π§ͺ Acid Number (TAN)
Determination of acid number (TAN) of petroleum products and lubricants by potentiometric titration method. Result expressed in mg KOH/g sample.
Overview
Acid number (TAN β Total Acid Number) is a measure of total content of acidic components in petroleum products and lubricating oils. It is expressed as the amount of potassium hydroxide (KOH) in milligrams needed to neutralize acids contained in 1 gram of sample. It is one of the key parameters for assessing lubricating oil condition in service.
TAN increase during oil service indicates its oxidation and degradation β oxidation products (organic acids, aldehydes, ketones) increase oil acidity. High TAN signals risk of corrosion of metal components (especially bearings and cylinders), formation of deposits and varnishes on hot surfaces and accelerated oil consumption. In engine oils, critical TAN value is established by engine manufacturer β typically alarm at doubling of initial value.
Standard PN-EN ISO 6618 describes method using color indicator, while in laboratory practice potentiometric titration (per ASTM D664/ISO 6619) is more often used, which is more objective β does not depend on subjective assessment of color change. Potentiometric titration is preferred for dark and colored samples where observation of indicator color change is difficult.
TAN is required in specifications of base oils, new and used lubricating oils, transformer oils, turbine and hydraulic oils. It is a key parameter in oil condition monitoring programs (condition monitoring/tribology).
Method principle
Oil sample is dissolved in mixture of solvents (toluene, isopropanol, water) and titrated with alcoholic solution of potassium hydroxide (KOH in isopropanol, concentration 0.1 mol/L). In color indicator method, end of titration is determined by color change of p-naphtholbenzein. In potentiometric method (ASTM D664), end point is determined from titration curve (inflection point) recorded using combined pH electrode immersed in solution. Result is expressed as mg KOH needed to neutralize acids in 1 g sample.
Applications
- Monitoring condition of lubricating oils in service (condition monitoring)
- Quality control of base oils and finished lubricant products
- Evaluation of transformer oils (PN-EN 62021) β oil degradation
- Control of turbine oils β alarm when TAN rises above limit
- Evaluation of hydraulic and compressor oils
- Testing biofuels β FAME has naturally elevated TAN
- Quality control of crude oil (naphthenic acid content β corrosivity)
Key parameters
| Parameter | Value |
|---|---|
| Result unit | mg KOH/g sample |
| Measurement range | 0.05β250 mg KOH/g (potentiometry) |
| Repeatability | Β±(0.044 Γ TAN + 0.01) mg KOH/g |
| Reproducibility | Β±(0.14 Γ TAN + 0.04) mg KOH/g |
| Sample weight | 1β20 g (dependent on expected TAN) |
| Determination time | 5β15 min per sample |
Standard
- Standard number
- PN-EN ISO 6618:2006
- Title (PL)
- Przetwory naftowe i Εrodki smarowe β Oznaczanie liczby kwasowej lub zasadowej β Metoda miareczkowania wobec wskaΕΊnika barwnego
- Title (EN)
- Petroleum products and lubricants β Determination of acid or base number β Colour-indicator titration method
Step-by-step procedure
1. Solvent preparation
Prepare mixture: 500 mL toluene + 495 mL isopropanol + 5 mL water. Mix thoroughly. Store in tightly closed bottle.
2. Titrant standardization
Weigh 0.1 g benzoic acid (or KHP). Dissolve in solvent mixture. Titrate with KOH/IPA 0.1 mol/L. Calculate exact titrant concentration.
3. Sample preparation
Weigh appropriate amount of sample (1β20 g, depending on expected TAN) on analytical balance. Larger sample for low TAN samples.
4. Sample dissolution
Transfer sample to titration beaker. Add 60β100 mL solvent mixture. Stir until complete dissolution.
5. Blank measurement
Titrate 100 mL solvent mixture alone (without sample). Record titrant consumption for blank.
6. Sample titration
Immerse electrode in sample solution. Start automatic potentiometric titration with KOH/IPA 0.1 mol/L. Dosing rate: 0.1β0.5 mL/min.
7. End point determination
Autotitrator automatically determines inflection point on titration curve. Record titrant volume at end point.
8. TAN calculation
TAN [mg KOH/g] = (V - Vb) Γ C Γ 56.1 / m, where V = titrant volume [mL], Vb = blank [mL], C = KOH concentration [mol/L], m = sample mass [g], 56.1 = KOH molar mass.
9. Repeat and QC
Perform second determination. Results should be within repeatability limits. Every series measure CRM (certified oil with known TAN).
10. Apparatus cleaning
Rinse electrode and titration vessel with solvent mixture, then with isopropanol. Store electrode in KCl solution.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Potentiometric autotitrator | Metrohm 848 Titrino plus, Metrohm 916 Ti-Touch, Mettler Toledo T5 | 25 000β60 000 PLN |
| TAN/TBN system for oils | Metrohm 848 Oil Titrino plus (dedicated package) | 35 000β70 000 PLN |
| Combined electrode for non-aqueous media | Metrohm Solvotrode easyClean, Mettler Toledo DGi115-SC | 1 500β3 500 PLN |
| Automatic burette | Exchangeable burette 20 mL with Β±0.01 mL accuracy | 3 000β6 000 PLN |
| Analytical balance | Mettler Toledo XPR205, Sartorius Quintix 224, accuracy 0.1 mg | 8 000β25 000 PLN |
| Magnetic stirrer | IKA C-MAG MS 4 or built into autotitrator | 1 500β4 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| KOH in isopropanol 0.1 mol/L | 1310-58-3 | Titrant β alcoholic solution of potassium hydroxide, 1 L bottle, standardized against benzoic acid or potassium hydrogen phthalate |
| Toluene | 108-88-3 | Purity β₯99.5%, solvent mixture component (50% vol.) |
| Isopropanol (propan-2-ol) | 67-63-0 | Purity β₯99.5%, solvent mixture component (49.5% vol.) |
| Deionized water | 7732-18-5 | Solvent mixture component (0.5% vol.), COβ-free |
| p-Naphtholbenzein (indicator) | 145-50-6 | 1 g/L solution in toluene/isopropanol mixture β color change indicator (ISO 6618 method) |
| Benzoic acid (for standardization) | 65-85-0 | Certified reference material for standardizing KOH titrant |
| Potassium hydrogen phthalate (for standardization) | 877-24-7 | KHP β alternative standard for KOH standardization, purity β₯99.95% |
Health and safety (OHS)
- Toluene β flammable, harmful when inhaled, irritating β work in fume hood
- Isopropanol β flammable, irritates respiratory tract
- KOH β corrosive (causes skin and eye burns), gloves and glasses mandatory
- Solvent mixtures β flammable, store away from ignition sources
- Safety glasses, nitrile gloves, laboratory coat mandatory