Tertiary Amine Value Determination Test: Principles, Methods, Standards, and Applications

Tertiary amine value determination is a core analytical test used to quantify the tertiary amine nitrogen content in polyurethane catalysts, epoxy curing agents, fatty amines, surfactant precursors, and gas-treating solvents. This article explains the test principle, standard potentiometric and indicator titration methods, sample preparation, calculation formulas, precision requirements, and typical industrial applications, helping laboratories and quality-control teams obtain accurate and reproducible results.

Technical Overview

The tertiary amine value is defined as the number of milligrams of potassium hydroxide equivalent to the tertiary amine basicity contained in one gram of sample, expressed in mg KOH/g. It is the standard quality metric for products in which the nitrogen atom carries three organic substituents and no N–H hydrogen, such as polyurethane gelling and blowing catalysts, epoxy accelerators, alkyl dimethylamines, and methyldiethanolamine (MDEA) gas-treating solvents.

Because primary and secondary amines coexist with tertiary amines in many commercial products, a simple acid titration only yields the total amine value. Selective determination of the tertiary fraction relies on a chemical blocking step: primary and secondary amines react quantitatively with salicylaldehyde to form imines and enamines that are essentially non-basic, while tertiary amines, which have no N–H bond, do not react. The remaining basicity is then titrated with standardized acid, giving the tertiary amine value directly.

Commonly referenced test standards include ASTM D2074 (Total, Primary, Secondary, and Tertiary Amine Nitrogen by Potentiometric Method), ASTM D6979 (Amine Number of Fatty Amines by Potentiometric Titration), and ASTM D2073 (Indicator Method). Accredited third-party laboratories and in-house QC laboratories typically adapt these standards to non-aqueous potentiometric titration, which offers objective endpoint detection and excellent reproducibility.

Testing Samples

Polyurethane Tertiary Amine Catalysts

Triethylenediamine solutions, bis(dimethylaminoethyl) ether formulations, dimethylcyclohexylamine, N,N-dimethylethanolamine, and blended amine catalyst packages. The tertiary amine value directly controls gelling and blowing activity and is used for incoming inspection and batch-to-batch consistency checks.

Fatty Tertiary Amines and Derivatives

Dimethyl cocoamine, dimethyl tallowamine, alkyl methyl amines, and amidopolyamine intermediates used to manufacture quaternary ammonium salts, amine oxides, fabric softeners, and asphalt anti-stripping agents. Tertiary amine value confirms conversion of primary amines to fully alkylated tertiary products.

Epoxy Curing Agents and Accelerators

Tris(dimethylaminomethyl)phenol, benzyldimethylamine, dimethylaminomethyl cresols, and tertiary amine-modified anhydride or polyamide hardeners. The tertiary amine value determines accelerator strength and therefore pot life and cure speed.

Gas-Treating and Fuel Additives

Methyldiethanolamine (MDEA) and formulated amine solvents for CO2 and H2S removal, as well as refinery corrosion inhibitors and gasoline detergents based on tertiary amine chemistry. Solvent strength monitoring by amine value is a routine process-control application.

Sample Preparation Requirements

• Homogenize the sample thoroughly; gently warm and mix solid or waxy amines without overheating.
• Determine water content separately if moisture exceeds specification, since water interferes with non-aqueous titration sharpness.
• Weigh the sample to 0.1 mg on an analytical balance; typical sample mass is 0.1–2 g, chosen so that the titrant consumption falls within an optimal range.
• Store samples in tightly sealed containers to prevent absorption of carbon dioxide and moisture from air.

Testing Items

• Tertiary amine value (mg KOH/g), the primary deliverable of the experiment.
• Total amine value by direct acid titration without blocking reagent.
• Primary and secondary amine content, calculated by difference between total and tertiary amine values.
• Tertiary amine nitrogen content (percent by mass), derived from the amine value.
• Supporting characterization frequently requested together with amine value: water content by Karl Fischer titration, color (APHA/Gardner), and ash or nonvolatile content.

