OHnumber

OHnumber, commonly written as hydroxyl number, is a measure used in polymer chemistry to quantify the amount of hydroxyl (-OH) groups in a substance. It expresses how many milligrams of potassium hydroxide (KOH) are equivalent to the hydroxyl functionality present in one gram of sample. The higher the OHnumber, the more hydroxyl groups are available for further reactions, such as polyurethane formation.

Measurement methods for OHnumber typically rely on acid–base titration after a chemical derivatization that converts hydroxyl groups into a form that can be titrated. The most common approach uses a standardized titration with a known concentration of KOH, with blank corrections to account for reagents and solvents. Results are usually reported in milligrams of KOH per gram of sample (mg KOH/g).

The calculation, in its simplest form, is OHnumber = (V × N × 56.1) / W, where V is the titrant volume used for the sample (in milliliters), N is the normality of the KOH titrant, 56.1 is the molar mass of KOH (g/mol), and W is the mass of the sample (in grams). This yields the mg KOH equivalent per gram of material.

Applications of OHnumber are prevalent in polyurethane chemistry, coatings, adhesives, and resins, where it helps determine reactivity and molecular weight of polyols and other hydroxyl-containing compounds. Typical OHnumbers for polyether and polyester polyols range from about 20 to 100 mg KOH/g, depending on desired flexibility and crosslinking. Limitations include moisture sensitivity, potential interference from acidic or basic impurities, and the need for standardized procedures to ensure comparability across laboratories.