Overview
ISO 7407:1983 specifies the reference method for preparing test solutions of fertilizers for the determination of acid‑soluble potassium. The method is based on boiling a weighed test portion with a dilute hydrochloric acid solution, then diluting and filtering the extract for subsequent potassium analysis. This standard defines reagents, apparatus and procedural steps to ensure reproducible, comparable results in fertilizer potassium testing.
Key topics and requirements
- Scope: Preparation of the test solution for subsequent determination of acid‑soluble potassium in fertilizers. (Pretreatment of samples is noted as the subject of a future standard.)
- Test portion: Weigh 5.000 g (nearest 0.001 g) of the prepared test sample.
- Reagents:
- Hydrochloric acid solution containing approximately 36.5 g HCl per litre.
- Preparation detail: add 50.0 ml HCl (density 1.18 g/ml) to about 400 ml water, cool, dilute to 500 ml and mix.
- Use reagent‑grade chemicals and water of equivalent purity.
- Procedure:
- Transfer the 5 g test portion to a 750 ml flat‑bottom flask containing 300 ml of the HCl solution.
- Fit a reflux condenser and boil for 30 minutes.
- Cool, transfer contents to a one‑mark 1 000 ml volumetric flask, dilute to the mark with the same HCl solution, mix and filter into a dry beaker.
- Discard the first 50 ml of filtrate; subsequent filtrate is used for potassium determination.
- Apparatus: Ordinary laboratory glassware plus a 750 ml flat‑bottom flask, reflux condenser, and a 1 000 ml one‑mark volumetric flask (ISO 1042 class B; max permitted error ±0.080 ml).
- Test report: Must include complete sample identification, reference to ISO 7407, and any non‑specified or optional operations or incidents that could affect results.
Applications and users
- Routine fertilizer quality control laboratories performing chemical analysis of potassium content.
- Regulatory and compliance testing bodies that evaluate fertilizer composition.
- Agricultural product manufacturers and suppliers ensuring product specifications.
- Research laboratories studying fertilizer composition, solubility and nutrient availability.
Keywords: ISO 7407, acid-soluble potassium, fertilizers, potassium determination, HCl digestion, test solution preparation, fertilizer analysis, reference method.
Related standards
- ISO 1042 - Laboratory glassware: one‑mark volumetric flasks (referenced for volumetric flask accuracy).
- Note: Sample pretreatment procedures for potassium determination are identified as the subject of a future International Standard.
