It is very important to use pure copper sulfate when growing crystals with it. This is because the purer your raw materials, the nicer the crystals will be.
The importance of purification
Crystals grown properly from pure solutions have flat faces and sharp edges. They are also quite translucent. On the other hand, crystals grown from dirtier solutions have rough faces, jagged edges and white inclusions.
Below is a photo of a crystal grown from an impure solution (left) vs a relatively pure one (right).
There are two situations when you’d want to purify the copper sulfate on hand.
- When your solution looks cloudy/dirty.
This is very easy to spot. A pure saturated solution is a striking shade of blue, with a very slight purple tint. Meanwhile, an impure solution of the same concentration is also blue – but is tinted greenish instead. The more it leans towards green, the more impurities it contains.
Note the color difference between an impure saturated solution (left) and a pure one (right).
- When you experience problems with crystal growing.
There are 3 ways impurities in copper sulfate solution negatively affect crystal growth.
- It leads to the formation of jagged and uneven faces.
This is likely due to the presence of iron salts. These salts also cause the shape of the crystals to flatten.
- It causes small needle-like crystals to form.
These are probably crystals of calcium sulfate, which are usually relatively insoluble in water. They stick all over the place, and often embed themselves inside your copper sulfate crystals.
- It encourages crystal crust to creep up the sides of the container.
This crust sucks out your solution and grows upwards. Eventually, the crust might cover the entire container and spread to the surface underneath. Oxidation of iron salts might also cause the crust to take on a yellowish color.
All 3 problems mentioned are visible in this photo.
The easiest way to purify copper sulfate is via re-crystallization.
In this process, crystals are allowed to grow in an impure solution. These crystals will consist of only copper sulfate ions while excluding impurities. Then, the plan is to take these crystals out, wash them, then re-dissolve to form a purer solution. We can repeat this process to obtain crystals of even higher purity.
First, dissolve a large amount of impure copper sulfate with hot water in the ratio of 50 g copper sulfate to 100 ml water. If the copper sulfate does not dissolve, heat it lightly and stir. My powder was bought online and had a bunch of dirty stuff inside.
Look at how murky the thing is!
After that, filter the warm solution using a coffee filter/filter papers. Collect the filtrate in a dish and discard the residue, which is mainly dust, sand and insoluble copper compounds.
If you added 50 g copper sulfate to 100 ml water, the solution is supersaturated at room temperature. Therefore, crystals should begin to form immediately. If that doesn’t happen, drop a few grains of crystal powder into the solution and crystallization should start immediately.
Now, don’t get too excited over the crystals. These are low-quality ones.
Place the solution into the fridge for a day. By cooling it down, you are decreasing the solubility of copper sulfate in water, forcing even more crystals to grow.
Note: Copper sulfate is mildly toxic, so putting it with your food is a bad idea.
After one day, there will be a lot of crystals at the bottom of the container. Pour the remaining solution into another container.
Wash the crystals using cold water, and store them properly. We will get back to these crystals later.
Heat the remaining solution to evaporate off even more water.
Wait until about 70% of the solution is left, then wait for it to cool down.
Sprinkle some crystal powder to induce crystallization, then put it back into the fridge.
Remove the crystals after a day and wash them with cold water. Store them together with the old batch.
Then continue heating the remaining solution.
Repeat these steps until your solution is down to 10-20% of its original volume. It is now “waste water” as it contains a lot of impurities.
Meanwhile, you should have collected a lot of purified crystals. These are the fruits of your labor. They are reasonably clean, and can be used for crystal growing by re-dissolving in water.
You might have noticed that your new solution is much clearer and bluer than your original one. This is proof that your purification is successful!
Compare the dirty waste water with the stunning blue solution. Also compare it with the dark, murky solution I began with. (pic above)
If you want even purer stuff, you can choose to do a second re-crystallization – or even a third one. For crystal growing purposes, 1-2 will be enough.
For my run, I did two re-crystallizations. I started with 148g of copper sulfate and ended up with 121g, which was a percentage yield of 81%.
Having purified our copper sulfate, it’s time to start growing actual beauties. I’ll show you exactly how in this article: The Best Way to Grow Big Copper Sulfate Crystals
Alternatively, you can use it to grow copper metal crystals.
If you have any questions/thoughts, drop me a comment below.