Characteristic effects can be observed when water or alcohol soluble oxidants are mixed with green suspensions of ferrous hydroxide. The same holds for nitro or nitroso compounds as manifested by a change from green to brown, due to the formation of ferric hydroxide. However, the color change from green ferrous hydroxide to brown ferric hydroxide is not sensitive.
In this study ammoniacal ferrous salt was colored with dimethylglyoxime and was disappeared by nitro compounds.
A drop of the test solution was mixed in a small test tube with ferrous salt solution and alcoholic dimethylglyoxime, and neutralized by ammonia, then allowed to stand a few minutes. Red color of ferrous dimethylglyoxime complex disappeared if nitro compound is present.