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Amine value

The amine value, also referred to as the amine number, is a key analytical parameter in organic chemistry that measures the total basicity attributable to amine groups in a sample, expressed as the milligrams of potassium hydroxide (KOH) equivalent to the amine nitrogen content per gram of material.^[1] It specifically quantifies the amount of acid required to neutralize the protonatable amine functionalities, providing an indicator of the sample's amine concentration and reactivity.^[2] This value is typically determined through nonaqueous acid-base titration, where the sample is dissolved in a solvent like glacial acetic acid and titrated with a strong acid such as perchloric acid to reach a sharp endpoint, often detected potentiometrically or via color indicators.^[1] Standardized methods, such as ASTM D2074, outline procedures for calculating total, primary, secondary, and tertiary amine values in fatty amines and related compounds, ensuring reproducibility across laboratories.^[3] The measurement avoids aqueous media to prevent interference with amine protonation, enhancing accuracy for low-concentration analyses.^[1] Amine value plays a critical role in industries like coatings, adhesives, and composites, where it guides the formulation of epoxy curing agents by determining the amine hydrogen equivalent weight (AHEW) for optimal stoichiometric mixing with epoxy resins.^[2] In polyurethane production, it quantifies hazardous curing agents like 4,4'-diamino-3,3'-dichlorodiphenylmethane (MOCA), aiding compliance with safety standards such as ASTM D6979 or ISO 25761.^[1] High amine values indicate greater reactivity and potential for faster curing but may also correlate with increased sensitivity to moisture or blushing in final products.^[2] Overall, precise amine value assessment ensures product quality, performance, and regulatory adherence in amine-functional materials.^[3]

Fundamentals

Definition

The amine value is a quantitative measure of the basic nitrogen content in organic materials, specifically defined as the number of milligrams of potassium hydroxide (KOH) equivalent to the basicity arising from amine groups present in 1 gram of sample.^[1] This metric captures the total amine functionality by measuring the capacity of the basic amine groups to neutralize an acid titrant.^[2] Expressed in units of mg KOH/g, the amine value plays a key role in evaluating the concentration of reactive amine groups in various compounds, including epoxy hardeners, fatty amines, and polymers.^[4]^[3]^[1] It is particularly valuable in quality control processes to verify the reactivity and purity of these materials in chemical formulations, ensuring consistent performance and preventing deviations that could affect end-product properties.^[4]^[2] The concept of amine value emerged in mid-20th century analytical chemistry as a standardized way to characterize amine-based materials, with early discussions appearing in industry literature by 1962.^[5] Standardization efforts began in the 1960s through organizations like ASTM, establishing reliable protocols for its determination, primarily via titration methods.^[3]^[1]

Chemical Basis

Amines are organic compounds derived from ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups, resulting in distinct classes based on the number of substituents attached to the nitrogen atom. Primary amines have the general formula RNH₂, where R represents an alkyl or aryl group; secondary amines are R₂NH, with two such groups; and tertiary amines are R₃N, featuring three. This classification reflects the progressive substitution at the nitrogen center, which influences their chemical behavior while maintaining the core tetrahedral geometry around nitrogen similar to ammonia.^[6]^[7] The basicity of amines stems from the lone pair of electrons on the nitrogen atom, which enables protonation by acids to form ammonium ions, a process central to their reactivity. In aliphatic amines, this lone pair is readily available, conferring moderate basic strength with pKa values for the conjugate acid typically ranging from 9 to 11, allowing effective interaction with protic acids. This protonation equilibrium underscores the amine's role as a nucleophilic base, where the nitrogen accepts a hydrogen ion to yield a positively charged ammonium species.^[8]^[9] Amine groups provide basic sites that react with acids such as hydrochloric acid (HCl) or perchloric acid (HClO₄) to form stable ammonium salts, exemplified by the reaction:

\mathrm{R_3N + HCl \rightarrow R_3NH^+ Cl^-}

This salt formation is reversible and driven by the electrostatic attraction in the ionic product, highlighting the fundamental acid-base chemistry of amines.^[10]^[11] Basicity varies significantly between aliphatic and aromatic amines, with the former being stronger bases due to the unhindered lone pair availability on nitrogen, whereas in aromatic amines, resonance delocalization into the aromatic ring diminishes electron density on nitrogen and reduces basicity. Tertiary amines generally form stable ammonium salts akin to their lower counterparts but can present challenges in reactivity distinction owing to the absence of N-H bonds, which affects their behavior in certain protonation scenarios without specialized approaches.^[12]

