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This report describes the development of foldable paper-based analytical devices (PADs) to perform redox titrations. Paper devices were designed to contain three spot tests, which were wax printed and folded to create a three-layer structured platform and to promote the sample transport based on vertical flow. The proposed devices were explored for the visual determination of the alcoholic content in whiskey samples. For this purpose, a classical permanganometry reaction was employed to allow the indirect determination of ethanol based on the required amount of oxalic acid to react with the excess of permanganate in acidic medium. The endpoint of the redox titrations performed in different alcoholic concentrations was measured and revealed a good linear behavior for the ethanol concentration range between 0% and 50% (R2 = 0.992), achieving a limit of detection equal to 2.1%. The alcoholic content was determined in a total of 44 whiskey samples seized by the Brazilian Federal Police. When compared to genuine samples and using an established cut-off limit, 73% of the seized samples were correctly classified as whiskeys containing adulterated alcoholic content. The proposed method was compared to a reference protocol and no difference was observed at the confidence level of 95%. The instrumental simplicity, the low cost, the sample volume requirement, the short analysis time and mainly the inherent portability make these devices quite attractive for on-site forensic applications.

This study describes the use of a smartphone for monitoring acid–base titrations on wax printed paper microzones. An array of twelve microzones of 5 mm diameter each was wax printed on filter paper. The analytical performance of the proposed devices was explored with acid–base titrations examples, where jaboticaba peel extract was used as a natural pH indicator. The color intensity was captured using a smartphone and analyzed through a free App named Photometrix®. Before titrations, color intensity versus pH was calibrated to be used as a reference in titrations as (i) strong acid versus strong base; (ii) strong base versus strong acid; and (iii) weak acid versus strong base. In all examples, images were obtained after the addition of each aliquot of titrant solutions. The obtained titration curves showed the same behavior as the conventional titration curves. After evaluating the feasibility of the proposed methodology, the concentration level of acetic acid was obtained in three vinegar samples. Although the obtained values ranged from 5% to 8% compared to the concentrations on the conventional method, the proposed methodology presented high analytical reliability. The calculated concentrations of acetic acid in three samples ranged from 3.87% to 3.93%, and the proposed methodology did not significantly differ from classic acid–base titration at a confidence level of 95%. The acid–base titration on paper-based devices is outstanding, since any titration can be completed within 5 min using 20 µL volumes. Besides, the use of a smartphone to capture images followed by analysis in a free app offers simplicity to all users. The proposed methodology arises as a new strand to be exploited in the diffusion of the analytical chemistry education field as well as an alternative for quantitative analysis with extremely simplified instrumentation.