Overview
ISO 6619:1988 specifies a potentiometric titration method to determine the neutralization (acid) number of petroleum products and lubricants. The method measures acidic constituents in samples that are soluble or nearly soluble in mixtures of toluene and propan-2-ol and reports results as milligrams of potassium hydroxide (mg KOH) per gram of sample. ISO 6619 is intended for acids with dissociation constants greater than about 10⁻⁹ and is widely used to monitor relative oil degradation under oxidizing conditions.
Key topics and technical requirements
- Principle: Dissolve a test portion in a toluene/propan-2-ol solvent with a small water content and titrate potentiometrically with alcoholic KOH using a glass indicating electrode and a calomel reference electrode. End points are taken at well-defined inflections or at meter readings corresponding to non‑aqueous acidic and basic buffer solutions.
- Definition: Acid number = mass of KOH (mg) required to titrate 1 g of sample to the specified end point.
- Reagents and solvent: Standardized alcoholic KOH titrant (e.g., 0.1 mol/L), alcoholic HCl standards, buffer stock solutions (y-collidine, m‑nitrophenol), and a titration solvent prepared from toluene, anhydrous propan-2-ol and a small amount of water.
- Apparatus: Manual and automatic potentiometric titration setups are described - including glass indicating electrode, calomel reference electrode, precision burettes, stirrers, and meters with required accuracy and shielding to avoid electrostatic interference.
- Sample preparation: Procedures for handling used oils (heating, suspending sediment, straining) to obtain representative test portions are included.
- Quality control: Instructions for electrode maintenance, standardization against non-aqueous buffer solutions, and recording titration curves are provided.
- Limitations: The standard warns that acid number is an indicator of acidic content or degradation but does not predict corrosiveness; no general correlation exists between acid number and bearing or metal corrosion.
Applications and users
ISO 6619 is used by:
- Lubricant and petroleum quality-control laboratories
- Oil formulators and additive manufacturers
- Maintenance and reliability teams monitoring oil condition (used-oil analysis)
- Petrochemical testing labs and third‑party testing services
Practical uses include monitoring oxidation/degradation, verifying additive levels, tracking relative changes in acid content during service, and supporting formulation QC.
Related standards
- ISO 6618 - color‑indicator titration method for acid number (comparable alternative mentioned in ISO 6619)
Keywords: ISO 6619, acid number, neutralization number, potentiometric titration, petroleum products, lubricants, toluene, propan-2-ol, non-aqueous titration, mg KOH/g, oil degradation.
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