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. 2021 Aug 5;7(3):109. doi: 10.3390/gels7030109

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Time-lapse images indicating the color change of the hydrogel doped with phenolphtalein, as a floating pH meter for NH₃ (upper) and NaOH (bottom) solutions.