Since nitrogen oxides can interfere with the thiosulfate titration, they must be removed by the addition of 10mL of concentrated sulfuric acid, as shown at the right. The presence of the concentrated sulfuric acid aids in the exclusion of the remaining nitrogen oxides from the hot solution. Those red oxides first boil off, then one sees fumes of sulfur trioxide beginning to escape, as at the left . The resulting sample is first cooled in ice, as on the left. Then concentrated ammonia is added dropwise until the dark blue copper tetraamine complex is formed, as on the right.
Potassium iodide is added and the solution, when swirled, looks like the image on the left.
The actual color you get will vary somewhat and it will depend on the amount of zinc in the brass. High concentrations of zinc give darker colors. The solution is titrated and it slowly loses its dark color. Notice the slightly lighter color at the right. Finally, it will become a light yellow-brown or mustard color, as shown below and left.