Overview
ISO 25475:2016 specifies a laboratory method for the determination of ammoniacal nitrogen in fertilizers. The procedure applies to nitrogenous and compound fertilizers where nitrogen is present as ammonium salts or as ammonium salts together with nitrates. It is explicitly not applicable to fertilizers containing urea, cyanamide or other organic nitrogenous compounds. The method uses base-induced displacement of ammonia, steam distillation, capture in a standard acid and back-titration to quantify ammoniacal nitrogen.
Key topics and technical requirements
- Principle: Conversion of ammonium to ammonia by excess NaOH, distillation of liberated NH3, absorption into standard H2SO4 (or alternative acids), and back-titration of excess acid with standard NaOH/KOH.
- Scope/limitations: Valid for fertilizers where nitrogen is exclusively inorganic (ammonium ± nitrate); excludes organic-N fertilizers (urea, cyanamide).
- Reagents: Analytical-grade reagents and CO2- and nitrogen-free water (ISO 3696 grade 3). Specified solutions include diluted HCl, standard H2SO4 (various molarities depending on variant), carbonate-free NaOH/KOH titrants, 30% NaOH for sample dissolution, and indicators (mixed methyl red/methylene blue or methyl red).
- Apparatus: Borosilicate steam distillation setups (several illustrated configurations), rotary shaker (35–40 r/min), volumetric glassware and pipettes, anti-bumping granules. Automatic distillation apparatus permitted if results are statistically equivalent.
- Procedure highlights: Weigh prepared sample (typical quantities 5–10 g), extract with HCl and water, shake and filter to 500 ml, distil an aliquot with NaOH added, collect ~250 ml distillate, titrate residual acid. Tables in the standard (Table 1–3) define weighing/dilution/aliquot choices and calculation factors for different N ranges.
- Quality controls: Blank and control tests, specified precision data (repeatability, reproducibility, inter-laboratory test results) and reporting requirements.
Applications and users
ISO 25475:2016 is used by:
- Fertilizer manufacturers - production quality control and batch verification.
- Analytical testing laboratories - routine analysis to verify ammoniacal N content for specification compliance.
- Regulatory and inspection bodies - enforcement of fertilizer labeling and trade requirements.
- Research and agronomy labs - formulation studies and fertilizer behavior investigations.
Practical benefits include reliable, standardized measurement of ammoniacal nitrogen for product specification, safety data, and trade conformity.
Related standards
- ISO 3696 - Water for analytical laboratory use
- ISO 14820-2 - Fertilizers - Sampling and sample preparation - Part 2
- ISO 14820-1 (recommended for sampling)
- EN 12944-1 / EN 12944-2 - Fertilizer vocabulary referenced for terms
Keywords: ISO 25475:2016, ammoniacal nitrogen, fertilizer analysis, ammonia distillation, titration, ammonium salts, laboratory method.
