Studies on the Absorptiometric Determination of Microamounts of Boron. V. The Necessity of Methyl or Ethyl Oxalate in the Color Reaction of Boric Acid with Curcumin

Abstract

Abstract The coloration of microamounts of boric acid with an ethanolic solution takes place in the presence of oxalic, but not in that of other acids. Near-ultraviolet, visible, and infrared absorption spectroscopic studies have revealed that the aging reaction of an ethanolic curcumin-oxalic acid reagent solution is simply the esterification of the oxalic acid with the solvent used. The resulting diethyl oxalate is necessary to the color reaction. The role of the ester has not been interpreted yet; however, it is so unique that it can be performed only by dimethyl oxalate, not by the other esters. The esterification of oxalic acid in an acetonic reagent solution may be attributed to the presence of alcoholic impurities present in aceton used. The composition of the colored reaction product, rubrocurcumin, presented by Spicer and Strickland has not been substantiated yet in relation to the necessity of the esters to the color reaction.