Measurement Methods

Titration Techniques

Titration techniques for determining amine value primarily involve acid-base reactions in non-aqueous media to ensure accurate measurement of basicity from amine groups. Potentiometric titration is a widely used method, employing perchloric acid (HClO₄) as the titrant in glacial acetic acid solvent, with endpoint detection via a pH meter or glass electrode for precise identification of the equivalence point. This approach enhances the acidity of the titrant and differentiates weak bases like amines, allowing for reliable quantification in polymers and other materials.^[13]^[14] Indicator-based methods provide a visual alternative, particularly for fatty amines, where dyes such as bromophenol blue are added to the sample solution in solvents like isopropyl alcohol, and titration proceeds with hydrochloric acid (HCl) until a color change indicates the endpoint. Bromophenol blue shifts from blue to yellow around pH 3.0-4.6, offering clear detection for total amine content in such samples. Methyl orange can also be used in similar setups for its transition from red to yellow at pH 3.1-4.4, though it is less common for amine-specific analyses due to potential overlaps in color change with other indicators.^[15]^[16] Sample preparation typically involves dissolving 1-5 g of the sample in 50-100 mL of the chosen non-aqueous solvent, such as glacial acetic acid or isopropyl alcohol, followed by addition of excess standard acid and back-titration with a base like potassium hydroxide if needed. To distinguish total amine value from individual types, primary and secondary amines can be selectively acetylated using acetic anhydride, converting them to neutral acetamides that do not interfere; the remaining tertiary amines are then titrated directly with perchloric acid. This step-wise approach enables isolation of tertiary amine content by subtracting it from the total value obtained in a separate direct titration.^[13]^[14] Precautions are essential to maintain accuracy, including the use of strictly non-aqueous media to prevent water interference, which acts as a competing base and broadens the endpoint; all glassware must be dried thoroughly. Titrations should be conducted at controlled temperatures of 20-25°C to minimize solvent volatility and ensure consistent reaction kinetics, and blank determinations on the solvent alone are required to correct for inherent acidity. Electrode conditioning in the solvent prior to use avoids calibration errors from residual water.^[13] These techniques have limitations, including potential interference from other basic compounds or weak acids in the sample that may alter the endpoint sharpness, necessitating sample purification for complex mixtures. They are most suitable for samples with amine content greater than 0.1 mg KOH/g, as lower levels may yield imprecise results due to sensitivity constraints in non-aqueous systems. These methods align with standardized procedures such as those from ASTM for consistent application.^[13]^[14]

Calculation and Units

The amine value is calculated from titration data using the general formula:

\text{Amine value (mg KOH/g)} = \frac{(V_\text{sample} - V_\text{blank}) \times N_\text{titrant} \times 56.1}{w_\text{sample}}

where V_\text{sample} and V_\text{blank} are the volumes of titrant in mL for the sample and blank titrations, respectively, N_\text{titrant} is the normality of the titrant, 56.1 is the equivalent weight of KOH, and w_\text{sample} is the sample weight in grams.^[13]^[17] In perchloric acid titrations, commonly used for non-aqueous media, the formula simplifies when a blank is negligible or absent:

\text{Amine value (mg KOH/g)} = \frac{V_\text{HClO}_4 \times N_\text{HClO}_4 \times 56.1}{m_\text{sample}}

where

V_\text{HClO}_4

is the volume of 0.1 M perchloric acid titrant in mL,

N_\text{HClO}_4

is its normality, and m_\text{sample} is the sample mass in grams.^[13] For samples containing volatile components, such as solvents in epoxy formulations, the amine value is adjusted by dividing by the non-volatile matter (NVM) fraction to reflect the active solid content:

\text{Adjusted amine value} = \frac{\text{Unadjusted amine value}}{\text{NVM fraction}}

where NVM fraction is the proportion of non-volatiles determined separately, typically by evaporation.^[18] Amine values greater than 200 mg KOH/g typically indicate high amine content, as seen in curing agents for epoxy resins with values in the 200–350 mg KOH/g range, making them suitable for applications requiring strong basicity and reactivity.^[19] Conversely, values below 10 mg KOH/g, such as 0.55 mg KOH/g in modified polyols, suggest minimal amine presence and low basicity.^[20] Key error sources in these calculations include imprecision in volume measurements, often limited to ±0.01 mL with calibrated burettes, and inaccuracies in titrant normality, which requires precise standardization against primary standards like potassium acid phthalate.^[21]^[22]

