Overview
ISO 5400:1984 defines a reduced molybdosilicate spectrometric method for the determination of total silicon in leather, expressed as SiO2. The method is designed to destroy organic matter, solubilize mineral content and develop a molybdosilicate complex whose blue coloration is measured spectrometrically at 630 nm. The procedure is intended for routine laboratory use in tanning, quality control and material analysis.
Key Topics
- Principle: Oxidation of a test portion under alkaline conditions (Parr bomb) to remove organics and solubilize mineral matter, followed by formation of the molybdosilicate complex and spectrometric measurement at 630 nm.
- Reagents: Concentrated and diluted sulfuric acid, glycerol, sodium sulfate solution, ammonium molybdate, tin(II) chloride solution, and a defined fusion mixture (sodium peroxide, potassium nitrate and powdered sucrose).
- Apparatus: Ordinary laboratory glassware and equipment plus a 22 ml Parr bomb (or alternative Wurzschmitt bomb), one-mark volumetric flasks, pipettes, analytical balance (0.001 g), spectrometer/photometric absorptiometer and pH meter.
- Sampling and test portions: Sampling follows ISO 2418 and ISO 2588. The standard describes preparing test samples in accordance with ISO 4044 and treating a measured test portion with the fusion mixture in the bomb.
- Calibration and measurement: A calibration curve is prepared using fused pure silica; colour development uses ammonium molybdate and tin(II) chloride, with colour stability up to 1 hour and absorbance read at 630 nm.
- Accuracy: Repeatability and reproducibility are specified: duplicate determinations by the same operator in one lab should not differ by more than 0.2% (on original leather mass), and inter-laboratory results should not differ by more than 0.5%.
- Notes: The method reports total silicon as SiO2 and highlights that organo‑silanes can be largely extracted by organic solvents; an ash residue from zirconium isolation may be substituted at the fusion stage if declared in the test report.
Applications
This ISO method is applicable for:
- Quality control of leather and tanned products to monitor silicon levels.
- Material compliance testing where silicon content affects performance or regulatory requirements.
- Laboratory analysis in research, troubleshooting, and supplier verification.
Practical advantages include a standardized oxidation and colourimetric measurement route that yields comparable results across laboratories when sampling and procedure are followed.
Related Standards
- ISO 2418 - Leather: Laboratory samples - location and identification.
- ISO 2588 - Leather: Sampling - number of items for a gross sample.
- ISO 4044 - Leather: Preparation of chemical test samples.
- ISO 1042 and ISO 835 - Laboratory glassware specifications.
For implementation, laboratories should follow the reagent, apparatus and sampling details in ISO 5400:1984 and report any deviations or additional extraction steps used to distinguish organo‑silicon species.
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