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Association of Nitrate Ion with Metal Cations in Aqueous Solution: a UV-Vis Spectrometric and Factor-Analytical Study

Ion association in aqueous solutions of varied concentrations of 3 LiNO , 3 NaNO , 32 Ca(NO ) , 32 Sr(NO ) , 32 Ba(NO ) , 32 Mn(NO ) , 32 Co(NO ) , 32 Ni(NO ) , 32 Cu(NO ) , 32 Zn(NO ) , and 32 Cd(NO ) was studied by means of UV spectrometry. In all cases, except for 32 Zn(NO ) (aq), the principal- components (PCA) and evolving-factor (EFA) analyses of the spectra indicated formation of associated ion pairs -1 3 (MNO ) z (M z denotes the cation) at higher salt concentrations. The (dis)similarities of the -1 3 (MNO ) z spectral profiles (computed by EFA) were examined by means of cluster analysis. The influ- ence of transition-metal ions on the nitrate UV spectrum was found to be remarkably different compared to that of non-transition cations. That could be a consequence of different types of cation-anion interac- tions. The ion pairs containing non-transition metal ions could be regarded as classical electrostatic ones, whereas a weak coordination interaction was presumably present in the ion pairs formed by the nitrate and the cations of transition metals.