Overview
ISO 5315:1984 specifies a titrimetric method for the determination of total nitrogen in fertilizers. The method combines chemical reduction of nitrate, mineralization (or hydrolysis) of organic and urea nitrogen to ammonium sulfate, distillation of released ammonia, and back-titration of the absorbed ammonia. It is intended for routine laboratory analysis used in quality control and regulatory compliance of fertilizer products. Note: the method is not recommended for materials containing more than 7% organic matter.
Key Topics
- Principle: Reduction of nitrate to ammonia using chromium powder in acid medium; conversion of organic and urea nitrogen into ammonium sulfate by digestion with concentrated sulfuric acid and a catalyst; distillation of ammonia from an alkaline solution and absorption in excess standard acid, followed by back-titration with standard NaOH.
- Main reagents: chromium metal powder, concentrated sulfuric acid, catalyst (potassium sulfate and copper(II) sulfate pentahydrate), concentrated hydrochloric acid, sodium hydroxide solution, and standard titrated acid solutions. Indicators such as methyl red or mixed methyl red/ methylene blue are specified.
- Apparatus: Kjeldahl-type distillation flask (800–1 000 ml), Allihn condenser with expansion reservoir, distillation fittings, boiling regulators or anti-foaming devices, burettes (Class A), and glass beads as necessary.
- Procedure phases: sample weighing (typically 0.5–2.0 g depending on expected nitrogen), optional reduction of nitrate, hydrolysis (for urea/cyanamide forms), mineralization for other organic nitrogen forms, distillation of ammonia into acid, and titration of excess acid.
- Quality checks: run a blank and a control using a prepared ammonium nitrate solution (100 mg N) to verify apparatus performance and reagent purity.
- Performance: repeatability (r) and reproducibility (R) were established in interlaboratory studies; the standard reports r ≈ 0.36% (as N) and R ≈ 1.3% (as N) at 95% confidence for the tested levels.
Applications
- Laboratory quality control of fertilizer production for monitoring total nitrogen content.
- Regulatory compliance testing where standardized, repeatable measurement of total nitrogen is required.
- Method selection guidance: suitable for mineral fertilizers and many formulated products; avoid when organic matter exceeds 7% or when other specialized nitrogen analyses are preferable.
Related Standards
- Standards on sample preparation for fertilizers (not covered here) and related Kjeldahl or distillation procedures.
- ISO/IEC guidance on laboratory quality and method validation can support implementation and verification of ISO 5315:1984 in accredited testing environments.
This overview highlights the practical steps and controls embedded in ISO 5315:1984 to ensure reliable, titrimetric measurement of total nitrogen in fertilizer samples for operational and compliance use.
Open preview (PDF)Fertilizers — Determination of total nitrogen content — Titrimetric method after distillation
