A fast and accurate UV–vis method for the quantification of polylactic acid in biodegradable plastics

Polymer blends are used to achieve tailored properties, but the quantification of their components remains difficult. We developed an ultraviolet–visible spectrophotometric method with a two-step pretreatment that involves the conversion of polylactic acid (PLA) into methyl lactate via alcoholysis and then into sodium lactate through alkaline hydrolysis, followed by detection with ferric chloride at wavelengths from 400 to 410 nm. The method demonstrated linearity, with a correlation coefficient above 0.99. For PLA and polypropylene blends with proportions that ranged from 10 to 100 wt%, the bias was less than ± 4%, and the relative standard deviation was less than 7%. Conversion and identity were confirmed via Fourier transform infrared and proton nuclear magnetic resonance spectroscopies. Selectivity was validated in blends with polybutylene adipate–coterephthalate, polyhydroxybutyrate, and cellulose acetate, with absorbance deviations within ± 0.03. This cost-effective method enables reliable quantification for biodegradable plastic certification.