Paliwal, R.K. “Study of Mixed Complexes of Cu(II) With Propylene Diamine, Malonate and Maleate Ions Using Polarographic Technique”. Asian Journal of Chemistry, vol. 20, no. 7, Mar. 2010, pp. 5531-5, https://asianpubs.org/index.php/ajchem/article/view/12891.