Potassium ferrioxalate produces striking green crystals that are both beautiful and easy to grow.
Potassium ferrioxalate is a chemical that can be made from relatively common materials. You can prepare it by dissolving rust in oxalic acid, which is often used in cleaning, and then neutralizing the brown solution with potash (potassium carbonate).
The resultant solution will look bright green, and when it evaporates, crystals start to grow.
Hi, I’m Chase, and I love to grow crystals at home. Not many people know that it’s easy to grow crystals with household chemicals like table salt, Epsom salt and sugar. I first started this hobby in high school and I’ve been loving it ever since. Today, I’ll share how I grew potassium ferrioxalate crystals with you.
If you want to try it out, note that while the compound is not particularly toxic, it’s still an irritant. Also, making it involves dissolving rust in acid, so wear gloves during this step.
If you’re looking for a fun activity to do with your kids, making green rupees probably isn’t the best choice. Consider checking out my guide on how to grow alum crystals, which are both beautiful and food safe.
Now, let’s get started.
As mentioned, you need rust, oxalic acid and potassium carbonate to make this compound.
I made the rust myself, and bought the other two online. Oxalic acid is commonly used in cleaning and bleaching. Potassium carbonate is sometimes used to make certain foods. It also acts as a drying agent.
How to make rust?
Rust is iron(III) oxide. It can actually be bought as well, but I thought it’d be fun to make it myself.
I first soaked some fine steel wool in a dish containing salt water. Salt solution speeds up the rusting process, just like how objects rust faster near the sea.
However, it was still going to take a long time. Although I only needed about 5-10 g of rust to grow decent crystals, it would take weeks – even months for the piece of steel wool to crumble completely into rust.
To further speed things up, I added some hydrogen peroxide to the steel wool. Hydrogen peroxide is an oxidizer, which helps convert the iron to iron(III) hydroxide.
The next day. I added hydrogen peroxide only to the dish on the left.
I continued to periodically add hydrogen peroxide to the dish on the left. After 2 weeks, the treated steel wool had completely disappeared, while the untreated one wasn’t even halfway there.
I filtered off the salt water from the dish on the left, and washed the iron hydroxide a few times to remove traces of salt.
The iron hydroxide looks almost exactly like the floor. Oh well.
Then, I heated the iron hydroxide strongly at 200°C for 1 hour to convert it into iron oxide. I generally heat it for twice as long as the time it takes for it to dry completely to ensure all of the iron hydroxide has reacted.
Upon heating, the color turned from black to reddish brown. Pure iron oxide is actually red, but small amounts of impurities aren’t a big issue for crystal growing.
The initial piece of steel wool weighed roughly 1 gram. I ended up with 4 grams of dry iron (III) oxide powder, so the yield was good.
A more efficient way of making rust is via electrolysis. The disadvantage is that it requires a power supply.
To carry out electrolysis, I first prepared dilute salt solution as the electrolyte. Then, I used a piece of iron as the positive electrode and a steel wire as the negative electrode.
Once I turned on the power supply, the iron started dissolving slowly, while bubbles of hydrogen gas formed on the wire. Hydrogen gas is explosive, so I did it outside. Some of you might have concerns that chlorine gas will develop, but because chlorine is so reactive, and because the salt solution is dilute, it’s safe for this setup.
A brownish precipitate soon formed at the bottom of the container. After half a day, I filtered and dried it, just like the steps above, yielding 6 grams of rust.
This video by NurdRage explains the process in more detail.
Enough about rust. This article isn’t titled How to Make Rust.
We’re here to grow shiny green potassium ferrioxalate crystals.
Making the potassium ferrioxalate
To prepare the solution, I dissolved 45 g oxalic acid in 200 ml of warm water. Then, I added 6 g of iron oxide, stirring as the solution slowly turned reddish brown.
Note: If you’re using anhydrous oxalic acid, just 30 g is enough. It’s usually sold as the dihydrate version, which requires 45 g.
