Determination of Physico-Chemical Properties of Pesticides: Test Items, Methods and Standards

Pesticide physico-chemical property determination provides the core technical data required for the registration of active substances and formulated products, quality control, hazard classification, and environmental risk assessment. Accredited third-party laboratories measure parameters such as melting point, vapor pressure, water solubility, n-octanol/water partition coefficient, hydrolysis behavior, and flammability in accordance with internationally recognized guidelines from OECD, CIPAC, ISO, ASTM, and GB. This article summarizes the scope of pesticide physico-chemical testing, the standard methods applied to each parameter, and the main regulatory and industrial applications of the resulting data.

Technical Overview

The determination of the physico-chemical properties of pesticides is the systematic measurement of the intrinsic physical and chemical characteristics of active substances, technical materials, and formulated products. These data confirm substance identity, define product behavior during manufacture, storage, transport, and field application, and supply the key input parameters for environmental fate and exposure modeling. Because such data underpin efficacy, safety, and regulatory decisions, physico-chemical testing is a mandatory element of pesticide registration dossiers in virtually every jurisdiction.

Testing is performed under internationally harmonized frameworks. The OECD Test Guidelines (Nos. 101–123) cover the physical-chemical characterization of chemical substances, the US EPA OCSPP 830 series provides equivalent product-property test guidelines, and EU Regulation (EC) No 283/2013 defines the data requirements for pesticide active substances. In China, the NY/T 1860 series of agricultural industry standards specifies the study principles for pesticide physico-chemical property determination in support of registration. Formulation performance tests follow the CIPAC methods referenced in FAO/WHO specifications, and many parameters can also be determined under ISO, ASTM, and GB national standards.

Studies intended for regulatory submission are normally conducted by accredited third-party laboratories operating under Good Laboratory Practice (GLP). The test substance is characterized for purity and impurity profile before testing begins, and every measurement is reported together with the method, test conditions, validation data, and quality assurance statements, ensuring traceability and acceptance by registration authorities.

Testing Samples

Physico-chemical testing programs cover the full range of pesticide sample types, from purified active ingredients to complex multi-component formulations.

Technical Materials and Technical Concentrates (TC/TK)

Technical materials with a defined purity and known impurity profile are tested for the complete set of intrinsic endpoints, including melting point, boiling point, vapor pressure, water solubility, organic solvent solubility, partition coefficient, dissociation constant, and stability characteristics. These values constitute the reference identity and hazard profile of the active substance.

Formulated Products

Finished formulations are tested for both intrinsic parameters and application-relevant performance properties. Typical sample types include:

• Emulsifiable concentrates (EC), emulsions in water (EW), and micro-emulsions (ME)
• Suspension concentrates (SC), suspo-emulsions (SE), and capsule suspensions (CS)
• Wettable powders (WP), water-dispersible granules (WG/WDG), and soluble granules (SG)
• Soluble liquids (SL), aqueous solutions (AS), oil dispersions (OD), and flowable concentrates for seed treatment (FS)
• Granules (GR), tablets, dusting powders (DP), and aerosol products

Impurities, Metabolites, and Degradation Products

Relevant impurities and major degradation products are characterized individually where data gaps exist, supporting same-source equivalence assessments, impurity limit justification, and toxicological evaluation.

Reference Materials and Packaging Components

Certified or purified reference substances are used for spectroscopic identity confirmation (IR, UV-Vis) and method calibration, while representative container and packaging materials may be included in corrosion compatibility studies.

Testing Items

The scope of physico-chemical determination for pesticides is typically organized into the following categories of endpoints.

• Identity and general properties: appearance (color, physical state, odor), UV-visible absorption spectrum, pH, acidity/alkalinity, and water content
• Thermal and volatility properties: melting point and melting range, boiling point and boiling range, thermal stability, vapor pressure, and evaporation behavior
• Density and interfacial properties: relative density, bulk density and tap density, surface tension of aqueous solutions, viscosity, and wettability
• Solution and partition behavior: water solubility, solubility in organic solvents, n-octanol/water partition coefficient (log Kow), dissociation constant (pKa), and fat solubility
• Stability and reactivity: hydrolysis as a function of pH, aqueous photolysis, stability in air, storage stability (ambient, accelerated, and low temperature), and stability toward metals and metal ions
• Safety classification endpoints: flash point, flammability of liquids and solids, auto-ignition temperature, self-heating behavior, oxidizing properties, explosive properties, and corrosivity to metals
• Particle and formulation performance: particle size distribution, dustiness, wet sieve residue, suspensibility, emulsion stability, persistent foam, dispersion stability, attrition of granules, and flowability

The exact endpoint list depends on the substance type and the target registration jurisdiction, and is usually defined by the OECD, EPA, EU, or NY/T guideline matrices.

Testing Methods

Each parameter is measured using validated instrumental or classical methods selected according to the physical state of the sample, its expected magnitude range, and the governing standard.

Appearance and Identity Confirmation

Color, physical state, and odor are assessed by visual and organoleptic examination under standardized conditions, following guidance such as US EPA OCSPP 830.6302 and the corresponding part of the NY/T 1860 series. Identity is corroborated by UV-visible spectrophotometry (OECD 101) and infrared spectroscopy against reference spectra.

Melting Point and Melting Range

Determination is performed by the capillary tube method, hot-stage microscopy, or differential scanning calorimetry (DSC) in accordance with OECD 102 and ASTM E794. Thermal analysis is preferred for substances that decompose before melting.

Boiling Point and Thermal Stability

Boiling point is measured by ebulliometry, the Siwoloboff method, or differential thermal analysis (OECD 103). Thermal stability and stability in air are screened by DSC/TGA following OECD 113, which also supports explosive-property risk screening.

