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Effect of Temperature on UV Spectra of Concentrated NaNO3 Aqueous Solutions

The effect of temperature on the   ← n transition band of nitrate ion in NaNO3 aqueous solutions (0.2  c/mol dm– 3  4.2) was studied in the temperature range (15 70) °C,   = 5 °C. For each solution, the spectral maximum at  300 nm was found to shift bathochromically with increasing temperature, and a well-defined isosbestic point occurred. Accordingly, the result of principal-components analysis (PCA) of the corresponding spectral matrices was the effective rank (pseudorank) of 2 in all cases. The spectral changes were proven to be reversible. Recently, it has been shown that two forms of 'free' nitrate ion ( -NO3 and  -NO3 ) are present in dilute aqueous solutions differing in the structure of the hydration shells. Since in solutions with higher electrolyte concentrations the two forms can be expected to form ion pairs, four equilibrium reactions were assumed to take place  interconversion between free nitrate forms, ion-pairing reactions of  -NO3 and  -NO3 , and interconversion between the ion pairs {; ; ; ; ; ; Na+( -NO3 )}; ; ; ; ; ; and {; ; ; ; ; ; Na+( - NO3 )}; ; ; ; ; ; . It was shown that the observed spectral changes caused by change in temperature could be explained if the ion- pairing reactions were taken to be approximately temperature independent (i.e. the corresponding  rH  0). The spectrum of NaNO3(aq) solution could be expressed as a linear combination of two temperature independent 'hybrid' spectra. That could explain the results of PCA as well as the existence of the isosbestic point over the entire temperature range examined.

UV spectrometry ; temperature effec t ; sodium nitrate ; ion association ; principal-components analysis ; van’ t Hoff equation

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