Crude Oil Acid Number Testing: Principles, Standards, and Industrial Applications

Crude oil acid number testing, most commonly reported as the Total Acid Number (TAN), quantifies the acidic constituents in crude oil and petroleum feeds by measuring the milligrams of potassium hydroxide required to neutralize the acids in one gram of sample. Accurate acid number determination is critical for evaluating naphthenic acid corrosion risk, feedstock acceptability, and the commercial valuation of crude oil cargoes. This article presents the technical principles, applicable standards such as ASTM D664 and GB/T 18609, sample requirements, testing procedures, and practical applications of crude oil acid number analysis in the petroleum industry.

Technical Overview

The acid number of crude oil is defined as the quantity of base, expressed in milligrams of potassium hydroxide (KOH), required to neutralize the acidic constituents present in one gram of sample. The acidic species in crude oil are predominantly naphthenic acids (cycloalkane carboxylic acids), along with smaller contributions from phenolic compounds, fatty acids, dissolved carbon dioxide, hydrogen sulfide, and occasionally inorganic acids. Because crude oil is dark and opaque, the acid number is almost always determined by potentiometric titration rather than visual color-indicator methods.

Total Acid Number (TAN) measures both weak and strong acids, while Strong Acid Number (SAN) isolates the mineral acid fraction through a separate titration with alcoholic acid. Crude oils are conventionally classified as high-acid crudes when the TAN exceeds 1.0 mg KOH/g, although many refiners apply stricter internal thresholds such as 0.5 mg KOH/g depending on unit metallurgy and sulfur content. Elevated acid numbers are strongly correlated with reservoir biodegradation and are a leading indicator of naphthenic acid corrosion, which attacks carbon steel equipment in the 220 to 400 degree Celsius range of atmospheric and vacuum distillation units. Reliable acid number data therefore underpins crude purchasing decisions, refinery blend planning, corrosion mitigation programs, and custody-transfer quality disputes.

Testing Samples

Conventional and Light Crude Oils

Standard crude oil cargoes, wellhead samples, and tank composites with TAN values typically below 0.5 mg KOH/g. These samples are analyzed to confirm feedstock specifications and to verify that acidity remains within refinery acceptance limits.

Heavy and Extra-Heavy Crude Oils

Viscous crudes that require heating to 40 to 60 degrees Celsius for representative sampling and homogenization. Larger sample masses and careful mixing are essential because acidic species can concentrate in the heavier fractions and uneven distribution leads to biased results.

High-TAN (High Naphthenic Acid) Crudes

Crude oils with TAN values above 1.0 mg KOH/g, often sourced from heavily biodegraded reservoirs. These feedstocks trade at a discount and require dedicated testing to define blending ratios and metallurgical safeguards before processing.

Condensates, Gas Liquids, and Refined Intermediate Streams

Natural gas condensates, kerosene and diesel fractions, atmospheric and vacuum gas oils, and residual streams are also tested for acid number to monitor corrosion risk in specific boiling ranges and to check compliance with product acidity specifications.

Sample Submission Requirements

A minimum of 100 to 250 mL of homogenized sample is typically requested, supplied in sealed glass or metal containers with minimal headspace. Samples should be protected from light, heat, and air oxidation, and should be accompanied by expected TAN range, sampling point, and any known additive or caustic treatment history, since these can affect titration behavior.

Testing Items

• Total Acid Number (TAN), expressed in mg KOH/g, covering the full range of weak and strong organic and inorganic acids
• Strong Acid Number (SAN), quantifying mineral acid contamination separately from weak organic acids
• Acid number distribution across distillation cuts, obtained by combining fractionation (for example, ASTM D2892 or D5236) with TAN determination of each fraction to locate the naphthenic acid concentration profile
• Naphthenic acid characterization studies supporting corrosion modeling and neutralization treatment design
• Comparative acid number data for crude blends to confirm that blended feed TAN meets the refinery target ceiling

Testing Methods

Potentiometric Titration - ASTM D664 and GB/T 18609

The reference method for crude oil is ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. The Chinese national standard GB/T 18609, covering acid number determination of crude oil by potentiometric titration, is technically aligned with this approach, as is ISO 6619 for neutralization number determination. In the procedure, a weighed quantity of sample, typically between 1 and 20 grams depending on the expected acid number, is dissolved in a mixed solvent of toluene, isopropyl alcohol, and water (approximately 50:49.5:0.5 by volume). The solution is titrated with standardized 0.1 mol/L alcoholic potassium hydroxide while the cell potential is monitored with a glass electrode and reference electrode system. The endpoint is detected as a potential inflection, and the acid number is calculated as TAN equals (A minus B) times normality of KOH times 56.1 divided by the sample mass in grams, where A and B are the titrant volumes for the sample and blank. The method is applicable across a broad range from roughly 0.1 to 150 mg KOH/g.

