Are there any sulphides, sulphates, carbonates that cannot be dissolved by aqua regia?

Aqua Regia: Does This “Royal Water” Dissolve Everything?

Aqua regia. Sounds impressive, right? The name literally means “royal water” in Latin, and it earned that title because it can dissolve gold and platinum—metals fit for a king! This stuff is a potent mix of concentrated nitric and hydrochloric acids, and you’ll find it used for everything from refining precious metals to giving lab glassware a sparkling clean. But here’s the million-dollar question: is there anything that can stand up to this “royal water”? Let’s find out, shall we?

So, what’s the deal with aqua regia, chemically speaking? Well, it’s usually made with hydrochloric acid and nitric acid mixed in a roughly 3:1 ratio. When you combine them, things get interesting. You end up with nitrosyl chloride, chlorine gas, and water. It’s this combo of an oxidizing agent (nitric acid) and a complexing agent (hydrochloric acid) that makes aqua regia so powerful. The nitric acid starts by oxidizing metals, turning them into ions. Then, the hydrochloric acid swoops in, grabs those ions, and whisks them away, which clears the way for even more oxidation to happen. Think of it like a chemical tag team! Gold, for example, gets dissolved like this:

Au + 3 HNO3 + 4 HCl → AuCl4− + 3 NO2 + H3O+ + 2 H2O

Now, let’s talk about the main event: sulfides, sulfates, and carbonates.

Generally speaking, aqua regia is pretty good at dissolving sulfide minerals. I’ve read that using a mix of potassium chlorate and hydrochloric acid first, followed by nitric acid, can dissolve pretty much any sulfide you throw at it. However, sulfides can sometimes throw a wrench in the works. If you’re trying to extract gold using aqua regia and there are sulfides present, they can hog the acid, using it up before it gets a chance to dissolve the gold. Annoying, right?

What about sulfates? Well, most will dissolve, but there’s always a troublemaker in the bunch. Barium sulfate, also known as barite, is notorious for being a pain to dissolve. Water? Forget about it. Most acids? Nope. Even aqua regia often struggles with it. I remember reading about a case where gold was trapped inside barite, and the aqua regia just couldn’t get to it. Talk about frustrating!

Carbonates, on the other hand, are usually pushovers when it comes to acids. Aqua regia is sometimes even used to get rid of carbonates. Just be careful, because the reaction between carbonates and the nitric acid in aqua regia can be pretty intense, so you definitely don’t want to go mixing them without knowing what you’re doing.

Now, before you go thinking aqua regia is some kind of ultimate solvent, there are materials that can laugh in its face. Certain metals, like tantalum, ruthenium, and a whole bunch of others, are surprisingly resistant. And don’t even think about using it on certain plastics! PVC, Teflon, and a few others will shrug it off like it’s nothing.

So, there you have it. Aqua regia is a chemical powerhouse, no doubt. It can dissolve gold, for crying out loud! But it’s not all-powerful. While it’ll make short work of most sulfides and carbonates, sulfates like barite can be a real problem. Plus, there’s a whole list of materials that are simply immune to its charms. Knowing these limitations is key, whether you’re refining gold, running experiments, or just trying to clean your lab equipment. After all, a little knowledge can save you a lot of trouble!