Purity is a key indicator of the quality of an original pesticide, representing the percentage of the active ingredient present. The higher the purity, the better the quality of the product. According to the quality standards set by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO), the minimum purity for most pesticides should be above 90%. In China, the purity of the original drug typically meets or exceeds this standard. However, when the purity is low, the concentration of impurities increases, which can lead to several negative consequences.
First, excessive 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 crop damage. Although bensulfuron-methyl is generally considered safe for rice seedlings, the incident was linked to high levels of impurities, specifically JP-003 and JP-004, which were found to be harmful. Second, impurities can increase the toxicity of pesticides to humans. Methamidophos, for instance, has a pure LD50 value of 30 mg/kg in rats, but some 50% emulsifiable concentrates have an LD50 as low as 13.6 mg/kg, indicating a higher level of toxicity. This is due to the presence of five impurities in the crude oil that are more toxic than the pure compound itself.
Third, impurities can interfere with chemical analysis methods used to determine the active ingredient content. These methods often measure elements or atomic groups within the active ingredient molecule. Since impurities may contain similar components, they can lead to inaccurate results, making it difficult to assess the true concentration of the active ingredient. Fourth, impurities can affect the processing of the pesticide, particularly in powder formulations. They may lower the freezing point of the original drug, making it harder to grind into a fine powder. Lastly, impurities can reduce the stability of the active ingredient over time, and when released into the environment, they may contribute to pollution. Therefore, increasing the purity of the original pesticide and minimizing impurities is essential for both safety and effectiveness.
In addition to purity, pH is another important quality parameter for both the original pesticide and its formulation. The pH refers to the concentration of free acid or base in the product, or the hydrogen ion concentration. Controlling the pH helps prevent the degradation of active ingredients during storage and ensures the physical properties of the formulation remain stable. It also helps avoid phytotoxic effects when applied and serves as a reference for assessing the corrosiveness of the pesticide on packaging materials. FAO standards express this in terms of acidity (as Hâ‚‚SOâ‚„) or alkalinity (as NaOH), typically requiring the original powder to have less than 0.1% to 0.2%. In China, pH values are commonly used for formulations, while acidity or alkalinity is often specified for the original drug.
Moisture content is another critical quality index for both the original pesticide and its preparations. Controlling moisture is essential to prevent the decomposition of the active ingredient and maintain its chemical stability. The Chinese Pesticide Network emphasizes that limiting moisture in powders and wettable powders helps ensure good dispersion when sprayed, allowing the product to spread evenly on plant surfaces for optimal performance.
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