(1) The reaction of Gries Romijin reagent with NO
2- ion was accelerated by adjusting the pH of reaction mixture at 4.0. At this pH, the color developed was stable. The estimation of micro-amounts (0.01∼0.4 ppm) of NO
2- ion was performed with a good sensitivity. (2) NO
3- was quantitatively reduced to NO
2- with Mn
2+ in ammonia alkali solution and reacted with G.R. reagent at pH 4.0. The estimation of 0.02∼0.6 ppm of NO
3- was thus performed. Since coexisting NO
2- is stable in the reduction conditions, NO
3- and NO
2- in one mixture can be estimated separately.
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