Awadallah, R.M., et al. “ Physico-Chemical Studies on Vanadium(V), Niobium(V) and Tantalum(V) Complexes With Eriochrome Cyanine-R and Methylene Blue”. Asian Journal of Chemistry, vol. 2, no. 2, Mar. 2010, pp. 155-67, https://asianpubs.org/index.php/ajchem/article/view/25305.