🧪 Acid Number (TAN)

Petrochemistry PN-ISO 6618

Overview

STATUS (as of 30.10.2026). PN-ISO 6618:2011 — Polish version, implements ISO 6618:1997 [IDT] and the corrigendum ISO 6618:1997/Cor 1:1999 [IDT]; published on 11.08.2011, it replaced PN-C-04066:1985; in the PKN catalogue it has no withdrawal note. It implements only the ISO standard, without a European standard, hence the designation “PN-ISO” without “EN”. ISO 6618:1997 is the second edition of the standard (published on 06.02.1997, it replaced ISO 6618:1987), last confirmed by ISO on 18.10.2023.

SCOPE (clause 1). Colour-indicator titration for determining the acidic or basic constituents of petroleum products and lubricants soluble in mixtures of toluene and propan-2-ol. Acids and bases with dissociation constants in water greater than 10⁻⁹ are determined; weaker ones do not interfere, and salts react if their hydrolysis constants are greater than 10⁻⁹. As acidic constituents of new and used oils the standard counts, among others, organic and inorganic acids, esters, phenolic compounds, lactones, resins, salts of heavy metals and additives (inhibitors, detergents); as basic ones — among others, organic and inorganic bases, amino compounds, salts of weak acids (soaps), salts of heavy metals and additives. The method is not suitable for measuring the basic constituents of many basic additive-type lubricating oils — ISO 3771 serves this purpose. Oils in which the colour change of the indicator cannot be seen — for example many cutting oils, rust-preventive oils and similar compounded oils, or excessively dark oils — are tested according to ISO 6619; the results of the two methods may differ numerically but are generally of the same order of magnitude.

WHAT THE RESULT MEASURES (clauses 1 and 3). The acid number is the quantity of base, in milligrams of KOH per gram of sample, required to titrate the acidic constituents in 1 g of sample under prescribed conditions; the strong acid number covers only the strongly acidic constituents; the base number is the quantity of acid expressed as the equivalent number of milligrams of KOH per gram. The result indicates relative changes occurring in an oil during use under oxidizing conditions; it is not an absolute measure from which the performance of the oil in service can be predicted, and no general relationship between bearing corrosion and the acid or base number is known.

REAGENTS AND APPARATUS (clauses 5–6). Titration solvent: 500 ml of toluene and 5 ml of water added to 495 ml of anhydrous propan-2-ol (water at most 0.9 % (V/V)). Titrants: alcoholic KOH solution 0.1 mol/l (6 g KOH in about 1 l of propan-2-ol, gentle boiling for 10–15 min, addition of at least 2 g Ba(OH)₂ and boiling again for 5–10 min, standing for several hours and filtering; storage in a chemically resistant bottle with a soda-lime guard tube, not in a glass bottle) and alcoholic HCl solution 0.1 mol/l (9 ml of concentrated HCl per 1 l of propan-2-ol). Both solutions are standardized frequently enough to detect a change of 0.000 5 mol/l — KOH against 0.1–0.15 g of potassium acid phthalate in about 100 ml of carbon-dioxide-free water with phenolphthalein, HCl preferably electrometrically — at a temperature close to that of the titrations and close to 20 °C. Indicator: 1.0 g of p-naphtholbenzein in 100 ml of titration solvent, meeting the requirements of Annexes A and B (in a 1992 study only one commercial source met the requirements of Annex A); for the strong acid number — methyl orange, 0.1 g in 100 ml of water. Apparatus: a 50 ml burette graduated in 0.1 ml or a 10 ml burette graduated in 0.05 ml.

COURSE OF THE TEST (clauses 7–12). Used oil is heated in its original container to (60 ± 5) °C and agitated until the sediment is homogeneously suspended, then filtered through a 150 µm wire mesh. A test portion depending on the type of oil and the expected number is weighed into a 250 ml conical flask (Table 1: new or light-coloured oils — 20 ± 2 g for a number up to 3, 2 ± 0.2 g for a number from 3 to 25, 0.2 ± 0.02 g for a number from 25 to 250; used or dark-coloured oils — 2 ± 0.2 g for a number up to 25, 0.2 ± 0.02 g above), 100 ml of solvent and 0.5 ml of indicator are added and the flask is swirled until dissolution. A yellow-orange colour means determining the acid number: titration immediately, below 30 °C, with the KOH solution added in increments, until the colour changes from orange to green or green-brown and persists for 15 s or reverses on addition of two drops of the HCl solution; with dark oils the flask is shaken vigorously to produce a slight foam momentarily, and the end point is observed under a white fluorescent lamp. A green or green-brown colour means determining the base number — titration with the HCl solution until the colour changes to orange. In both cases a blank titration is made on 100 ml of solvent and 0.5 ml of indicator. The strong acid number is determined in the aqueous extract of about 25 g of sample (three extractions with boiling water: 100 ml and twice 50 ml), titrated with the KOH solution against methyl orange to a golden-brown colour; when the extract does not turn pink or red, the strong acid number is zero. Calculation: acid number = (V₁ − V₀) · c(KOH) · 56.1 / m; the strong acid number and the base number — according to formulas 11.2–11.3. The result is reported to the nearest 0.1.

