ตำรายาของประเทศไทย
Thai Pharmacopoeia
สำนักยาและวัตถุเสพติด กรมวิทยาศาสตร์การแพทย์ กระทรวงสาธารณสุข
Bureau of Drug and Narcotic, Department of Medical Sciences, Ministry of Public Health5.11 DETERMINATION OF ESTERS
Boil a convenient amount of ethanol thoroughly to expel carbon dioxide and neutralize it to phenolphthalein TS. Unless otherwise directed in the monograph, weigh accurately about 2 g of the test substance, or other suitable quantity, so that the volume of 0.5 M ethanolic potassium hydroxide VS to be added is at least twice that theoretically required, dissolve it in 5 ml of the neutralized ethanol contained in a hard-glass flask, and neutralize the free acid in the solution with 0.1 M ethanolic potassium hydroxide VS, using 0.2 ml of phenolphthalein TS as indicator. Add 25.0 ml of 0.5 M ethanolic potassium hydroxide VS, attach the flask to a reflux condenser, and boil on a water-bath for 1 hour. Add 20 ml of water, and titrate the excess of alkali with 0.5 M hydrochloric acid VS, using a further 0.2 ml of phenolphthalein TS as indicator. Repeat the operation without the test substance. The difference between the titrations is equivalent to the alkali required to saponify the esters.