Determination of Metals in Natural Waters by Inductively Coupled Plasma Optical Emission Spectroscopy after Preconcentration on Silica Sequentially Coated with Layers of Polyhexamethylene Guanidinium and Sulphonated Nitrosonaphthols

Table 4

pKa values of 1-nitroso-2-naphthol-3,6-disufonic acid and 2-nitroso-1-naphthol-4-sufonic acids and stability of their complexes with transition metals (25°C, μ = 0.1) [32].

	1-Nitroso-2-naphthol-3,6-disufonic acid			2-Nitroso-1-naphthol-4-sufonic acid
	= 6.87			= 6.15
Metal
Mn(II)	2.68			2.19^a
Cd(II)	3.42	6.00		2.45
Zn(II)	4.46	7.10		3.15
Pb(II)	4.64	7.83		3.94
Co(II)	5.40			4.34
Ni(II)	6.90	12.5	17.3	5.50	10.42	14.45
Cu(II)	9.90	15.6		7.90	13.10

– μ = 0