Standardization

ASTM Standards

ASTM International has established several standards for the determination of amine values, primarily through titration methods tailored to specific materials such as fatty amines, petroleum products, and polyurethane raw materials. These standards ensure consistent measurement for quality control in chemical and industrial applications. ASTM D2074 provides alternative indicator-based procedures for measuring total, primary, secondary, and tertiary amine values in fatty amines, excluding fatty amidoamines and fatty diamines. This visual endpoint titration method uses reagents like hydrochloric acid and indicators such as bromophenol blue, offering a simpler approach compared to potentiometric techniques when instrumentation is limited. First issued in 2007 and reapproved in 2019, the standard includes provisions for interference mitigation to enhance reliability in routine testing.^[3] ASTM D2896 specifies a potentiometric titration method using perchloric acid in glacial acetic acid to determine the base number, which encompasses amine values, in petroleum products and lubricants. Applicable to samples with base numbers up to 300 mg KOH/g or higher with adjustments, it is widely used for assessing additive effectiveness and degradation in fuels and oils. The current version, approved in 2021, incorporates updates for improved precision and applicability to modern formulations.^[23] ASTM D6979 describes a titration procedure for quantifying free amine content, expressed as percent nitrogen, in polyurethane raw materials like polyols. The method targets basic constituents soluble in glacial acetic acid and reactive with perchloric acid, suitable for samples containing 0.3% to 10% nitrogen. Developed in 2003 and reapproved in 2018, it supports quality assurance in polymer production by addressing potential interferences from other basic species.^[24] These ASTM standards have evolved through reapprovals to refine precision, reduce interferences from sample matrices, and align with advancements in analytical equipment, thereby facilitating standardized quality control across amine-related industries.

Other Standards

In addition to ASTM standards, several international and industry-specific guidelines address the determination of amine value through titration methods tailored to particular applications. The International Organization for Standardization (ISO) standard EN ISO 9702:1998 specifies a titration procedure for measuring the primary, secondary, and tertiary amine group nitrogen content in aliphatic or aromatic amine hardeners used for epoxy resins, emphasizing the role of these curing agents in polymer formulations.^[25] The American Oil Chemists' Society (AOCS) Official Method Tf 2a-64, reapproved in 2024, provides a titration-based approach to quantify primary, secondary, and tertiary amine values in fatty amines, where the primary amine value represents the milligrams of potassium hydroxide equivalent to the primary amine basicity per gram of sample, and similar definitions apply to secondary and tertiary values; this method is particularly adapted for the oils and fats sector, differing from general polymer-focused techniques by accounting for lipid-derived amine structures.^[26] In the European context, DIN 53176:2002 outlines the determination of amine value in water-thinnable binders for paints and varnishes, including those based on polyamides and epoxies, using titration to assess free amine content that influences curing and film properties.^[27] Pharmacopeial methods, such as those in the United States Pharmacopeia (USP) General Chapter <541> Titrimetry, utilize non-aqueous titration for assessing amine content in pharmaceutical substances, where bases like amines are titrated in solvents such as glacial acetic acid with perchloric acid to enhance endpoint detection in drug formulations containing amine functionalities.^[28] Efforts toward harmonization between ISO and ASTM standards have promoted global consistency in amine value testing, with post-2000 updates incorporating automation in titration protocols to improve precision and reproducibility across industries.

Applications

In Epoxy Resins and Coatings

Amines function as essential curing agents, or hardeners, in epoxy resin systems, reacting with epoxide groups to form a cross-linked polymer network. The amine value plays a pivotal role in formulation by determining the amine hydrogen equivalent weight (AHEW), calculated as AHEW = 56100 / (amine value × average number of active H’s per nitrogen), which allows for precise stoichiometric mixing to achieve optimal curing ratios with the epoxy resin's equivalent weight.^[29] In coating applications, high amine values greater than 300 mg KOH/g are selected for fast-curing formulations that enable low-viscosity systems suitable for rapid application and reduced processing times. Monitoring the amine value during quality control is critical to avoid under-curing, which can lead to insufficient cross-linking and poor adhesion, or over-curing, resulting in brittle films with reduced flexibility.^[2]^[30] Polyamide and cycloaliphatic amine hardeners, characterized through amine value testing, are widely employed in corrosion-resistant epoxy coatings for marine and industrial environments, providing enhanced durability against chemical exposure and moisture. These hardeners contribute to robust barrier properties, minimizing substrate degradation in harsh conditions.^[2] A representative application occurs in two-part epoxy paints, where the amine value ensures stoichiometric equivalence—typically a 1:1 ratio—with the epoxy value, thereby maximizing mechanical strength, such as tensile properties, and chemical resistance to solvents and acids.^[30]

