Determination of Acid Value in Diesel Fuel: Standards, Methods, and Quality Significance

Acid value determination is a critical quality control test for diesel fuel, quantifying acidic constituents that can cause corrosion in fuel injection systems and signal oxidative degradation during storage. This technical article provides a comprehensive overview of diesel acid value testing, covering applicable standards such as ASTM D664, ASTM D974, ISO 6619, and GB/T 258, titration procedures, result interpretation, and practical guidance for fuel producers, distributors, and quality assurance laboratories.

Technical Overview

The acid value (also referred to as acid number or neutralization value) is a fundamental quality parameter used to evaluate the concentration of acidic compounds present in diesel fuel. It is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the acidic constituents in one gram of sample, and is expressed in mg KOH/g. For diesel fuels, the acid value reflects the combined contribution of naturally occurring organic acids, oxidation products, residual refining acids, and acidic contaminants introduced during production, transport, or storage.

An excessively high acid value in diesel fuel is a strong indicator of oxidation, degradation, or contamination. Acidic compounds promote corrosion of fuel injection pumps, nozzles, storage tank linings, and other engine components. They can also react with fuel additives, form deposits, and accelerate gum formation, all of which compromise combustion performance and equipment reliability. Consequently, most diesel fuel specifications, including the Chinese national standard GB 19147 and the European standard EN 590, impose strict limits on acidity, and routine acid value determination is a standard requirement in refinery laboratories, fuel terminals, and accredited third-party laboratories.

Modern acid value determination relies primarily on titrimetric techniques, performed either by potentiometric endpoint detection or by visual color-indicator titration. Both approaches quantify the amount of strong base (standard alcoholic potassium hydroxide solution) consumed when neutralizing the acids dissolved in a mixed organic solvent system.

Testing Samples

Refinery Diesel Products

Straight-run diesel, hydro-treated diesel, and blended diesel grades (such as No. 0, No. -10, and No. -35 diesel under GB 19147) are routinely sampled at production units, blending facilities, and product shipment points to verify conformity with acidity limits prior to release.

Biodiesel and Biodiesel Blends

Fatty acid methyl ester (FAME) biodiesel and its blends with petro-diesel (such as B5 and B7) are particularly susceptible to the presence of free fatty acids. Biodiesel standards such as EN 14214 and ASTM D6751 specify acid value limits, making blend verification an essential step in the supply chain.

Stored and Aged Diesel

Diesel held in long-term storage tanks, emergency generator reserves, or marine bunker tanks is periodically tested for acid value to monitor oxidation stability and determine whether the fuel remains fit for use or requires polishing and reconditioning.

Imported and Traded Diesel

Commercial consignments moving through import, export, and spot trading channels are sampled and tested by accredited third-party laboratories to certify compliance with contractual and regulatory specifications.

Contaminated or Disputed Fuel

Fuel involved in quality disputes, equipment failure investigations, or suspected contamination events is analyzed to establish whether elevated acidity contributed to corrosion or performance problems.

Testing Items

Acid value testing services for diesel fuel typically encompass the following analytical items:

• Total acid number (TAN) — the quantity of base, expressed as mg KOH/g, required to titrate a sample from its initial state to a specified endpoint, covering both strong and weak acidic species.
• Strong acid number — the portion of acidity attributable to strong inorganic acids, determined by separate titration to an intermediate endpoint.
• Acidity expressed as mg KOH/100 mL — an alternative reporting format used in GB/T 258 for light distillate fuels such as diesel.
• Conformity assessment against specification limits in GB 19147, EN 590, ASTM D975, and related fuel standards.
• Comparative assessment of oxidation degree for condition monitoring of stored fuel.

Testing Methods

Potentiometric Titration — ASTM D664 / ISO 6619

The sample is dissolved in a mixed solvent of toluene and isopropyl alcohol containing a small amount of water, then titrated with standardized alcoholic potassium hydroxide solution while a glass electrode and reference electrode monitor the potential of the solution. The endpoint is located at the inflection point of the titration curve. This method is suitable for both clear and dark-colored diesel samples, eliminates subjective color judgment, and delivers high precision with a typical repeatability of a few percent relative. It is the preferred technique for automated titration systems in modern laboratories.

Color-Indicator Titration — ASTM D974 / ISO 6618

The sample is dissolved in the titration solvent with p-naphtholbenzein indicator added, and titration with alcoholic KOH proceeds until the indicator changes from orange to green or green-blue. The method is relatively rapid and requires simple equipment, but it is restricted to samples that are light enough in color for the endpoint color change to be observed clearly, making it less suitable for dark or heavily degraded diesel.

GB/T 258 — Acid Value by Ethanol Extraction and Indicator Titration

Widely applied in China for gasoline, diesel, and kerosene, GB/T 258 extracts the acidic constituents into hot aqueous ethanol, after which the extract is titrated with standard potassium hydroxide solution using phenolphthalein or an alkali blue indicator to a defined color endpoint. Results are commonly reported as mg KOH/100 mL. GB 19147 specifies that diesel acidity determined by this method must not exceed 7 mg KOH/100 mL.

Procedure and Key Operational Points

• Sample handling: warm the sample to ambient or slightly elevated temperature, homogenize thoroughly, and filter out particulates or free water before weighing.
• Solvent preparation: the titration solvent must be neutralized before use, and carbonate contamination from atmospheric carbon dioxide must be minimized to avoid endpoint drift.
• Sample mass: select the sample size according to the expected acid number so that titrant consumption falls within the optimal burette or autotitrator range.
• Electrode maintenance (potentiometric method): clean and condition the glass electrode regularly, as electrode fouling by heavy hydrocarbons is the leading cause of distorted curves and missed inflection points.
• Titration speed: control the titrant delivery rate to allow equilibrium to be reached near the endpoint, particularly for weak organic acids.
• Blank determination: run solvent blanks under identical conditions and subtract the blank volume from the sample titration in the calculation.

Calculation

The acid value is calculated as the product of the net titrant volume, the normality of the KOH solution, and 56.1 (the molar mass of KOH), divided by the sample mass in grams. For GB/T 258, the result is converted to mg KOH/100 mL of fuel. Corrected endpoint volumes and validated blank values are essential for accurate reporting.

Applications

Diesel acid value determination serves a wide range of industrial purposes:

• Refinery quality control — confirming that hydrotreating and finishing operations have reduced acidity to specification levels before product release.
• Fuel specification compliance — demonstrating conformity with GB 19147, EN 590, ASTM D975, and customer contractual requirements for commercial fuel batches.
• Biodiesel blend management — verifying that FAME components and finished blends meet acid value limits in EN 14214 and ASTM D6751, and monitoring blend stability over time.
• Storage condition monitoring — tracking oxidation of strategic reserves, generator fuel, and marine bunker stocks to schedule fuel replacement or stabilization treatment.
• Failure investigation — providing evidence in root cause analysis of injection pump corrosion, filter plugging, and deposit-related engine malfunctions.
• Research and development — evaluating oxidation stability, additive effectiveness, and alternative fuel formulations during new product development.
• Trade and custody transfer — supporting inspection and certification of fuel cargoes at loading and discharge ports by accredited third-party laboratories.

FAQ

What is the acceptance limit for diesel acid value?

Under GB 19147, diesel acidity determined by GB/T 258 must not exceed 7 mg KOH/100 mL. For biodiesel, EN 14214 limits the acid value to 0.50 mg KOH/g, and ASTM D6751 specifies a maximum of 0.50 mg KOH/g as well. Always confirm the applicable limit with the governing specification for the fuel grade in question.

What does a high acid value indicate?

An elevated acid value usually signals oxidation of the fuel, contamination with acidic species, degradation during prolonged storage, or incomplete removal of acidic compounds during refining. High acidity increases the risk of corrosion in fuel system components and the formation of gums and deposits.

Which method should be chosen: potentiometric or color-indicator titration?

Potentiometric titration (ASTM D664 / ISO 6619) is preferred for dark, deeply colored, or degraded diesel samples because the endpoint is detected electrically rather than visually. Color-indicator titration (ASTM D974 / ISO 6618) is suitable for light-colored samples where the indicator transition can be observed without interference.

How does sample water content affect the results?

Free water in the sample can cause emulsification, sluggish endpoints, and inaccurate volume readings. Water in the titration solvent must be controlled within the specified ratio, and the solvent must be neutralized prior to titration to prevent biased results.

How frequently should stored diesel be tested for acid value?

For long-term storage, an initial baseline test followed by periodic retesting — commonly every six to twelve months, or more frequently in hot and humid climates — is recommended so that oxidation trends can be identified and corrective action taken before the fuel becomes non-compliant.

Can acid value testing distinguish between strong and weak acids?

Yes. ASTM D664 provides both a total acid number and a strong acid number by evaluating distinct endpoints on the titration curve, which helps differentiate strong inorganic acid contamination from weak organic acids formed by oxidation.