Sulfur dioxide (SO2) is a common air pollutant with harmful effects on human health and the environment. In previous studies, various gas sensors using functionalized polymers have been developed, and Fourier transform infrared (FTIR) spectroscopy has been widely applied for monitoring gas adsorption. However, limited work has quantitatively linked FTIR absorbance with SO2 uptake using eco-friendly materials. We hypothesized that a polyvinyl alcohol (PVA)-based membrane functionalized with m-phenylenediamine (M-PDA) can be a sustainable and accurate sensor for SO2 adsorption. In this research, a series of PVA/M-PDA membranes were fabricated and exposed to SO2 under controlled concentrations and durations. FTIR spectra were recorded to analyze characteristic peaks at 1250 and 1723 cm−1, which were correlated with the amounts of SO2 adsorbed measured using a quartz crystal microbalance. Among all samples, the direct chamber standard test (DCST) membrane exhibited the highest linearity (R2 = 1.000) with only a 2% deviation between theoretical and actual adsorptions, compared with errors as high as 40.5% in other samples. The strong linear relationship between FTIR absorbance at 1250 cm−1 and SO2 uptake demonstrates the membrane’s sensing capability. These findings suggest that the eco-friendly PVA/M-PDA sensor provides a reliable and quantitative approach to SO2 monitoring. The proposed material combines environmental compatibility with analytical precision, making it a promising platform for real-time gas sensing applications.