Fully automated potentiometric determination of the hydroxyl number (HN) according to ASTM E1899-08 and DIN 53240-2

Hydroxyl is an important functional group and knowledge of its content is required in many intermediate and end-use products such as polyols, resins, lacquer raw materials and fats (petroleum industry). The test method to be described determines primary and secondary hydroxyl groups. The hydroxyl number is defined as the mg of KOH equivalent to the hydroxyl content of 1 g of sample.The most frequently described method for determining the hydroxyl number is the conversion with acetic anhydride in pyridine with subsequent titration of the acetic acid released: H3C-CO-O-CO-CH3 + R-OH -> R-O-CO-CH3 + CH3COOH. However, this method suffers from the following drawbacks:   - The sample must be boiled under reflux for 1 h (long reaction time and  laborious, expensive sample handling)   - The method cannot be automated   - Small hydroxyl numbers cannot be determined exactly   - Pyridine has to be used, which is both toxic and foul-smellingBoth standards, ASTM E1899-08 and DIN 53240-2, offer alternative methods that do not require manual sample preparation and therefore can be fully automated: The method suggested in ASTM E1899-08 is based on the reaction of the  hydroxyl groups attached to primary and secondary carbon atoms with excess toluene-4-sulfonyl-isocyanate (TSI) to form an acidic carbamate. The latter can  then be titrated in a non-aqueous medium with the strong base tetrabutyl- ammonium hydroxide (TBAOH).  The method suggested in DIN 53240-2 is based on the catalyzed acetylation  of  the hydroxyl group. After hydrolysis of the intermediate, the remaining  acetic acid is titrated in a non-aqueous medium with alcoholic KOH solution. The present work demonstrates and discusses an easy way to determine the hydroxyl number according to ASTM E1899-08 or DIN 53240-2 with a fully automated titrimetric system for a great variety of industrial oil samples.