Abstract: A new sensitive colour system of Ethyl Rhodamin B-molybdophosphoric acid-PVA has been studied. The optimum conditions for pbosphorus determination are [HNO₃]=0.56mol/dm³,[MoO₄^2-]=2.4×10^-2mol/dm³, [ERB⁺]=8.0×10^-5mol/dm³ and 0.12% PVA. Theion-associate exhibits maximum absorptivity at 584nm. At the working wavelength of 586nm its apparent molar absorptivity was found to be 3.2×10⁵ L·mol^-1·cm^-1.Beer′s law isobeyed in the range of 0 to 1.5μg of P(V) per 25mL. The colour of the aqueous solutionwas stable for at least lh. The molar ratio of P(V) to FRB⁺ in ion-associate was estab-lished to be 1:4. Infrared spectra studies show that an ionic bond occurs between ERBand phosphomolybdate and that the structure of the dye molccule is not of effected by the for-mation of the ion-assosiate.The effect of inteifering ions, such as As(Ⅴ), Si(Ⅳ) can bedecreased by increasing the acidity and adding mixed reductants. Phosphorus in steel andsome chemical reagents were determined directly at 1×10^-5% level by the proposed methodwith satisfactory results.

Key words: Ethyl Rhodamine B, Heteropolymolybdophosphoric acid, Specetrophotometry