Palygorskite and solvatochromic dye in solid-state colorimetric devices for rapid assessment of the amount of water in ethanol fuel

This study presents a novel method for quantitatively determining water content in fuel ethanol utilizing a colorimetric device composed of the methylene blue solvatochromic dye adsorbed on palygorskite, supported by filter paper. The quantitative analysis was based on digital images of the prepared support, wherein the hydrated ethanol interacted with the colorimetric devices, generating different colors according to the water content in the fuel. Digital images were acquired through an innovative system consisting of a photographic camera and compartments for liquid or solid samples coupled to a smartphone. The images were then processed using image processing software (ImageJ v.1.52n) installed on an external laptop, which enabled analysis of the color channels R (red), G (green) and B (blue), as well as the color properties H (hue), S (saturation) and V (brightness). The proposed method was partially validated and compared to the reference method (Karl Fischer ASTM E203/16) for determination of water in fuel ethanol. The results showed good linearity, with determination coefficients of 0.9613, 0.9431 and 0.8835 for R , G and S , respectively. The detection limits of the R , G and S channels were 0.329%, 0.306% and 0.557% m/m, respectively; the quantification limits for the same channels were 0.998%, 0.927% and 1.688% m/m, respectively. The accuracy of the method, as assessed by recovery, was between 98 and 110%. Precision, assessed by the relative standard deviation, was 0.56%, 0.92% and 2.05%, respectively, for R , G and S . The paired t test showed no significant difference (with 95% confidence) between the proposed and reference methods. R was selected as the best color channel for water analysis in fuel ethanol as it exhibited the best validation parameters. This novel method offers a simple, accurate and cost-effective alternative to the conventional Karl Fischer method, potentially facilitating routine water analysis in fuel ethanol. Graphical abstract