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This work aimed to develop a technique for detecting Cu2+ in water samples with naked eyes by using betacyanin extracted from white dragon fruit peels as a reagent. In this method, 20 mL of betacyanin solution was coated on 10 grams of filter paper No.1 with 0.5 cm. in diameter for 10 minutes. Then the filter papers coated with betacyanin solution were left in the dark until they dried out, and Paper-Betacyanin was obtained. This product was applied for the determination of Cu2+ in water samples by only dropping 1 piece of Paper-Betacyanin in a mixture of 250 µL of pH 7 buffer solution and 100 µL of water sample. The tested solution was found to change from pink to old rose color. The detection limit was found at 30 µM. This developed method also determined Cu2+ in semiquantitative analysis, which was divided into 3 color shades depending on Cu2+ concentrations. Moreover, maximum lifespan of Paper-Betacyanin kept in dark and dry containers was more than 60 days.
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