It took a while for the rust to dissolve. I continued to heat the solution gently on the hot plate for a few hours, until most of the solid stuff had dissolved. By then, the solution, iron(III) oxalate, was dark brown.
I then added potassium carbonate into the solution, bit by bit. There was a lot of fizzing as the reaction produced carbon dioxide. In total, I added 15 grams of potassium carbonate, and eventually, the solution turned greenish.
I stirred it around a bit, turned off the heat and waited for the precipitate to settle.
Note that at this point, the solution was supposed to be a striking fluorescent green, but it was a dirty olive green instead. Based on my previous runs, this could be due to 2 reasons:
- Not enough potassium carbonate was added
In this case, there’s a lot of brown iron oxalate solution left over, so naturally the solution wouldn’t look very green.
- Not enough oxalic acid was added
Provided there’s enough potassium carbonate, it seems like a small amount of excess acid makes the solution more stable. Basic solutions quickly turn brown and form precipitate. After adding a little oxalic acid, they immediately revert to green.
Solution with extra acid (left) vs solution without extra acid (right).
So I just added 2-3 g more acid, stirred it around, and filtered the glorious green solution into a dish. Then, I placed the dish inside the storeroom to provide a sheltered, stable condition for the crystals to grow.
Growing potassium ferrioxalate crystals
As water evaporates, the solution slowly becomes more concentrated. Once it reaches saturation, the extra solute has nowhere to go, and it crystallizes out.
My solution was a little dilute – I guess I should have heated more water away at the start to speed up the crystallization. Regardless, after 5 days, tiny green crystals started forming on the surface.
They eventually dropped down and continued growing at the bottom. Here is what they looked like after a week:
The crystals aren’t very big, but gosh, they’re so pretty!
Also note that the solution became much greener than before. I’ll explain more about this later.
By the 11th day, I had three green rupees sitting nicely in the solution, while many smaller crystals formed at the side. To prevent them from growing into each other, I blew them apart using a plastic dropper.
Observe that the crystals have straight edges and flat faces without being cut or polished. This is the natural crystal structure of potassium ferrioxalate. Meanwhile, other crystals such as table salt grow into cubes, while alum forms crystals shaped like octahedrons.
By day 17, the crystals were getting quite large, and mold had started to form in the solution. This is because some bacteria are able to use oxalate as a food source. I didn’t appreciate that though, and I wanted to try out a different technique, so I decided to remove them from solution.
First, I poured the solution into a different container. Then, using tweezers, I picked them out and placed them on a piece of filter paper to dry.
And the results were breathtaking.
But I wasn’t done yet.
I wanted to grow bigger crystals.
Growing big potassium ferrioxalate crystals
A common crystal growing technique is to tie a seed crystal to a string, and hang it from solution. This way, other crystals won’t stick to it, and it will also grow more symmetrically.
I picked a small, but beautiful crystal from the batch I had grown earlier, and tied it to the end of a fishing line. Then, I taped the other end of the fishing line to a stick, and lowered the crystal into a cup containing the potassium ferrioxalate solution.
I also covered the top of the cup with some cling wrap to slow down evaporation and discourage extra crystals from forming.
The crystal immediately began to grow, and after a few days, a nice clear layer had formed around the original crystal.
You can still see the outline of the original crystal.
Meanwhile, lots of smaller crystals also started forming at the bottom of the cup. These would compete with the main crystal and slow down its growth, so I transferred it to a new container.
With fewer crystals acting as competition, it began to grow much faster. Also, the sides widened out nicely. By the 1-week mark, it looked like this:
Unfortunately, some temperature changes due to the weather caused small cracks to form inside the crystal. They’re only visible when I shine at it with a torch.
Also, as mentioned previously, mold likes to form on the surface of the solution. Although they didn’t really affect the crystal growth, I removed them periodically with tweezers.
