Rapid colorimetric micromethod for free fatty acids.

Free fatty acids (FFA), the uranyl ion, and the basic dye Rhodamine B form colored complexes, which are extractable into toluene or benzene. Fatty acids of different chain lengths above C(10) and different degrees of unsaturation gave constant molar yield. Complexes in toluene alone are unstable, especially in the light, but a small amount of aqueous uranyl acetate stabilizes them sufficiently for determination.