Overview
ISO 23381:2020 specifies a simple, low-cost laboratory procedure to determine the salt out temperature (SOT) - also called the crystallization temperature (CT) - of liquid (fluid) fertilizers. The method uses a cooling medium (commonly a dry ice / alcohol bath) to induce crystallization and records the temperature behavior as the sample cools and then warms. The standard is intended as a practical test for assessing the temperature at which solids form or re-dissolve in liquid fertilizers, helping stakeholders manage storage, handling and transport in cold conditions.
Key topics and technical requirements
- Test principle: Cool a measured aliquot of fertilizer in an alcohol–dry ice bath while stirring; once crystals form, allow the sample to warm and record the temperature at which crystals first appear (FCTA / TCT) and when the last crystal disappears (LCTD). The LCTD is typically taken as the SOT unless crystals persist on warming - in which case the FCTA is reported.
- Scope & limitations: Applies to many liquid fertilizers but may not be applicable to some binary or ternary blends (exceptions and specific guidance are discussed in Clause 8).
- Reagents & safety:
- Use IPA, methanol, ethanol or acetone with dry ice; alternatives such as an ice–salt bath may be used if dry ice is unavailable.
- Safety warnings: dry ice causes frostbite; alcohols/acetone are flammable; perform operations in a fume hood and use insulated gloves.
- Equipment:
- Stir motor setup with thermometer replacing the stir blade, glass test tubes (≈50 cm noted), silver-lined Dewar vacuum flask, thermometers rated to at least −40 °C, thin stainless wire and latex tubing for mounting.
- Typical sample volume and set points in the standard: ~15 ml sample, ~250 ml solvent in Dewar; stirring at ~50–60 RPM.
- Procedure notes: Repeated cooling/warming cycles and careful observation of the last crystal to dissolve (LCTD) are central to obtaining reproducible SOT values.
Applications and users
ISO 23381:2020 is valuable for:
- Fertilizer manufacturers - product specification, quality control, R&D (formulation stability).
- Logistics and storage planners - establishing cold-chain limits to avoid salting out during transport or storage.
- Agronomy and extension services - advising end-users on safe handling and field application during cold seasons.
- Regulatory bodies and testing labs - harmonized method for comparative testing and compliance checks.
Keywords: ISO 23381:2020, salt out temperature, SOT, crystallization temperature, liquid fertilizers, test procedure, dry ice/methanol bath, LCTD, FCTA.
Related standards
- ISO 8157 (definition of liquid fertilizers)
- ISO 3696 (water for laboratory use)
- ISO 1392, ISO 3016 and referenced ASTM methods (e.g., ASTM D6660, D1177-17, D97) and OECD guidance are cited for context and alternative techniques.
