Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 237

Fig. 2.32 Near ultraviolet spectrum of nitrite in aqueous solution. 10 −3 mol L −1
NaNO 2 ; H 2 O as reference. Mod. 15 Cary spectrophotometer; optical
path 10cm
Source: Reproduced with permission from Marcel Dekker Inc. [540]
The near-ultraviolet spectrum of sodium nitrite in aqueous solution is shown in Fig. 2.32.
In aqueous solution the same general characteristic spectrum as in the nitrate medium is
maintained, but a small blue shift is observed. At low concentrations, the very intense
higher energy bands do not interfere with the lower energy bands of either nitrate or
nitrite.
From spectra obtained independently it might be inferred that the broadening of the
nitrate band would not strongly interfere with that of the nitrite at λ max if the mutual
concentrations did not exceed 10 −1 mol L −1 nitrate and 10−2mol L −1 nitrite. However, in
reality the analysis of a mixture containing these proportions is unfeasible because the
broadened nitrate band deforms the nitrite band causing a large error.
In the spectrophotometric analysis of nitrite in the presence of nitrate, it is fortunate
that the nitrite ion in molten nitrate, as well as in aqueous solutions, has an absorption
minimum in the region of the nitrate band, and an absorption maximum at 355nm.
When the concentration of nitrite is greater than 10 −2 mol L −1 if the concentration of
nitrate does not exceed 10 −1 mol L −1 the absorbances of nitrite in the presence of nitrate
can be measured at the maximum wavelength vs a suitable reference solution. The
reference solution must contain approximately the same concentration of nitrate as in the
unknown sample. For concentrations of nitrite 10 −2 mol L −1 or less, and 1mol L −1 nitrate,
the resulting error caused in the molar absorptivity of nitrite is no longer negligible. This
error can be avoided by using the