid, the color changes to orange. The concentration of salicylic acid is directly proportional to the length of the orange-colored band on the paper device. However, the specificity of colorimetric band formation still requires further study. It is anticipated that Fe³⁺ can still react with ascorbic acid, resulting in a fading of the color. Therefore, more research is needed to explore the specificity of these reactions, as well as to further improve the detection efficiency of the equipment.