Growing silver nitrate crystals

[g_axelsson]

Before anyone have issues about this not being a chemical process, it is. Look it up, it's called physical chemistry.

It all started a couple of months ago, I had some silver and copper solution and instead of cementing it on copper and put the silver away as I usually do, I thought, "Why don't I just leave it to evaporate, it should be easy to collect some pure silver nitrate crystals."
The last couple of months have contained a lot of fun watching the crystals grow, a lot of frustration when the crystals refused to grow large and just created a mass of smaller grains and a lot of learning to find out what I never thought about came back and bit me hard in the end.

This is not an effective way of purify silver or even making pure silver nitrate, that can be made with nitric acid and electrolytic silver. But it is a method that doesn't use any additional nitric acid, so very conservative for those that can't source nitric acid easily. And if you forget about it for a while, just add water and restart it.

This was the separation I have reached two days ago, three months into the experiment. All of these solutions / solids started out from a common source.

To the left is intensely blue copper nitrate crystals and some liquid after washing.
Second is an evaporating dish with some copper nitrate and silver nitrate crystals growing. I remove crystals from here to the beaker at the left.
Third is mostly silver nitrate but contaminated with copper too. It isn't enough copper to crystallize until the liquid is mostly gone, so that is when I pour off the liquid to the left and the silver nitrate crystals goes to the right.
Fourth is almost pure, you can see the white silver nitrate at the bottom, the solution is saturated. Same here, liquid to the left, crystals (when pure enough to the right).
Last is another evaporating dish with my stock silver nitrate that I had collected. I dissolved it to see if I could make some nice large crystals again but with higher purity than I've done before.

... and it worked. On Thursday morning it was just liquid in the dish, this is what I found when I came home in the evening...

I suspect that the solution was super saturated and when it started to crystallize it went fast. I've had similar experience before and I would like to put up a camera just to catch some nice videos whenever the process starts.

One of the earliest tests produced some nice crystals too, I saved the largest one but it's full of mechanically entrapped copper nitrate, giving it a pale blue tone. I just kept it because of the pure crystal form and it's size.


Here are some more pictures of silver nitrate crystals.

Hard to see but there's a large and water clear square crystal in the evaporating dish.


If I get any more spectacular crystals I'll post them here too.

Göran

 

[FrugalRefiner]

Very nice! I grew crystals for a science fair in school, but nothing like yours.

What were the differences between the early impure crystals, and the later very pure crystals?

Dave

 

[g_axelsson]

The biggest difference is the amount of contamination. It seems that most crystals are a bit porous, look at the blue dots lining one side of the oldest crystal, it was drawn out of the crystal when it dried up. If the crystal was very solid then that couldn't happen, the liquid would be captured inside.

So far I don't know what governs the form of the crystals. Initially there are a couple of large crystals forming but after that there are a lot of tiny ones forming all over and that makes it hard to wash in any practical way.

Göran

 

[FrugalRefiner]

So far I don't know what governs the form of the crystals. Initially there are a couple of large crystals forming but after that there are a lot of tiny ones forming all over and that makes it hard to wash in any practical way.

I had similar mixed results in an experiment I was doing with copper acetate. I made a number of small batches, and in some the crystals grew larger while others were all tiny. The conditions were essentially the same for each batch. I eventually redissolved them all into one batch and allowed the solution to evaporate slowly expecting larger crystals, but again the results were mixed.

My goal wasn't necessarily to grow large crystals, so I didn't pursue it any further.

Dave

 

[yellowfoil]

By many recrystallizations, you can get the purest substances achievable in chemistry. I came across article
in science magazine, about amateur crystal growth, some time back. For large flawless crystals, you need to control temperature and evaporation rate. To get certain orientation of final crystal, you have to introduce seed somewhere in the middle of the container. For separation of two materials, you need to find temperature dependent solubility of both materials and keep the temperature somewhere in the middle, then keep evaporation to minimum. You can even pull crystal out of the container and check on it. If it sprouts new crystal on the side, just knock it off and crystal will heal itself. I have no personal experience with this at all, but it fascinates me, that is why I remember it so well. As Göran mentioned, nitric acid, in many places, is not readily obtainable, and in many cases, not very cost effective. This could be another way, how to separate silver nitrate from contaminated solution, for use in silver cell. I still have this magazine, so if anybody is interested, I will look into it closer. I wonder, if this could be done with gold? Göran, thank you for directing me to this thread and keep us posted. Thanks.

 

[g_axelsson]

Crystallization as a method is a classical way of refining in some areas.

The electrolyte in copper refineries is cleaned from nickel by crystallization. I don't remember the details just now, but in principle either copper or nickel sulfate can be recovered in a relatively clean product as one starts to crystallize a long time before the other.

The rare earth metals are very similar in chemical terms so a couple of them are hard to separate. The first time some of the REE-metals were separated they started with a solution of ree-metal with some unknown contamination. By studying the crystals in a microscope they could see slight differences between the optical properties of the crystals and was able to isolate another element.

I would like to build a temperature controlled chamber where I can regulate the temperature up and down in periods. Small crystals dissolves easier than bigger ones and the big ones should grow faster, so having a chamber like that should have the potential to make bigger crystals. Just as a damaged crystal can self heal over time as the atoms have it easier to attach to the crystal at the damaged spot.
... or I could just wrap a beaker with some plastic foil, make a pinprick with a needle and put it in the window and let the sun do the temperature cycling while slowly evaporating the water.

Right now I'm working too much so the experiment has to rest for a while.

Göran