The method of determining the hydroxyl number in polyols and oxo alcohols specified in DIN EN ISO 4629-2 derives its theory from the catalytic acetylation of the hydroxyl group. Once the intermediate product is hydrolyzed, the residual acetic acid is titrated using alcoholic KOH solution under non-aqueous conditions with the Solvotrode.
Complete homogenization of the sample is achieved prior to the determination.
A blank determination is carried out as described in the section Sample.
The calculated amount of sample is precisely weighed out directly into a 120-mL PP beaker. After introducing the stirring bar and a Dis-Cover lid, the beaker is placed on the sample rack. 30 mL of the catalyst solution and 10 mL of the acetylation mixture are added automatically, following which the beaker is sealed again.
After this, the sealed sample is stirred for 15 minutes or more depending on the type of sample. The lid is again lifted automatically, and 3 mL deionized water is added at this point. The beaker is resealed and stirred for 12 minutes more. The solution is now titrated to find the hydroxyl number using c(KOH) = 0.5 mol/L in methanol until the equivalence point is passed.
Upon completing the titration, the Solvotrode EasyClean is immersed in acetonitrile within a PP beaker until the next measurement. The membrane kept immersed in deionized water.
Once the series of measurements is complete, the electrode is automatically put into a beaker of acetonitrile and kept immersed until the next analysis begins.
This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
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