Abstract
The anomalous waves in the polarographic reduction of Eosine (Food Red No. 103) and Phloxine (Food Red No. 104) appeared in acidic aqueous solutions as previously reported, and these phenomena were unfavourable for the polarographic method to be applied to the analyses of these coloring materials. The purpose of the present study was to eliminate these phenomena and to be able to use the polarographic method for microdetermination of these two xanthene-type food colors.
The electrolytic solution was prepared by adding 4 ml of buffer solution, 1 ml of 1M tetramethylam-monium chloride as a supporting electrolyte and 4 drops of 1% gelatin solution as a maximum suppressor to 5 ml of the solution of Eosine or Phloxine dissolved in dioxane-water (3 : 1) mixture. In the case of Eosine, the polarographic measurements were performed at pH 1.014.0, while in the case of Phloxine, they could not be performed at less than pH 3.8 because of precipitation of Phloxine. Eosine and Phloxine in this electrolytic solution at pH 3.8 showed the well defined polarographic reduction waves without any anomalous waves, and their halfwave potentials were -0.61 and -0.65 V vs. SCE, respectively. When pH of electrolytic solution became higher in the acidic range, their half-wave potentials shifted remarkably to negative, while in the alkaline range, they were almost constant and the second and third waves appeared with the rise of pH value.
For analytical application of the polarographic reduction of these compounds, the fundamental investigations were carried out at pH 3.8. The polarographic reduction waves of these two xanthene-type food colors could be considered as the reduction of their quinoid forms, but it could not be concluded that their reductions were reversible electrode reactions because their relationships of log [i/(id-i)] to E did not show the exchanges of two electrons as expected. It could be concluded that the limiting currents of these two food colors were diffusion currents, because their temperature coefficients from 15°C to 35°C were both 2.1% per degree and their limiting currents were proportional to square roots of bights of mercury reservoir. Moreover, the diffusion currents of Eosine and Phloxine were proportional to the concentrations from about 10-3 to 10-4M of their two compounds, and their diffusion current constants were 1.46 and 1.60, respectively.
By this polarographic procedure, micro-amounts of Eosine and Phloxine can be determined.
