⛽ Total Base Number (TBN)

Petrochemistry PN-ISO 3771

Overview

STATUS (as of 30.10.2026). PN-ISO 3771:2012 — Polish version, published 05.11.2012, implements ISO 3771:2011 [IDT]; it replaced PN-C-04163:1976. The PKN catalogue card has no withdrawal note — the standard is current; Polish title according to the card: “Przetwory naftowe — Oznaczanie liczby zasadowej — Metoda potencjometrycznego miareczkowania kwasem nadchlorowym”. ISO 3771:2011 (third edition) replaced ISO 3771:1994 after a technical revision.

SCOPE AND DEFINITION (clauses 1, 3). Determination of basic constituents in petroleum products — organic and inorganic bases, amino compounds, salts of weak acids (e.g. soaps), basic salts of polyacid bases and salts of heavy metals. The base number is the quantity of perchloric acid, expressed as the equivalent number of milligrams of potassium hydroxide, required to titrate 1 g of sample dissolved in the specified solvent to a well-defined inflection point. Precision was established for unused oils with base numbers 3–45, additive concentrates 5–45 and used oils 3–30.

SOLVENT AND ELECTRODE (5.9, 6.2, 8.3). Titration solvent: 6 volumes of toluene, 3 volumes of glacial acetic acid and 1 volume of acetone — the 2011 edition does not use chlorobenzene. Combination pH-Ag/AgCl electrode for non-aqueous media: low membrane resistance (100–200 MΩ), flexible ground-sleeve diaphragm, shielded reference electrode, non-aqueous electrolyte (e.g. 0.4 mol/l tetraethylammonium bromide in ethylene glycol or a saturated solution of lithium chloride in ethanol). The electrode is checked with pH 4 and pH 7 buffers — a potential difference of at least 150 mV.

STANDARDIZATION OF THE TITRANT (5.10). The 0.1 mol/l perchloric acid solution in acetic acid can be bought or prepared from concentrated perchloric acid, acetic acid and acetic anhydride (Table 1) and stored for at least 24 h. The titre is determined against tris(hydroxymethyl)aminomethane or potassium hydrogen phthalate for each new solution, at least once a week; certified solutions may be used instead of standardization.

WHAT WE DO NOT GIVE HERE. The publicly available text of the standard ends with the choice of sample mass (Table 2, 9.1). We do not give the further course of the titration, the endpoint determination, the calculation (clause 10) or the precision. The American method ASTM D2896 on the same subject states that the base number is sometimes used as a measure of lubricant degradation in service, but condemning limits must be established empirically; that method uses chlorobenzene and gives a chlorobenzene-free solvent as an alternative with poorer precision.

Method principle

The sample is dissolved in an essentially anhydrous mixture of toluene, acetone and glacial acetic acid and titrated with a standard volumetric solution of perchloric acid in glacial acetic acid using a potentiometric titrimeter with a combination pH-Ag/AgCl electrode. The meter readings are plotted against the volume of titrant, and the endpoint is taken as the last inflection of the curve (clause 4).

Applications

• Unused oils — base number 3–45 (precision range, clause 1)
• Additive concentrates — base number 5–45 (clause 1)
• Used oils — base number 3–30 (clause 1); condemning limits are established empirically (ASTM D2896)

Key parameters

Standard

Standard number

PN-ISO 3771:2012 (Polish version; adopts ISO 3771:2011 [IDT])

Title (PL)

Przetwory naftowe — Oznaczanie liczby zasadowej — Metoda potencjometrycznego miareczkowania kwasem nadchlorowym

Title (EN)

Petroleum products — Determination of base number — Perchloric acid potentiometric titration method

Step-by-step procedure

1. Preparing the titrant

   Perchloric acid is dissolved in 500 ml of acetic acid, acetic anhydride is added in the amount from Table 1 and the solution is made up to 1 l with acetic acid; it is stored for at least 24 h (5.10.1).

2. Standardization

   About 0.05 g of tris(hydroxymethyl)aminomethane or 0.1–0.2 g of potassium hydrogen phthalate, weighed to 0.1 mg, is dissolved in 60 ml of acetic acid and titrated with the same electrode system and the same endpoint method as the samples; the titre is determined for each new solution, at least once a week (5.10.2–5.10.3).

3. Preparing the electrode

   The electrolyte is topped up as high as possible, and measurements are made with the filling plug removed; a new electrode is conditioned in the solvent for 12 h. If in doubt, the electrode is checked with pH 4 and 7 buffers — a difference of at least 150 mV (8.1, 8.3).

4. Preparing the sample

   The sample must be representative — sediment may be acidic or basic; if necessary the sample is warmed to 60 °C to aid mixing, and used oils are shaken vigorously (clause 7).

5. Test portion

   The sample mass is chosen so that about 5 ml of titrant is used at the inflection: 2.00 g for base number 3–15, 1.00 g for 15–30, 0.25 g for 30–45 (Table 2).

6. Titration

   The sample dissolved in the solvent is titrated with perchloric acid while the potentiometric curve is recorded; the endpoint is the last inflection. The further procedure and the calculation are in clauses 9–10 of the standard, outside the publicly available text (clause 4).

7. Cleaning and storing the electrode

   Between titrations the electrode is thoroughly washed with solvent — washing with toluene, then petroleum ether, then toluene again is effective; no oil film may remain. Before long-term storage the electrode is cleaned and kept in the same electrolyte that is used to fill it, with the filling plug closed (8.2, 8.4).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard warns that its use may involve hazardous materials, operations and equipment, and that the user establishes the safety and health practices (warning before clause 1)
• The 0.1 mol/l perchloric acid solution is not harmful under the test conditions, but concentrated perchloric acid is a powerful oxidant when dry or heated — contact with organic matter under such conditions is avoided and spills are washed off immediately with plenty of water (5.10.1)
• Toluene, acetone, acetic acid and acetic anhydride — according to their safety data sheets

Frequently asked questions