PRECISION (clause 13, Table 2). Repeatability and reproducibility: for a number up to 0.1 — 0.03 and 0.04; from 0.1 to 0.5 — 0.05 and 0.08; from 0.5 to 1.0 — 0.08 and 15 % of the level; from 1.0 to 2.0 — 0.12 and 15 % of the level. The data apply only to new light-coloured straight mineral oils and to new and used inhibited steam turbine oils; they do not apply to oils so highly coloured that the colour change cannot be seen.

WHAT WE DO NOT GIVE HERE. The requirements for p-naphtholbenzein in Annexes A and B or the content of the corrigendum ISO 6618:1997/Cor 1:1999 — they are outside the available text of the standard.

Method principle

The test portion is dissolved in a mixture of toluene and propan-2-ol containing a small amount of water, and the resulting single-phase solution is titrated at room temperature with a standard alcoholic base (acid number) or alcoholic acid (base number) solution to the end point indicated by the colour change of p-naphtholbenzein — orange in acid, green-brown in base.

For the strong acid number, a separate portion of the sample is extracted with boiling water and the extract is titrated with KOH solution against methyl orange.

Applications

• Petroleum products and lubricants soluble in a mixture of toluene and propan-2-ol — acid number, strong acid number and base number (clause 1)
• New and used oils — relative changes during use under oxidizing conditions (clause 1)
• New light-coloured straight mineral oils and new and used inhibited steam turbine oils — precision has been established for them (clause 13)
• Outside the scope: basic constituents of many basic additive-type oils (ISO 3771) and oils in which the colour change cannot be seen (ISO 6619)

Key parameters

Standard

Standard number

PN-ISO 6618:2011 (Polish version; adopts ISO 6618:1997 [IDT] and Cor 1:1999)

Title (PL)

Przetwory naftowe i środki smarowe — Oznaczanie liczby kwasowej i zasadowej — Metoda miareczkowania wobec wskaźników barwnych

Title (EN)

Petroleum products and lubricants — Determination of acid or base number — Colour-indicator titration method

Step-by-step procedure

1. Preparing used oil

   Heating in the original container to (60 ± 5) °C, agitating until the sediment is homogeneously suspended, filtering through a 150 µm mesh; used oil is tested as soon as possible after sampling (clause 7).

2. Test portion and dissolution

   Portion according to Table 1 in a 250 ml flask, 100 ml of solvent and 0.5 ml of indicator; yellow-orange colour — acid number, green or green-black — base number (8.1).

3. Titration

   Immediately, below 30 °C: with KOH solution until the colour changes to green or green-brown (acid number) or with HCl solution until it changes to orange (base number); end point — when the colour persists for 15 s or reverses with two drops of HCl (8.2, 9.1).

4. Blank titration

   100 ml of solvent and 0.5 ml of indicator, KOH added in 0.05 ml or 0.1 ml increments until the colour changes from orange to green (8.3, 9.2).

5. Strong acid number

   About 25 g of sample is extracted with boiling water (100 ml and twice 50 ml), and the extract is titrated with KOH solution against methyl orange to a golden-brown colour; blank with 200 ml of boiling water (clause 10).

6. Calculation and result

   Acid number = (V₁ − V₀) · c(KOH) · 56.1 / m; strong acid number and base number according to 11.2–11.3; result to the nearest 0.1 (clauses 11–12).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard may involve hazardous materials, operations and equipment and does not address all hazards — safety measures are established by the user (warning of the standard)
• The KOH solution is prepared by boiling it in propan-2-ol (5.5.1), and the strong acid number is determined in an extract made with boiling water (10.1) — risk of burns

Frequently asked questions