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Simple and economical colorimetric strips for measuring manganese (Mn) in natural waters are described. For their construction, leucomalachite green (LMG) embedded in a Nafion® polymeric matrix was immobilized on a polyvinylchloride surface. Upon immersing the strips in the sample, any soluble manganese(II/III) present catalyzed the oxidation of the LMG base to malachite green by adding sodium periodate. The observed color change is related to the manganese concentration and can be quantified using a field device constructed using commercial red–green–blue sensors and an Arduino® board. A linear response from 0.1 to 0.6 μmol L⁻¹ manganese (limit of detection: 0.1 μmol L⁻¹) was observed. The signal for strips prepared on 7 d gave a relative standard deviation equal to 13%. The strips showed good agreement with results obtained by ICP‐MS in porewater collected on Spiekeroog Island …