⛽ Total Base Number (TBN)
Determination of Total Base Number (TBN) of lubricating oils by potentiometric titration with perchloric acid in glacial acetic acid.
Overview
Total Base Number (TBN) is a key parameter for assessing lubricating oil condition, especially engine oils. Expressed in milligrams of KOH per gram of sample (mg KOH/g), it determines the oil's ability to neutralize acids formed during fuel combustion and oil oxidation. Fresh engine oils have TBN in the range of 5–15 mg KOH/g (passenger car oils) to 30–70 mg KOH/g (marine oils for high-sulfur fuels).
Basic additives (metallic detergents — calcium and magnesium sulfonates, phenates and salicylates) neutralize sulfuric acid, organic acids and other degradation products, protecting engine elements from corrosion and deposits. During oil service, TBN gradually decreases — when it drops to critical level (typically 50% of fresh oil value or below 3 mg KOH/g), oil requires replacement.
The method according to ISO 3771 involves potentiometric titration of oil sample dissolved in glacial acetic acid and chlorobenzene mixture with perchloric acid (HClO₄) solution in glacial acetic acid. The titration endpoint is determined as the inflection point of the potentiometric curve (electrode potential vs. titrant volume).
TBN monitoring is a fundamental element of oil condition monitoring and predictive maintenance programs in vehicle fleets, marine fleets, power plants and heavy industry.
Method principle
A weighed oil portion (0.5–5 g, depending on expected TBN) is dissolved in 60 mL of solvent mixture (glacial acetic acid:chlorobenzene, typically 1:2 V/V). A combined glass electrode (pH-Ag/AgCl) is immersed in the solution and titration is conducted with perchloric acid (HClO₄ 0.1 mol/L in glacial acetic acid) using an automatic burette. The electrode potential dependence on added acid volume is recorded. The endpoint is determined mathematically as the inflection point of the titration curve. TBN is calculated from the HClO₄ volume consumed to the endpoint.
Applications
- Engine oil condition monitoring — base reserve assessment
- Marine oil analysis (TBN 30–70 mg KOH/g) — high-sulfur fuels
- Predictive maintenance programs for vehicle and industrial machinery fleets
- Fresh lubricating oil quality control — manufacturer declaration verification
- TBN determination of turbine, hydraulic and transmission oils
- R&D research — new additive package formulations
- Oil compatibility assessment with variable sulfur content fuels (IMO 2020)
Key parameters
| Parameter | Value |
|---|---|
| Determination range | 2–300 mg KOH/g (typically 5–70) |
| Titrant | HClO₄ 0.1 mol/L in glacial acetic acid |
| Solvent | Glacial acetic acid : chlorobenzene (1:2 V/V) |
| Electrode | Combined glass pH Ag/AgCl |
| Repeatability | 0.04 × TBN (for TBN 5–70) |
| Sample mass | 0.5–5 g (selected according to expected TBN) |
Standard
- Standard number
- PN-EN ISO 3771:2011
- Title (PL)
- Przetwory naftowe — Oznaczanie liczby zasadowej — Metoda miareczkowania potencjometrycznego kwasem nadchlorowym
- Title (EN)
- Petroleum products — Determination of base number — Perchloric acid potentiometric titration method
Step-by-step procedure
1. Titrant calibration
Weigh 80 mg KHP (potassium hydrogen phthalate), dissolve in 20 mL warm glacial acetic acid, add 40 mL chlorobenzene, cool. Titrate with HClO₄. Calculate titer.
2. Blank determination
To empty beaker add 60 mL solvent (acetic acid:chlorobenzene 1:2). Immerse electrode and stirrer. Conduct titration with HClO₄ to inflection point.
3. Oil sample weighing
Weigh oil sample to 0.1 mg accuracy. Mass depends on expected TBN: ~5 g for TBN <10; ~2 g for TBN 10–30; ~0.5 g for TBN >30.
4. Sample dissolution
Add 60 mL solvent (acetic acid:chlorobenzene) to beaker with sample. Stir until oil completely dissolves.
5. Electrode immersion
Immerse combined pH electrode in sample solution. Turn on magnetic stirrer at low speed. Wait for potential stabilization.
6. Titration
Start automatic titration with HClO₄ 0.1 mol/L. Titrator adds titrant in portions and records potential. Continue to clear inflection point on curve.
7. Endpoint determination
Titrator automatically determines inflection point from first or second derivative curve. Read titrant volume at endpoint.
8. TBN calculation
TBN [mg KOH/g] = [(V_sample − V_blank) × c_HClO₄ × 56.1] / m_sample, where 56.1 = KOH molar mass.
9. Quality control
Perform duplicate (difference ≤ 0.04×TBN). Measure CRM every series. Verify titrant titer weekly.
10. Cleaning
Rinse electrode sequentially: chlorobenzene, isopropanol, deionized water. Store in 3 mol/L KCl solution. Wash beakers with acetone.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Potentiometric titrator | Metrohm 905 Titrando, Mettler Toledo T5, SI Analytics TitroLine 7000 | 30 000–80 000 PLN |
| Combined pH electrode (for non-aqueous media) | Metrohm Solvotrode EtOH, SI Analytics type N 6280 | 1 500–4 000 PLN |
| Automatic burette | Exchangeable burette 20 mL, integrated with titrator | included with titrator |
| Analytical balance (d=0.1 mg) | Mettler Toledo ME204, Sartorius Secura 224 | 5 000–15 000 PLN |
| Glass beakers 250 mL | Tall borosilicate beakers, Duran/Simax | 15–40 PLN/pc. |
| Magnetic stirrer | IKA C-MAG MS 4, Heidolph MR Hei-Standard | 1 500–4 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Perchloric acid (HClO₄) 0.1 mol/L in glacial acetic acid | 7601-90-3 | Titrant, ready standardized solution or to prepare from 70% acid |
| Glacial acetic acid | 64-19-7 | Analytical grade, ≥99.8%, solvent and titrant component |
| Chlorobenzene | 108-90-7 | Analytical grade, solvent component for dissolving oil sample |
| Potassium hydrogen phthalate (KHP) | 877-24-7 | Standard substance for titrant calibration, dried at 110°C |
| Acetic anhydride | 108-24-7 | For removing traces of water from titrant (optional) |
| CRM reference oil | — | Certified reference material with known TBN for method validation |
Health and safety (OHS)
- Perchloric acid — strong oxidizer, corrosive, do not mix with organic materials in concentrated form
- Glacial acetic acid — corrosive, irritating, pungent odor, work under fume hood
- Chlorobenzene — toxic, flammable, irritating, avoid vapor inhalation
- Acetic anhydride — corrosive, reacts violently with water, store tightly closed
- Chemical-resistant gloves (butyl or Viton), safety glasses and coat mandatory
- Waste after titration — collect into containers for chlorinated organic waste