Testing Methods

Potentiometric Titration Method (Reference ASTM D2074 / ASTM D6979)

The sample is dissolved in a non-aqueous solvent mixture such as isopropanol–toluene or chlorobenzene–isopropanol. An excess of salicylaldehyde is added, and the mixture is allowed to stand for 15–30 minutes so that primary and secondary amines are converted to non-basic condensation products. The solution is then titrated with standardized hydrochloric acid in isopropanol (typically 0.1 mol/L or 0.5 mol/L) using an automatic titrator equipped with a glass electrode system suitable for non-aqueous media. The endpoint is located from the potentiometric inflection point, and a reagent blank is run in parallel.

Visual Indicator Titration Method (Reference ASTM D2073)

For laboratories without automatic titrators, the blocked sample solution is titrated manually with standardized alcoholic HCl using bromophenol blue or bromocresol green as the indicator. The endpoint is the persistent color transition, typically from blue to yellow across neutralization. Visual endpoint detection is slightly less precise than potentiometric detection and is sensitive to sample color, so it is best suited to pale-colored products.

Calculation Formulas

• Tertiary amine value (mg KOH/g) = (V − V0) × N × 56.1 / m, where V is the titrant volume for the sample, V0 is the blank volume, N is the acid normality, 56.1 is the molar mass of KOH, and m is the sample mass in grams.
• Primary + secondary amine value = total amine value − tertiary amine value.
• Tertiary amine nitrogen (mass %) = tertiary amine value × 14.007 / (56.1 × 10), which equals approximately the amine value divided by 40.05.

Quality Control and Precision

• Standardize the hydrochloric acid titrant against a primary standard base and verify with a blank titration in every series.
• Run duplicate determinations; the difference between duplicates should typically not exceed 2–3 % of the mean for well-behaved samples.
• Keep the system anhydrous: dry solvents, condition the electrode per non-aqueous titration practice, and protect samples and titrant from atmospheric CO2.
• Report results to the nearest 0.1 mg KOH/g together with the method used, titrant concentration, and sample mass.

Applications

Tertiary amine value data are used across many industries:

• Polyurethane manufacturing: incoming inspection of amine catalysts, formulation dosing control, and troubleshooting of foam gelation and blow balance.
• Epoxy systems: verification of tertiary amine accelerator content in adhesives, coatings, composites, and electrical encapsulation compounds.
• Surfactants and home care: quality control of tertiary amine feedstocks for amine oxides and quaternary ammonium products.
• Oil and gas: monitoring MDEA-based solvent strength in natural gas sweetening units and controlling amine-based corrosion inhibitor concentrates.
• Fuel and lubricant additives: conformity assessment of detergents and dispersant intermediates.
• Chemical trade and regulatory compliance: batch certification, supplier qualification, and dispute resolution through accredited third-party laboratories.

FAQ

What is the difference between total amine value and tertiary amine value?

The total amine value measures the basicity of all amines (primary, secondary, and tertiary), whereas the tertiary amine value measures only the fraction that does not react with salicylaldehyde. The difference between the two reflects primary plus secondary amine content.

Why must salicylaldehyde be added before titration?

Salicylaldehyde reacts with the N–H hydrogens of primary and secondary amines to form imines and enamines with negligible basicity. Tertiary amines have no N–H bond and remain titratable, allowing selective quantification.

Why is a non-aqueous solvent used?

Many fatty and polymeric amines are poorly soluble in water, and non-aqueous media sharpen the potentiometric curve and suppress carbon dioxide interference, giving a clearer, more reproducible endpoint.

How should the sample mass be chosen?

Select the mass so the endpoint titrant volume is large enough for accuracy but within burette or titrator capacity; high amine value samples (for example, above 300 mg KOH/g) require smaller masses, while dilute amine solutions require larger masses.

What precision can be expected?

With standardized titrant, proper electrode conditioning, and careful blank correction, repeatability within about 2–3 % relative is normally achievable for potentiometric titration, which is suitable for most product specifications and release testing.