R. Agnihotri, N. Agnihotri, R. Kamal, and J. R. Mehta, “3-Hydroxy-2-(3´-methyl-2´-thienyl)-4-oxo-4H-1-benzopyran as an Analytical Reagent for the Spectrophotometric Determination of Tungsten(VI)”, ajc, vol. 20, no. 6, pp. 4469–4476, Mar. 2010.