A Colorimetric Method for Naked-eye Detection of 6PPD in Rubber Products and Wastes

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) has received increasing attention due to its ubiquitous environmental occurrence and the extreme aquatic toxicity of its quinone oxidation product. Given 6PPD's application as an antioxidant in a wide array of rubber products and wastes, cost-effective measurement of 6PPD is important for product and waste management. We herein developed a naked-eye colorimetric method for the rapid measurement (<10 min) of 6PPD in rubber products and wastes with low cost (<$1 per sample). The inspiration for this method stems from the observation of the formation of red-colored products when 6PPD is exposed to oxidants, at concentrations as low as 0.65 mg/L. The remarkable selectivity of the method was evaluated by 15 other structurally diverse phenols and anilines. LC-UV and mass spectrometry results corroborated N-1,3-dimethyl butyl-N’-phenyl quinone diamine (6QDI) as the primary oxidation product. Interestingly, we discovered that the protonated form of 6QDI, rather than its neutral counterpart, was the red-colored reaction product with λmax = 490 nm. We further validated the method by applying it to the measurement of 6PPD in a wide array of rubber products and wastes, with 6PPD detected in 22 out of 50 samples. Strong agreement was observed between the colorimetric method and LC-MS measurements, with the highest concentrations detected in car tire rubber. In summary, this study introduces a rapid and cost-effective approach for rapid screening of 6PPD in rubber products and wastes.