Overview - ISO 7537:1997 (acid number, semi-micro colour‑indicator titration)
ISO 7537:1997 specifies a semi‑micro colour‑indicator titration method to determine the acid number of petroleum products and lubricants. The method is designed for small sample volumes and for materials soluble in or suspendable in a toluene / propan‑2‑ol solvent system containing a small amount of water. It is intended for acids (and salts that hydrolyse) with dissociation or hydrolysis constants larger than 10‑9 and is especially useful when sample quantity is limited.
Key topics and technical requirements
- Method principle: sample dissolved in a toluene/isopropanol (propan‑2‑ol) solvent with a small water fraction and titrated under nitrogen with standardized alcoholic KOH. The endpoint is a stable green colour using p‑naphtholbenzein indicator.
- Indicator and endpoint: p‑naphtholbenzein (10 g/L in titration solvent). Colour changes orange → yellow → green; endpoint is the first stable green that does not revert within 15–20 s.
- Titrant: alcoholic KOH, c(KOH) = 0.01 mol/L (standardized against dried potassium hydrogen phthalate). Phenolphthalein used for standardization/blank titrations.
- Atmosphere control: nitrogen purge (≥99.5% pure, dry and CO2‑free) and soda‑lime/desiccant traps on burette/reservoir to prevent CO2 and moisture interference.
- Apparatus: micro‑burette (0.01 mL divisions), amber titrant reservoir, tall 100 mL beaker with specific stopper and purge arrangement, magnetic stirring, and accurate micro‑pipetting for indicator.
- Sample preparation: used oils heated to 60 ± 5 °C and agitated to disperse sediment, filtered through ~150 µm mesh when necessary to obtain a representative aliquot.
- Calculation: acid number AN (mg KOH/g) calculated by AN = 56.1 × c × (Vsample − Vblank) / m.
- Titration conditions: performed below 30 °C, blank titration daily, and reporting precision rules (see clause 11 of the standard).
Applications and users
- Who uses it: petroleum testing laboratories, lubricant manufacturers, tribology researchers, QA/QC and R&D teams that monitor oil degradation or oxidative change when only small sample volumes are available.
- Typical applications:
- Monitoring acid number changes during oxidation tests and limited‑volume wear or bench tests.
- Routine lab checks of small or precious samples where ISO 6618/6619 require too much sample.
- Comparative acid number analyses for used oils, hydraulic fluids, turbine oils and similar lubricants.
Related standards
ISO 7537:1997 provides a practical, small‑sample titration method for reliable acid number measurement when conservation of sample and sensitive endpoint control are required. Keywords: ISO 7537:1997, acid number, petroleum products, semi‑micro titration, p‑naphtholbenzein, alcoholic KOH, toluene/isopropanol, lubricant analysis.
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