I also tried scooping them up with copper wire, as copper is toxic to microorganisms. It worked for a few days, but the mold always came back eventually.
After 3 weeks, the crystal was getting big, and the level of the solution had greatly decreased. I decided it was time to harvest the crystal.
I removed the crystal from solution and dried it using a piece of filter paper. Then, I cut the string away using some scissors. The final crystal was 4.5 cm long and 2.6 cm wide.
It was undoubtedly one of the most beautiful crystals I had ever grown. The faces of the hexagonal crystal were sharp and crisp, and it glowed fluorescent green under the morning light.
Storing the crystals
The deep green color of potassium ferrioxalate is breathtaking. However, like all ferrioxalates, it is sensitive to light. This means it decomposes in the presence of bright light, with a reduction in the iron center and the oxalate ion oxidizing into carbon dioxide.
Green potassium ferrioxalate solution exposed to light will turn yellow, and then brown. After keeping them in the dark for some time, they become green again as the iron ions are oxidized by air, and the ferrioxalate complex reforms. Adding small amounts of oxalic acid greatly helps to speed up the process.
This explains why my initial solution was a little yellowish, and why it turned bright green after sitting in the storeroom for a few days.
The solution on the left was keep indoors, while the solution on the right was exposed to direct sunlight for 30 minutes.
On the other hand, the surface of crystals exposed to bright light will turn duller, and eventually white. I have found that occasional indoor lighting is fine. But this process becomes much faster under the sun.
Here’s the first batch of crystals that I grew, and brought outdoors for 30 minutes, compared to those that I did not take outside:
Actually, they don’t look that bad – there’s a different vibe to them.
When kept in the dark, these crystals are quite stable. I have kept them for a month or so, and they look the same.
How impurities affect crystal shape
You can also change the shape of potassium ferrioxalate crystals by adjusting the acidity, and ratio of potassium carbonate to iron (III) oxalate. Impurities also affect its structure, ranging from sticks to hexagonal crystals.
For example, this beautiful crystal by Reddit user u/cwdcloud has a different shape from mine.
You can also find pictures of much longer, stick-shaped crystals online.
I have seen small octahedral crystals before, growing in a solution with a large excess of oxalic acid (I added too much by accident).
You can see the octahedrons, as well as some clusters of oxalic acid crystals intertwined with potassium ferrioxalate.
I have yet to pin down the exact conditions affecting its growth – I’ll have to make more experiments. Crystal growing is both a science and an art, and the results amaze me every time.
Edit: I’ve figured out the effect of excess acid on the shape of the crystal. To a saturated solution of potassium ferrioxalate, I added oxalic acid until the solution was also saturated in terms of oxalic acid. Then, I used the same procedure as above. Here are the results:
They aren’t actually octahedrons. Compare it to the rupee shaped ones above. The striations on the top and bottom of the crystal remain, but they have been “squashed” lengthwise so that the crystal is no longer quite as long, but much thicker.
There are pros and cons to growing them from a saturated oxalic acid solution. The good news is that there was significantly less mold in the highly acidic solution. The bad news is that oxalic acid crystals also formed simultaneously, and the two types of crystals frequently got stuck together:
The transparent plates are crystals of oxalic acid, grown into the green potassium ferrioxalate crystals.
In short, if you want long crystals, grow them from a less acidic solution. If you want thicker and stockier ones, grow them from a more acidic solution.
That’s all I have to say about impurities.
Finally, I’d like to mention that it’s also possible to grow crystals from other ferrioxalate compounds, like sodium ferrioxalate and lithium ferrioxalate. Substituting potassium carbonate, the preparation and growing process is exactly the same.
They have different crystal structures, and I plan to try them out in the future.
I hope you enjoyed the article. Feel free to drop a comment if you’d like to ask me anything.
Also, if you’d like to read more articles like this one, consider signing up for my mailing list. Or check out my post on how to grow stunning crystal clusters from fertilizer.
As always, happy growing.