Why the ratio very important in making Aqua regia, in other words why it is 3:1?
1. The molar ratio HCl:HNO3 is 3:1.But if you calculate on the concentrated acids, their molar conc etc, the volume ratio between the commercially available acids will be 4:1. Remember that concentration of HCl is ~35%, and concentrated HNO3 is ~65%. 2. The acids react to form a water solution of nitrosyl chloride (NOCl) and chlorine. The originally colourless mixture is soon going yellow and dark orange red (by the NOCl), and emits an intense smell of chlorine. Fume hood is needed, goggles and gloves too. There's a tiny temperature rising during the reaction. The mixture can be stored for a short time in a glass bottle, but there is a risk that both the components will be gassing off above room temperature. And consequently pressure increases inside a closed bottle! Should be stored for a short time only at low temp, unstable even in cold. Aqua regia is most often used freshly prepared because of the decomposition.