Accurate determination of aldehydes in amine catalysts or amines by 2,4-dinitrophenylhydrazine derivatization

• Aldehydes in amine catalysts can add to toxic emission from polyurethane products.
• 2,4-Dinitrophenylhydrazine derivatization of aldehydes in amines is optimized.
• Complete derivatization occurs in acidic medium but not in neutral or basic solutions.
• Aldehydes can be measured accurately at pH 2 with heating period of 20 min at 40 °C.
• Mechanisms for the formation of hydrazones in acidic and basic media are discussed.

Carbonyl compounds, specifically aldehydes, present in amine catalysts or amines are determined by reversed-phase liquid chromatography using ultraviolet detection of their corresponding 2,4-dinitrophenylhydrazones. The primary focus has been to establish optimum conditions for determining aldehydes accurately because these add exposure concerns when the amine catalysts are used to manufacture polyurethane products. Concentrations of aldehydes determined by this method are found to vary with the pH of the aqueous amine solution and the derivatization time, the latter being problematic when the derivatization reaction proceeds slowly and not to completion in neutral and basic media. Accurate determination of aldehydes in amines through derivatization can be carried out at an effective solution pH of about 2 and with derivatization time of 20 min. Hydrochloric acid has been used for neutralization of an amine. For complete derivatization, it is essential to protonate all nitrogen atoms in the amine. An approach for the determination of an adequate amount of acid needed for complete derivatization has been described. Several 0.2 M buffer solutions varying in pH from 4 to 8 have also been used to make amine solutions for carrying out derivatization of aldehydes. These solutions have effective pHs of 10 or higher and provide much lower aldehyde concentrations compared to their true values. Mechanisms for the formation of 2,4-dinitrophenylhydrazones in both acidic and basic media are discussed.