||Triangular silver nanoplates with sharp corners exhibit fascinating optical properties, but their shapes are not thermodynamically stable. However, this intrinsic instability can be used to realize novel applications. In the current work, we designed such application, one in which the spontaneous rounding of the sharp corners of the Ag nanoplates exposed to bromide ions (Br-) was used to develop a novel sensing platform for the quantification of water in water-ethanol mixtures. This relationship, which was attributed to the previously determined dependence of the solubility of KBr on the volume fraction of water in water-ethanol mixtures, allowed for the quantification of water in water-ethanol mixtures. We believe that the nanocrystal-based sensing platform holds great potential for use as a analytical tool for the quantification of water in the fine chemical, pharmaceutical, and food and beverage industries.