Mostafavi, A., et al. “Simultaneous Separation and Preconcentration Trace Amounts of Cd2+, Mn2+, Ni2+, Zn2+ and Fe2+ onto Modified Amberlite XAD-4 Resin That Loaded With Pyrocatechol Violet”. Asian Journal of Chemistry, vol. 18, no. 3, Mar. 2010, pp. 2303-9, https://asianpubs.org/index.php/ajchem/article/view/20988.