Table 2. Some physico-chemical features of the cations used in this study (from www.webelements.com).

Cation	Distance ion – O (Å)	Ionic radius (Å)	Preferred coordination	pKa
K+	2.80	1.41	4, 6, 8
Mg2+	2.09	0.86	6	11.4
Mn2+	2.19	0.81	6	10.6
Co2+	2.11	0.79	6	10.2
Co3+		0.68	6
Zn2+	2.10	0.88	4, 6	9.0
Sr2+	2.64	1.32	8
Ca2+	2.42	1.14	6, 8	12.9
Ba2+	2.80	1.49	8	13.5
Cd2+	2.30	1.09	4, 6	9.6
Pb2+		1.33	6	7.7
Pt4+		0.71	8
Au3+		0.82	4
Ru3+		0.82	6