In Polymers and Lubricants

In polyurethane raw materials, the amine value serves as a key indicator for quantifying unreacted amine groups in polyols and diamine chain extenders, enabling precise control over reaction stoichiometry to achieve desired molecular weights and viscosities in resulting foams and elastomers.^[31] For instance, dimer diamines like Priamine, with an amine value of approximately 205 mg KOH/g, are incorporated into non-isocyanate polyurethane formulations, where monitoring residual amine content post-reaction ensures optimal cross-linking and mechanical properties without excess free amines that could compromise foam stability.^[32] This measurement is particularly critical in rigid and flexible foam production, as deviations in amine value can lead to variations in viscosity, affecting processability and final product performance. In lubricant formulations and additives, the amine value evaluates the basicity of amine-based components, such as those used in rust inhibitors and detergents for engine oils, where it correlates with their ability to neutralize acids and provide corrosion protection.^[33] Ethyleneamines, for example, are employed as ashless dispersants in engine oils to mitigate sludge formation, with their amine value determining dispersancy efficiency and compatibility with other additives.^[33] Products like Amine O, exhibiting an amine value of 162 mg KOH/g, function as oil-soluble corrosion inhibitors, enhancing thermal and oxidative stability in lubricating oils by forming protective films on metal surfaces.^[34] For quality control in polyamides, a low amine value signifies the completion of condensation reactions between diamines and dicarboxylic acids, indicating high molecular weight and minimal end-group termination by amines, which is essential for achieving target mechanical strength and thermal properties.^[35] In contrast, elevated amine values may denote incomplete polymerization, hydrolytic degradation, or contamination by free amines, prompting adjustments in synthesis conditions to prevent brittleness or reduced durability in applications like fibers and engineering plastics.^[36] This parameter is routinely assessed via titration to verify batch consistency, as seen in the production of nylon-type polyamides where amine end-group control directly influences viscosity and melt flow characteristics. In metalworking fluids, the amine value is utilized to monitor the performance of amine-derived emulsifiers, ensuring stable oil-in-water emulsions that facilitate lubrication while preventing corrosion on ferrous and non-ferrous metals.^[37] For example, multifunctional emulsifiers like CORRGUARD SA-100, with an amine value around 150 mg KOH/g, provide hard water tolerance and low foaming, while ongoing measurement of amine content in the fluid helps detect depletion due to use or contamination, thereby maintaining pH balance and corrosion inhibition efficacy.^[38] Such monitoring supports extended fluid life in machining operations by signaling the need for replenishment of amine components to sustain emulsification and protective barriers against rust.^[39]

Primary, Secondary, and Tertiary Amines

The total amine value represents the sum of basic nitrogen from all amine types in a sample, determined by direct potentiometric or indicator titration with a standard acid such as hydrochloric acid in glacial acetic acid medium. This measures the overall basicity without distinguishing between amine classes. To isolate tertiary amine value, the sample is first acetylated with acetic anhydride, which reacts with primary and secondary amines to form non-basic acetamides by replacing their active hydrogens, while tertiary amines remain unaffected due to the absence of such hydrogens. The acetylated mixture is then titrated to quantify the remaining basicity attributable solely to tertiary amines. The value is expressed in milligrams of KOH equivalent per gram of sample. Primary amine value is determined by selective derivatization, commonly using salicylaldehyde, which condenses with primary amines to form non-basic Schiff bases, leaving secondary and tertiary amines intact for subsequent titration. The basicity of the derivatized sample (secondary + tertiary) is measured, and primary amine value is calculated as the difference from the total amine value. Alternatively, nitrous acid can be employed for aliphatic primary amines, as it converts them to non-basic alcohols with evolution of nitrogen gas, allowing quantification by difference after titration of the residual amines; this method is less common due to its applicability mainly to aliphatic systems. Secondary amine value is obtained indirectly as the difference between total amine value and the sum of primary and tertiary values.^[40]^[41] In practical applications, such as epoxy resin curing, primary and secondary amines are valued for their reactive hydrogens, which enable direct crosslinking with epoxy groups to form a robust polymer network, contributing to mechanical strength and adhesion. Tertiary amines, lacking these hydrogens, do not participate in crosslinking but enhance solubility, act as catalysts for epoxy homopolymerization, and improve formulation flexibility without altering the primary cure mechanism.^[42]

Comparison with Acid Value

The acid value, expressed in milligrams of potassium hydroxide (mg KOH) per gram of sample, measures the content of acidic groups such as carboxylic acids through titration with a base like alcoholic KOH, serving as the counterpart to the amine value's assessment of basic amine groups via titration with an acid.^[43]^[44] In oils and lubricants, amine value and acid value provide complementary insights into material stability, with the acid value (often denoted as Total Acid Number or TAN) indicating acidic degradation products from oxidation and the amine value contributing to the Total Base Number (TBN) through amine-based additives that neutralize acids; an imbalance, such as elevated TAN coupled with depleted TBN, promotes corrosion by allowing unneutralized acids to attack metal surfaces.^[45]^[44] These metrics differ fundamentally in methodology and focus: amine value titrations occur in non-aqueous media, such as glacial acetic acid with perchloric acid, to enhance the endpoint detection for weak bases like amines, while acid value titrations use alcoholic KOH solutions to solubilize and neutralize acidic components, targeting distinct reactive sites with no mutual interference.^[46]^[43] In epoxy resin systems, a low acid value in the resin (typically <0.5 mg KOH/g) is essential to avoid interference with amine hardeners, preventing premature curing reactions that could reduce shelf life, while monitoring both values together aids in forecasting formulation longevity and performance.^[2]

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