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Updated: Jun 28, 2025

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation
Published on: April 11, 2014
Cations and Anions Affect the Speed of Sound in Water Oppositely
Yiwei Jiang1, Theo G M van de Ven1
1Pulp & Paper Research Centre and Quebec Centre for Advanced Materials (QCAM) Department of Chemistry, McGill University, 3420 University Street, Montreal, QC H3A 2A7, Canada.
Abstract:
Identifying the composition of a solution using acoustics remains a challenge. It is known that for low salt concentrations the speed of sound in water increases linearly with the concentration of the electrolyte, but the contribution of individual cations and anions is unknown. We introduce the concept of intrinsic sound speed A to quantify the contribution of ions to the speed of sound. We found that cations increase the speed of sound in water whereas anions decrease the speed of sound. Hydration layers around the ions play a major role. Because cations have a hydration layer thicker than that of anions, their contribution to the speed of sound is larger than that of anions. Experimental data on salts not used to determine the contribution of individual ions are in quantitative agreement with the predicted values. Our method can be applied to various systems containing small quantities of ions, molecules, or particles. With the knowledge that cations increase the speed of sound, we were able to explain previously unexplained data in the literature.
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