Drug Research Information Bulletin - Buffering Oxytetracycline Hydrochloride Immersion-Marking Solutions with Sodium Phosphate Dibasic

Oxytetracycline hydrochloride (OTC-HCL) water soluble powder is approved in the U.S. for use in the skeletal marking of finfish fry and fingerlings by immersion at dosages of 200 -700 mg/L active OTC for 2 -6 h. This compound is acidic, and thus most OTC-HCL solutions will likely need to be buffered to prevent or minimize mortality in treated fish due to low pH. Anhydrous sodium phosphate dibasic (SPD) is commonly used as such a buffer, probably because it is relatively safe to humans (Stecher et al. 1968) and its buffering effects are relatively easy to control (Fielder 2002). Our study was designed to show how pH decreases when OTC-HCL is added to various “source waters” to produce 700 mg/L active OTC solutions and how pH increases when SPD is added to buffer such solutions to pH 7.