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Hydroxyl number

The hydroxyl number (OHN) indicates the number of mg KOH corresponding to the hydroxyl groups in 1 g sample.

The most frequently described method for determining the hydroxyl number is conversion of the sample with acetic acid anhydride in pyridine with subsequent titration of the released acetic acid. The one-hour boiling under reflux, the difficulty in automating the process and particularly the use of the health-hazardous pyridine are serious disadvantages.

An alternative is offered by the considerably simpler and more easily automated method according to ASTM E 1899. Primary and secondary hydroxyl groups are converted with toluene-4-sulfonyl-isocyanate (TSI) into an acid carbamate, which is then titrated with the strong base tetrabutylammonium hydroxide (TBAH) in a non-aqueous medium.

The method is especially suitable for neutral raffinates. Acidic products can falsely indicate too high values. In the same way products that contain bases can show too low values due to neutralization of the carbamate formed. 

Applications:
8.000.6047 Fully automated potentiometric determination of the hydroxyl number (HN) according to ASTM E 1899-08 and DIN 53240-2
AB-322 Fully automated potentiometric determination of the hydroxyl number (HN) according to ASTM E 1899-08 and DIN 53240-2