Saturday, January 05, 2008

Antique chemistry text sheds light on Lugol's solution shelf-life issue (thanks, Google Books!)

"Iodine is slightly soluble in water, one part of the element requiring about 3700 parts of water to dissolve it, forming a yellowish solution, which, on exposure to light, decolorizes with formation of hydriodic acid. The solubility of iodine in water is much increased by the presence of potassium iodide or hydriodic acid. The official Liquor lodi Compositus, U.S.P. (Lugol's Solution), is a preparation based on this property. In strong alcohol iodine is soluble to the extent of one part in ten of the solvent, forming a dark-brown liquid, which is stronger than the Tincture lodi, U.S.P., which contains 7 parts of iodine and 5 parts of potassium iodide in 100 of alcohol. ALCOHOLIC SOLUTION OF IODINE READILY UNDERGOES DECOMPOSITION UPON STANDING, THE LOSS IN FREE IODINE VARYING FROM 15 TO 25 PER CENT, IN ONE YEAR'S TIME OR EVEN LESS. THE IODINE IS CONVERTED INTO HYDRIODIC ACID AND ETHYL IODIDE. THIS DECOMPOSITION MAY BE PREVENTED BY THE PRESENCE OF POTASSIUM IODIDE AS DIRECTED IN THE U.S.P." [emphasis added]

from A Text-book of Chemistry: Intended for the Use of Pharmaceutical and Medical Students, p 177