Overview
ISO 6225-2:1990 - "Rubber, raw, natural - Determination of castor oil content - Part 2: Determination of total ricinoleic acid content by gas chromatography" specifies a gas‑chromatographic method for measuring the total ricinoleic acid (the main constituent of castor oil) in raw natural rubber. Applicable to all grades of natural rubber, the method quantifies both free ricinoleic acid and ricinoleic acid released by hydrolysis of castor oil triglycerides after chemical derivatization to the methyl acetate (acetate of methyl ricinoleate) and GC analysis.
Keywords: ISO 6225-2:1990, castor oil content, ricinoleic acid, gas chromatography, natural rubber test, raw rubber.
Key topics and technical requirements
- Scope and principle
- Measures total ricinoleic acid to estimate residual castor oil in raw rubber.
- Principle: extract free acid and glycerides, hydrolyse glycerides to ricinoleic acid, derivatize to the methyl acetate, and quantify by gas chromatography (GC) against a reference standard.
- Sample preparation
- Use thin milled rubber samples (per ISO 1795 sampling guidance).
- Reflux extraction in ethanolic potassium hydroxide, followed by hydrolysis and derivatization steps (methanolic sulfuric acid and pyridine/acetic anhydride).
- Reagents and purity
- Analytical‑grade reagents and distilled water conforming to ISO 3696.
- Reference solutions: castor oil and ricinoleic acid standards for calibration/comparison.
- Apparatus and GC conditions
- Gas chromatograph with flame‑ionization detector (dual FID recommended).
- Suitable polar or non‑polar packed columns that separate the methyl ricinoleate peak sharply.
- Practical column/inlet temperatures around 200 °C are noted as likely optimal.
- Calculation and reporting
- Result expressed as percentage by mass of ricinoleic acid or castor oil glycerides.
- Calculation uses the measured GC peak areas of test and reference solutions and the masses of reference and test portions:
% = (mR × AX) / (mX × AR) × 100
where mR = mass of ricinoleic acid or castor oil in reference; mX = mass of test portion; AR = area of reference peak; AX = area of test peak.
- Safety
- Follow all laboratory health and safety precautions during refluxing, solvent handling and GC operation.
Applications and users
- Quality control laboratories in rubber manufacturing to monitor residual castor oil used as a processing aid.
- Raw material inspection and incoming‑goods testing for natural rubber bales.
- R&D and analytical chemistry labs assessing rubber additives or degradation.
- Regulatory and standards bodies requiring a validated method for castor oil content determination.
Related standards
- ISO 6225 series (other parts cover alternative methods such as thin‑layer chromatography for castor oil glycerides).
- ISO 1795 - raw rubber sampling.
- ISO 3696 - water for analytical laboratory use.
This ISO method provides a reproducible GC‑based route to quantify residual castor oil (via ricinoleic acid) for process control, specification compliance and material evaluation in the rubber industry.