methyl orange is an indicator with a ka of
Methyl orange In an acid, it is reddish and in alkali, it is yellow. Methyl orange has a pKa of 3.47 in water at 25 °C (77 °F).[2]
Aqua regia Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, lit. "royal water" or "king's water") is a mixture of nitric acid and hydrochloric acid,[1] optimally in a molar ratio of 1:3. Aqua regia is a yellow-orange fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals.