Determination of Acid Value in Chemical Raw Materials: Principles, Methods, Standards, and Applicati

Acid value determination is one of the most fundamental and widely performed analyses for chemical raw materials, providing a quantitative measure of free acidity that directly reflects product quality, purity, and degradation status. This technical article explains the principle of acid value testing, applicable international and national standards, titration methodologies, sample types, and practical considerations for accurate measurement. It serves as a professional reference for manufacturers, quality control engineers, and laboratory personnel involved in raw material inspection and process monitoring.

Technical Overview

The acid value (also called acid number) is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the acidic constituents present in one gram of a sample. It is expressed in units of mg KOH/g and serves as a key quality indicator for a wide range of chemical raw materials, including resins, polyols, plasticizers, fatty acids, oils, and organic intermediates.

In chemical raw materials, acidity originates primarily from free carboxylic acids, residual acidic catalysts, acidic degradation products from hydrolysis or oxidation, and unreacted acid raw materials. A higher-than-specified acid value typically indicates incomplete reaction, contamination, moisture-induced hydrolysis, oxidative degradation, or improper storage. Elevated acidity can cause discoloration, corrosion of processing equipment, catalyst poisoning, undesirable side reactions during polymerization, and deterioration of the performance of downstream products such as coatings, adhesives, foams, and plastics.

Accurate acid value determination is therefore essential for incoming material inspection, batch release, process control, shelf-life evaluation, and failure analysis. Most test protocols are based on acid-base titration in non-aqueous solvent systems, following established standards published by ISO, ASTM, EN, and GB standardization organizations.

Testing Samples

Polymer Resins

Resin materials are among the most common samples for acid value testing, including epoxy resins, unsaturated polyester resins, alkyd resins, phenolic resins, acrylic resins, and amino resins. For these materials, the acid value reflects residual carboxyl groups from polyesterification, unreacted acids or anhydrides, and hydrolytic degradation products, and it is a decisive parameter for controlling curing behavior, viscosity, and film properties.

Polyurethane Raw Materials

Polyether polyols and polyester polyols used in polyurethane production require strict acid value control, because residual acidity neutralizes the amine or organotin catalysts used in foaming and elastomer formulations, leading to slow or incomplete curing. Isocyanate intermediates and prepolymers may also be monitored for acidic impurities.

Plasticizers and Ester Products

Ester-type plasticizers such as phthalate, adipate, sebacate, and trimellitate esters are tested for free acid content derived from incomplete esterification or hydrolysis. Excess free acid affects thermal stability, electrical properties, and compatibility with polymers.

Fatty Acids, Oils, and Fatty Acid Derivatives

Vegetable and animal oils, distilled fatty acids, fatty acid methyl esters, dimer acids, and fatty alcohols are routinely analyzed for acid value to assess refining efficiency, hydrolytic rancidity, and oxidation during storage and transport.

Solvents, Intermediates, and Other Chemicals

Organic solvents, surfactant intermediates, lubricant base stocks, coating materials, monomers, and miscellaneous fine chemicals may also be subject to acid value specification, particularly when trace acidity can compromise downstream reactions or product stability.

Testing Items

The core testing item is the acid value (acid number), reported in mg KOH/g. Depending on the material specification, the acid value test is often performed together with complementary parameters to give a complete acidity and composition profile:

• Acid value (acid number) of resins, polyols, plasticizers, oils, and fatty acids
• Free acid content expressed as a specific acid, such as free acid as adipic acid or phthalic acid
• Hydroxyl value, for polyols and resins, to characterize reactive hydroxyl content
• Saponification value, to evaluate total ester and free acid content
• Iodine value, for unsaturated oils and fatty acids, to assess unsaturation degree
• Water content (Karl Fischer titration), because moisture accelerates hydrolysis and increases acidity

Testing Methods

Color Indicator Titration Method

In the classical color indicator method, the sample is dissolved in a suitable solvent system, typically a mixture of ethanol, isopropanol, toluene, or diethyl ether, often with a small amount of water. The solution is titrated with standardized alcoholic potassium hydroxide (KOH) solution to the endpoint of a pH-sensitive indicator such as phenolphthalein, Alkali Blue 6B, or p-naphtholbenzein. The endpoint is the persistent color change observed in a non-aqueous medium. This method is simple, rapid, and cost-effective, and it is suitable for light-colored or transparent samples where the endpoint color change is clearly visible. Representative standards include ASTM D974 and GB/T 1668 for plasticizers, as well as corresponding national and industry methods for resins and oils.

Potentiometric Titration Method

In the potentiometric method, the sample is dissolved in a mixed solvent such as toluene and isopropanol with a small proportion of water, and titration is performed with standardized alcoholic KOH while monitoring the potential of a glass electrode-reference electrode pair. The endpoint is located from the inflection point of the titration curve using automatic titration equipment. This approach eliminates subjective color judgment, making it the preferred choice for dark-colored, opaque, or highly colored samples such as aged resins, oxidized oils, and used chemical products. It also offers better precision and lower detection limits for samples with very low acidity. Representative standards include ASTM D664 and related automatic titration procedures.

Applicable Standards

• ASTM D664 - Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
• ASTM D974 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• ASTM D1639 - Standard Test Method for Acid Value of Organic Coating Materials
• ASTM D1386 - Standard Test Method for Acid Number (Empirical) of Synthetic and Natural Waxes
• ASTM D4662 - Standard Test Methods for Polyurethane Raw Materials: Determination of Acidity as Acid Number for Polyether Polyols
• ISO 660 - Animal and vegetable fats and oils: Determination of acid value and acidity
• ISO 2114 - Plastics: Unsaturated polyester and epoxy resins: Determination of acid value
• GB/T 2895 - Plastics: Unsaturated polyester resins: Determination of acid value
• GB/T 1668 - Plasticizers: Determination of acid value and ester content
• GB/T 12008.5 - Plastics: Polyether polyols: Determination of acid value

Calculation and Data Treatment

The acid value is calculated from the volume of titrant consumed by the sample, corrected by a blank determination, according to the following general relationship:

AV = (V − V0) × c × 56.1 / m

where AV is the acid value in mg KOH/g, V is the titrant volume consumed by the sample in mL, V0 is the blank titrant volume in mL, c is the exact molar concentration of the KOH titrant in mol/L, 56.1 is the molar mass of KOH in g/mol, and m is the sample mass in grams. Results are typically reported to one or two decimal places, with duplicate determinations required to agree within the repeatability limits stated in the applicable standard.

Factors Affecting Accuracy

• Sample homogeneity and representativeness; viscous or solid resins should be warmed gently and mixed thoroughly before subsampling
• Appropriate sample mass selection based on expected acid value, so that the titrant consumption falls within the reliable working range
• Complete dissolution of the sample; incomplete dissolution causes low and scattered results
• Standardization of the KOH titrant and protection of alcoholic KOH from atmospheric carbon dioxide
• Carbon dioxide exclusion during titration, especially for low acid value samples, by using blank corrections and minimized exposure
• Correct endpoint detection; dark samples should be analyzed potentiometrically rather than by visual indicator
• Temperature control, as endpoint behavior in non-aqueous media depends on titration temperature specified by the method

Applications

Acid value determination plays a critical role throughout the chemical industry value chain:

• Incoming raw material inspection: verification of supplier conformity for resins, polyols, plasticizers, and fatty chemicals against purchase specifications
• Manufacturing process control: monitoring of esterification, polycondensation, and alkoxylation reactions to determine reaction completion and catalyst performance
• Product release testing: batch certification of finished resins, plasticizers, and intermediates against national, international, or customer standards
• Stability and shelf-life assessment: tracking acid value increase during accelerated aging, heat exposure, or long-term storage to evaluate hydrolytic and oxidative stability
• Failure analysis and troubleshooting: identification of acidity-related defects such as catalyst deactivation in polyurethane foaming, gel-time drift in unsaturated polyester systems, and corrosion or discoloration in coated articles
• Research and development: characterization of new monomers, bio-based polyols, recycled materials, and modified resins during formulation development
• Trade and regulatory compliance: provision of compliant test data for import-export inspection, customs clearance, and certification by accredited third-party laboratories

FAQ

What is the difference between acid value and acid number?

The two terms are synonymous. Both refer to the mass of KOH in milligrams required to neutralize the acidic components in one gram of sample, expressed as mg KOH/g. Different standards organizations and industries favor one term or the other, but the measurement principle and reporting unit are identical.

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

Color indicator titration is suitable for light-colored, transparent samples with moderate to high acidity, offering simplicity and low cost. Potentiometric titration is recommended for dark, opaque, or highly colored samples, for materials with very low acid values, and whenever higher precision, automated operation, or traceable objective endpoint detection is required. The applicable product standard or customer specification usually designates the required method.

Why do acid values increase during storage?

Acid value growth during storage is most commonly caused by hydrolysis of ester groups in the presence of moisture and by oxidative degradation of unsaturated structures. Elevated temperature, light, metallic contamination, and air exposure accelerate these reactions. An increasing acid value is therefore a sensitive early indicator of material aging and quality deterioration.

What solvents are used for sample dissolution?

Common solvent systems include ethanol-diethyl ether mixtures, toluene-isopropanol mixtures with a small amount of water, and ethanol-toluene mixtures. The solvent must fully dissolve the sample and the sample acidity must be completely released to the titration medium. The specific solvent system is prescribed by the applicable test standard for each material class.

How much sample is required for a test?

The required sample mass depends on the expected acid value. As a general rule, larger sample masses are used for low-acid-value materials so that titrant consumption remains measurable, while smaller masses are used for highly acidic samples to keep titrant consumption within practical limits. Typical sample masses range from about 1 g to 20 g; in total, approximately 50 g to 100 g of sample is usually sufficient to perform duplicate determinations with the necessary preparation.

Can acid value testing be performed on solid resin materials?

Yes. Solid or highly viscous resins are first crushed or cut into small pieces and then dissolved in a heated solvent mixture, such as a toluene-isopropanol or ethanol-toluene system, until a clear solution is obtained. Gentle warming, not exceeding the temperature limits of the method, may be used to aid dissolution before titration.

What repeatability can be expected?

Repeatability depends on the acid value level and the standard applied. For well-controlled potentiometric titration of resins and polyols, duplicate results typically agree within a few percent relative deviation. Each published standard specifies the permissible difference between duplicate determinations, and laboratories should demonstrate compliance through method validation, control charts, and regular participation in proficiency testing.