pH Dynamics of Tricaine Methanesulfonate (MS‐222) in Fresh and Artificial Seawater

Tricaine methanesulfonate (MS‐222) solutions are used routinely for clinical and experimental anesthesia of a variety of aquatic organisms. It is commonly accepted that MS‐222 alters pH when added to freshwater, and buffering freshwater anesthetic solutions is considered to be the standard of care. However, buffering saltwater solutions is considered by many to be unnecessary; some individuals question whether any water with high natural mineral hardness needs to be buffered when MS‐222 is added. To assess the need for buffering, we studied the dynamics of pH change when adding clinically relevant amounts of MS‐222 to artificial seawater and to freshwater of varying hardness. We found that artificial seawater does not possess adequate buffering capacity to prevent a physiologically significant drop in pH when MS‐222 is added, and hardness does not accurately predict the reaction of water pH to the addition of MS‐222 in clinically relevant concentrations. Our data for salt water suggest that MS‐222 solutions should be buffered to saturation with sodium bicarbonate and in freshwater should be titrated to the particular water source to satisfy the optimal pH of the species being anesthetized.