Vapor Pressure

Depending on the expected pressure range, the static method, isoteniscope, gas saturation technique, Knudsen effusion method, or spinning rotor gauge is applied (OECD 104). Measurements at several temperatures are extrapolated using the Clausius-Clapeyron relationship.

Density and Bulk Density

Relative density of liquids is determined with the pycnometer or hydrostatic balance (OECD 109, ISO 758) or by oscillating U-tube digital densitometry (ISO 12185, ASTM D4052). Bulk and tap density of powders and granules are measured with graduated cylinders and tamping volumeters.

Surface Tension and Viscosity

Surface tension of aqueous solutions is measured by the Du Noüy ring, Wilhelmy plate, or stalagmometric method (OECD 115, ISO 304). Viscosity is determined by rotational viscometry (ISO 2555, ISO 3219, ASTM D2196) or by capillary kinematic methods (ISO 3104, ASTM D445), according to OECD 114.

Water Solubility and Solvent Solubility

Water solubility is determined by the flask-shaking method or, for very poorly soluble substances, the column elution technique (OECD 105), typically at 20–25 °C and as a function of pH. Solubility in representative organic solvents is measured by the saturated-solution flask method.

Partition Coefficient and Dissociation Constant

The n-octanol/water partition coefficient is determined by the shake-flask method (OECD 107), estimated by HPLC screening (OECD 117), or measured by the slow-stirring method for highly hydrophobic substances (OECD 123). Dissociation constants are obtained by titrimetry, spectrophotometry, or conductometry (OECD 112).

Hydrolysis and Aqueous Photolysis

Hydrolysis is studied at pH 4, 7, and 9 at 25 °C (or elevated temperature for stable compounds) with kinetic evaluation of degradation products (OECD 111). Direct aqueous photolysis is evaluated under simulated sunlight with quantum yield determination (OECD 316).

Flash Point and Flammability

Flash point is measured by Pensky-Martens closed cup (ISO 2719, ASTM D93), Tag closed cup (ASTM D56), rapid equilibrium closed cup (ISO 3679), or Abel closed cup (ISO 13736) depending on viscosity and expected range. Flammability of solids, auto-ignition, and self-heating are assessed with the burning-rate and cube tests described in the UN Manual of Tests and Criteria.

Explosive, Oxidizing, and Corrosive Properties

Thermal screening by DSC (OECD 113) is followed, where required, by mechanical sensitivity and thermal explosibility tests, oxidizing liquid and solid tests, self-heating tests, and corrosion-to-metal tests performed according to the UN Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria.

pH, Water Content, and Particle Performance

pH is measured potentiometrically (OECD 122, GB/T 1601) and water content by Karl Fischer titration (ISO 760, ASTM E203, GB/T 1600). Particle size distribution is determined by laser diffraction (ISO 13320); wet sieve residue by CIPAC MT 169 and GB/T 16150; suspensibility by CIPAC MT 171 and GB/T 14825; emulsion stability by GB/T 1603; persistent foam by CIPAC MT 184; and accelerated storage stability and cold stability by CIPAC MT 46.3, GB/T 19136, and GB/T 19137 respectively.

Applications

Physico-chemical determination data serve multiple regulatory and industrial purposes throughout the pesticide product life cycle.

• Registration support: mandatory data packages for active substance and product authorization in major markets, and same-source equivalence assessment for new technical material manufacturers
• Hazard communication: GHS classification, safety data sheet authoring, and transport classification under UN dangerous goods regulations
• Quality control: specification setting for manufacturers, batch release testing, and compliance verification against FAO/WHO and national specifications in trade
• Formulation development: solvent and co-formulant selection based on solubility, partition, and interfacial data; milling and granulation endpoint control through particle size monitoring
• Storage and packaging: shelf-life definition via ambient and accelerated storage studies, and packaging compatibility confirmation through corrosion testing
• Environmental assessment: input of log Kow, water solubility, vapor pressure, hydrolysis, and photolysis data into fate and exposure models used in risk assessment

FAQ

Q: Which physico-chemical endpoints are mandatory for registering a new pesticide active substance? A: Typical core requirements include appearance, melting/boiling point, vapor pressure, relative density, surface tension, water solubility, organic solvent solubility, partition coefficient, dissociation constant, UV-visible spectrum, hydrolysis as a function of pH, photolysis, stability data, flash point, and safety classification endpoints. The precise list is defined by the OECD, EU, EPA, or NY/T 1860 guideline applicable to the target market.

Q: What purity should the test substance have? A: Intrinsic endpoints are normally measured on the purest available material (commonly at least 98% purity for technical material studies), with a documented impurity profile, whereas formulation endpoints are measured on the commercial product as manufactured.

Q: Is GLP compliance required? A: For registration of new active substances and plant protection products, most authorities require physico-chemical studies to be conducted under GLP by accredited third-party laboratories, with full raw-data traceability and quality assurance audits.

Q: How much sample is needed and how long does testing take? A: Simple physical parameters generally require a few grams and one to three weeks, whereas hydrolysis, photolysis, and full registration packages may require several months and larger sample quantities depending on the endpoint matrix.

Q: Are OECD methods and Chinese standards mutually acceptable? A: The NY/T 1860 series is largely harmonized with the corresponding OECD guidelines, and data generated under one system can usually be supplemented with method-equivalence justifications to satisfy the other, subject to the accepting authority's policy.

Q: Why is the n-octanol/water partition coefficient so important? A: Log Kow indicates the lipophilicity and bioaccumulation potential of an active substance and directly governs sorption, mobility, and uptake predictions used in environmental and human exposure modeling.