Aqua regia preparation
Aqua regia preparation is corrosive mixture of nitric acid and hydrochloric acid, used as an attack agent, for certain analytical chemistry procedures, and to refine gold. It dissolves gold, platinum and palladium, but not other noble metals . Here is what you need to know to prepare aqua regia and use it safely.
Reaction to make Aqua Regia
Here is what happens when nitric acid and hydrochloric acid are mixed:
HNO 3 (aq) + 3HCl (aq) → NOCl (g) + 2H 2 O (l) + Cl 2 (g)
Over time, nitrosyl chloride (NOCl) will decompose into chlorine gas and nitric oxide (NO). Nitric acid automatically oxidizes to nitrogen dioxide (NO 2 ):
2NOCl (g) → 2NO (g) + Cl 2 (g)
2NO (g) + O 2 (g) → 2NO 2 (g)
Nitric acid (HNO 3 ), hydrochloric acid (HCl) and aqua regia are strong acids . Chlorine (Cl 2 ), nitric oxide (NO) and nitrogen dioxide (NO 2 ) are toxic.
Aqua Regia Security
Preparation with aqua regia consists of mixing strong acids. The reaction produces heat and gives off toxic vapors, so it is important to follow the safety protocols during the manufacture and use of this solution:
- Make and use the aqua regia solution inside a hood, with the shutter down as much as possible to contain the vapors and protect against injury in case of splashes or broken glassware.
- Prepare the minimum volume required for your application.
- Make sure your glassware is clean. In particular, you don’t want organic contaminants because they can produce a vigorous or violent reaction. Avoid using glassware that could be contaminated with a chemical containing a CH bond. Do not use the finished solution on a material containing an organic.
- Wear safety glasses.
- Wear a lab coat.
- Wearing gloves.
- If you get drops of any of the strong acids on your skin, wipe them off immediately and rinse with plenty of water. If you spill acid on clothing, remove it immediately. If inhaled, move to fresh air immediately. Use the eye wash and consult a doctor in case of eye contact. If swallowed, rinse mouth with water and do not induce vomiting.
- Neutralize any spills with sodium bicarbonate or a similar compound. Remember, it is best to neutralize a strong acid with a weak base and not a strong base.
Prepare Aqua Regia Solution
- The usual molar ratio between concentrated hydrochloric acid and concentrated nitric acid is HCl: HNO 3 of 3: 1. Keep in mind that concentrated HCl is around 35%, while concentrated HNO 3 is d about 65%, so the volume ratio is usually 4 parts of concentrated hydrochloric acid to 1 part of concentrated nitric acid. A typical total final volume for most applications is only 10 milliliters. It is unusual to mix a large volume of aqua regia.
- Add nitric acid to hydrochloric acid. Do not add hydrochloric acid to nitric acid! The resulting solution is a smoking red or yellow liquid. It will smell strongly of chlorine (although your hood should protect you from this).
- Remove the remaining aqua regia by pouring it over a large amount of ice. This mixture can be neutralized with a saturated solution of sodium bicarbonate or 10% sodium hydroxide. The neutralized solution can then be safely poured down the drain. The exception is the solution used which contains heavy metals. Heavy metal contaminated solution should be disposed of in accordance with local regulations.
Once you have prepared aqua regia, use it when it is fresh. Store the solution in a cool place. Do not store the solution for an extended period of time as it becomes unstable. Never store clogged aqua regia as pressure buildup could break the container.
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