Studies on the chemistry of the determination of boron with curcumin

Abstract

The stability of the reagent curcumin (H 3A) in sulphuric acid-acetic acid medium both in the presence and absence of boric acid has been studied. Kinetic measurements show that protonated curcumin decomposes according to a second-order rate law. The decomposition is inhibited by the addition of boric acid which forms a complex with curcumin. This complex has a much slower decay rate. The complex was analyzed and found to be a 1:2 boric acid-curcumin compound. The chemical behaviour as well as the physical data are in good agreement with results from the investigation of rosocyanin synthesized as described by Spicer and Strickland. The necessity of water removal and the use of curcumin in excess in the analytical procedures is explained. The acid dissociation constant of curcumin in water-ethanol solution has been determined and K a= ([H +][H 2A -])/[H 3A]=10 -7.9. For roso-cyanin, B(H 2A) 2 +, the acid dissociation constant for the equilibrium B(H 2A) 2 +→2 H + + B(H 2A)(HA) was found to be lO -5·7 in the same medium. From two-phase experiments with chloroform, rosocyanin in the form of an ion-pair with chloride was found to dissolve in the organic phase just as well as the neutral form. The consequences ofthe results for analytical purposes are discussed.