Three color reactions for detecting tung oil, a maleic anhydride reaction (Reaction with maleic anhydride in chloroform), the Storch & Morawsky reaction (Reaction with sulfuric acid in acetic anhydride) and an antimony trichloride reaction (Reaction with 10% antimony trichloride-chloroform solution) were examined for the detection of tung oil.
These reactions were applied to various vegetable oils present with tung oil as a mixture and to oils containing conjugated trienoic fatty acids prepared from linseed oil. The maleic anhydride reaction was observed to be stable with no change in the specific yellow color following the reaction period. However, this reaction was not suitable for deep colored oils.
The Storch & Morawsky reaction soon showed a strong purplish red color which changed to a different color after 2030 s. The color resulting from the antimony trichloride reaction was very deep, but nearly 1h was required for this reaction to sufficiently take place.
These reactions can be used to detect tung oil when present at a 5% concentration in vegetable oils.
The color noted deeping the course of the above reactions was due to the presence of conjugated trienoic fatty acids in tung oil. Thus conjugated fatty acids prepared from linseed oil showed the same color reaction as tung oil. In the case of oil mixtures containing more than 5% tung oil, the above simple reactions should be used for tung oil detection instead of the rather complicated methods of gaschromatography or ultraviolet spectrophotometry. However, care must be taken since other oils containing conjugated trienoic fatty acids will show the same color reaction.
