🧪 Acid Number (TAN)

Petrochemistry PN-EN ISO 6618

Overview

Acid number (TAN — 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 — 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 — 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 — 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) — oil degradation
• Control of turbine oils — alarm when TAN rises above limit
• Evaluation of hydraulic and compressor oils
• Testing biofuels — FAME has naturally elevated TAN
• Quality control of crude oil (naphthenic acid content — corrosivity)

Key parameters

Standard

Standard number

PN-EN ISO 6618:2006

Title (PL)

Przetwory naftowe i środki smarowe — Oznaczanie liczby kwasowej lub zasadowej — Metoda miareczkowania wobec wskaźnika barwnego

Title (EN)

Petroleum products and lubricants — Determination of acid or base number — 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.

   ⏱ Time: 5 min

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.

   ⏱ Time: 15 min

3. Sample preparation

   Weigh appropriate amount of sample (1–20 g, depending on expected TAN) on analytical balance. Larger sample for low TAN samples.

   ⏱ Time: 2 min

4. Sample dissolution

   Transfer sample to titration beaker. Add 60–100 mL solvent mixture. Stir until complete dissolution.

   ⏱ Time: 3–5 min

5. Blank measurement

   Titrate 100 mL solvent mixture alone (without sample). Record titrant consumption for blank.

   ⏱ Time: 5 min

6. Sample titration

   Immerse electrode in sample solution. Start automatic potentiometric titration with KOH/IPA 0.1 mol/L. Dosing rate: 0.1–0.5 mL/min.

   ⏱ Time: 5–15 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) × C × 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.

    ⏱ Time: 5 min

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Toluene — flammable, harmful when inhaled, irritating — work in fume hood
• Isopropanol — flammable, irritates respiratory tract
• KOH — corrosive (causes skin and eye burns), gloves and glasses mandatory
• Solvent mixtures — flammable, store away from ignition sources
• Safety glasses, nitrile gloves, laboratory coat mandatory

Frequently asked questions