Increasing the Biological Activity of Weak Acid Herbicides by Increasing and Decreasing the pH of the Spray Mixture

Abstract The pH of the spray mixture controls the solubility and ionic state of weak acid herbicides and thus influences their uptake and biological activity. When the pH of the spray water is below the pKa of the herbicide, increasing pH can increase solubility and improve activity when herbicide solubility limits uptake. However, raising the pH above the pKa makes the weak acid anionic and thus may make it more difficult to penetrate into the lipophilic cuticle and the negatively charged membrane and cell wall. Decreasing the pH below the pKa converts the weak acid into a neutral or unionized form and thus makes it easier to penetrate these lipophilic and negatively charged barriers. pH also influences other herbicide properties including chemical stability, volatility, and chemical compatibility. Thus, manufacturers need to balance a number of properties when they adjust the pH of their adjuvant and herbicide formulations. These studies show significant differences in the biological activity of several herbicides when the pH is increased and decreased with a range of surfactant types. These results support the concept that the physicochemical properties of the herbicide and adjuvant should be matched for optimum activity.