Color-Indicator Titration - ASTM D974 and ASTM D3339

ASTM D974 determines acid number by titration with p-naphtholbenzein indicator, and ASTM D3339 is a semi-micro version requiring smaller sample sizes. Both are reliable for pale, transparent distillates but are generally unsuitable for dark crude oils because the natural sample color obscures the indicator endpoint. They remain useful for refined product streams associated with crude evaluation programs.

Method Selection and Quality Control

For crude oil, potentiometric titration is the preferred and referee technique owing to its objectivity, sensitivity, and applicability to dark samples. Good laboratory practice requires pre-warming and thorough homogenization of viscous samples, exclusion of atmospheric carbon dioxide, blank titrations of the solvent, duplicate determinations meeting repeatability limits, and routine maintenance and calibration of electrode systems. Accredited third-party laboratories commonly cross-check results against certified reference materials and can perform both ASTM and GB method variants to support international and domestic trading requirements.

Supplementary Rapid Techniques

Fourier-transform infrared (FTIR) spectroscopy and near-infrared (NIR) chemometric methods are increasingly used for high-throughput screening once calibrated against ASTM D664 titration data. These rapid techniques are valuable for screening large numbers of cargo or blend samples, but titration remains the arbiter method for specification and dispute resolution.

Applications

Crude oil acid number data are applied throughout the petroleum value chain. In crude trading and valuation, TAN is a contractual quality parameter, and high-acid crudes are priced at a discount reflecting processing risk. In refinery planning, acid number results define blending strategies that keep blended feed TAN below limits consistent with unit metallurgy, often targeting values below 0.5 mg KOH/g for conventional carbon steel equipment. In corrosion engineering, boiling-range acid number profiles guide the placement of corrosion-resistant alloys, injection of corrosion inhibitors and high-temperature neutralizers, and inspection priorities in furnaces, transfer lines, and vacuum columns. In catalyst and operations management, acid data help prevent fouling, soap formation, and catalyst poisoning caused by naphthenic acid salts. Acid number testing also supports pipeline integrity programs, produced-fluid surveillance in maturing fields, verification of caustic or esterification treatment performance, and quality certification for custody transfer and marine cargo inspection.

FAQ

What is considered a high acid number for crude oil?

Crude oil is generally regarded as high-acid when the TAN exceeds 1.0 mg KOH/g. Many refiners impose stricter internal acceptance limits around 0.5 mg KOH/g, particularly when processing through units with standard carbon steel metallurgy or when sulfur levels are also elevated.

Are acid number, acid value, and TAN the same thing?

Yes. Acid value is an alternative name for acid number, and TAN is the standard industry abbreviation for Total Acid Number determined on the whole sample. All three terms express acidity in mg KOH per gram of sample.

Why is potentiometric titration preferred over colorimetric methods for crude oil?

Crude oil is dark and opaque, which masks color-indicator endpoints used in methods such as ASTM D974. Potentiometric titration detects the endpoint electrically, providing objective, reproducible results that are insensitive to sample color.

Do sulfur compounds interfere with acid number results?

Hydrogen sulfide and certain mercaptans can register in the titration and contribute to the apparent acid number. Degasification and consistent sample handling minimize this effect, and results should be interpreted alongside total sulfur data because sulfur species strongly influence the severity of naphthenic acid corrosion.

How much sample is needed and what is a typical reporting turnaround?

Most laboratories request 100 to 250 mL of homogenized crude oil. Routine TAN determinations are typically completed within three to five working days, with expedited services available for cargo inspection and operational decisions.

How can the acid number of a crude feed be reduced?

The most common industrial approach is dilution blending with low-TAN crudes. Other options include caustic washing, adsorption treatment, and chemical esterification of naphthenic acids, each of which must be evaluated for cost, byproduct formation, and downstream impact.