The neutral species of the weak base trimethoprim is more toxic to willow trees (Salix viminalis) than the cation

The acute toxicity of the veterinary antibiotic trimethoprim (TMP) to willow trees was tested at three different pH levels in hydroponic solutions with TMP concentrations of 1, 10, 100 or 1000 mg/L. The pH variation was achieved by using ammonium (pH 4.3, low) or nitrate (pH 6.4, medium) as nitrogen source, and by additional tritration with KOH (pH 8.15, high). TMP is a weak base that dissociates at pKa 7.2. A statistically significant higher toxicity of the neutral form was observed, i.e. a higher toxicity at medium and high pH than at low pH. A toxic effect of the neutral form was observed at an external concentration of 100 mg/L, while the toxicity of the cation appeared only at the concentration of 1000 mg/L. The result of the study shows that the toxicity of TMP to willow trees is low, but also that the toxicity of weak bases varies with pH.