Purity is a critical factor that reflects the concentration of the active ingredient in the original pesticide formulation, typically expressed as a percentage. It serves as a primary quality indicator, with higher purity generally indicating better product quality. According to the quality standards set by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO), the minimum acceptable purity for pesticides should be over 90%. In China, the purity of most original pesticide formulations also meets this standard. However, when the purity is low, the presence of excessive impurities can lead to several negative consequences.
Firstly, high levels of impurities may cause phytotoxicity to crops. For example, in 2000, the use of bensulfuron-methyl in rice fields in Meihekou City, Jilin Province, resulted in severe crop damage. Although bensulfuron-methyl is usually safe for rice seedlings, researchers found that the excessive presence of impurities—JP-003 and JP-004—was likely responsible for the toxicity. Secondly, impurities can increase the toxicity of pesticides to humans. Methamidophos, for instance, has a pure LD50 of 30 mg/kg in rats, but some domestic emulsifiable concentrates have an LD50 as low as 13.6 mg/kg, indicating a significant increase in toxicity. This is due to the presence of five impurities in the crude amine-phosphorus oil, which are more toxic than the pure compound itself.
Moreover, impurities can interfere with chemical analysis methods used to determine the active ingredient content. Since these impurities often contain similar elements or atomic groups, they can skew test results, leading to inaccurate assessments of the true active ingredient concentration. Additionally, impurities can make the processing of powders more difficult, as they lower the freezing point of the original drug, making it harder to grind into fine particles. Finally, impurities can reduce the stability of the active ingredient and contribute to environmental pollution once the pesticide is applied.
In addition to purity, pH is another essential quality parameter for both the original pesticide and its formulations. The pH measures the amount of free acid or base present in the formulation or the hydrogen ion concentration. Controlling pH helps prevent the degradation of active ingredients during storage and avoids changes in the physical properties of the preparation, which could lead to phytotoxic effects when applied. It also helps assess the potential corrosiveness of the pesticide on packaging materials. FAO standards often express this in terms of acidity (as Hâ‚‚SOâ‚„) or alkalinity (as NaOH), with the original powder typically requiring less than 0.1% to 0.2%. In China, the moisture content of powders and wettable powders is also strictly controlled to maintain the stability and dispersibility of the formulation, ensuring effective application and good coverage on plant surfaces.
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