Study on the Determination of Traces of Nitrite Ion in Single Crystals of Sodium Nitrate by Ion Chromatography

For keeping good optical property and its service life, it is necessary to restrict the content of NO₂^- in large sized single crystal of NaNO₃ to a very low concentration, and hence it is of urgent importance to offer methods with high practicability and good serviceability for determination of trace amounts of NO₂^- in single crystals of sodium nitrate. Two ion chromatographic methods with different ways of separation of traces of NO₂^- and the matrix of NO₃^-, by gradient elution (mode A) and by value switching (mode B) were proposed. A portion of 0.200 0 g of the sample was taken and dissolved in water, and its volume was made up to 100.0 mL with water. After filtering through 0.22 μm filtering membrane, the filtrate was used for IC-analysis by either of the 2 modes. In mode A, 25 μL of the sample solution was passed through AS11-HC anion exchange column. The column was rinsed with 8 mmol·L^-1 KOH solution at a flow-rate of 1.0 mL·min^-1 within an interval of 9 min to elute NO₃^- from the column and the eluate was used for determination of NO₂^- with conductivity detector. Afterwards, the column was eluted with 30 mmol·L^-1 KOH solution to remove macro amounts of NO₃^- as matrix of the sample from the column, thus to avoid interference of NO₃^- to NO₂^- in the next determination. In mode B, by switching of a 6-way valve of the instrument with an extra-10-way valve at definite time intervals, the separation of NO₂^- from NO₃^- of the matrix was attained. Method of integration of peak area was adopted in calculation of results of determination. Linear relationships between values of integration of peak areas and mass concentration of NO₂^- were found in the same range of 0.10-2.00 mg·L^-1, and values of detection limits (3S/N) found were 8×10^-4% for mode A and 5×10^-4% for mode B. Tests for precision was carried out by 6 paralle determinations of a same sample for its NO₂^- contents, results of RSDs (n=6) found were 4.2% (for mode A) and 3.2% (for mode B). Recovery was tested by addition of standard solution of NO₂^- to 6 NaNO₃ single crystal samples and their contents of NO₂^- were determined by the proposed methods, giving values of recovery in the ranges of 92.5%-108% (for mode A) and 95.5%-104% (for mode B). It was shown that 2 modes in the method of IC determination of NO₂^- proposed in this paper have their own virtues and defects.