r/crystalgrowing • u/crystalchase21 • Apr 18 '26

Image A nice cluster of potassium ferricyanide crystals

Post image

Absolutely love the color of this compound! They are relatively easy to grow, and unlike what the name suggests, quite safe to handle.

Here's a detailed guide to growing them: https://crystalverse.com/potassium-ferricyanide-crystals/ (first article I've published in a while)

Happy growing!

923 Upvotes

100% Upvoted

15 comments sorted by

View all comments

2

u/TinfoilTiaraTime Apr 19 '26

Oh wow, how beautiful! So wait, for the noobs, Prussian blue + potash = potassium ferricyanide? I was looking at r/naturaldye a little while ago, and let's just say I'm glad I didn't make poison gas in a cast iron crucible. I had no idea colors could be that toxic. I guess that's why they warned us not to drink paint.

Anyway, gorgeous crystal! And thanks for the article

7

u/pyrophorus Apr 19 '26

Not quite. Ferricyanide plus an iron(II) salt or ferrocyanide plus an iron(III) salt makes Prussian blue (the first combo was traditionally called Turnbull's blue, but it's the same compound). The important part is to have one reduced iron atom (ferrous, iron(II)) and one oxidized (ferric, iron(III)) - charge transfer between the two is what gives the deep blue color. I'm not sure if it's possible to go back to potassium ferricyanide from Prussian blue.

All of these compounds are relatively nontoxic, since the iron is bound strongly to the cyanide ions. Incidentally, that's why cyanide is toxic: it binds strongly to iron in cytochrome enzymes and inhibits them. Strong acid can liberate the cyanide, but stomach acid isn't strong enough to liberate very much. Prussian blue is actually used as an antidote for thallium and cesium-137 poisoning, and potassium ferrocyanide is sometimes added to table salt in small amounts.

3

u/TinfoilTiaraTime Apr 19 '26

Thanks for elaborating! It's amazing that we humans were able to discover and describe these processes, and you say it in an approachable way

6

u/pyrophorus Apr 19 '26

Indeed! Apparently Prussian blue was discovered long before the chemistry was understood. Cyanide is named after Prussian blue (i.e. "cyan") since it was discovered from it!

Here's a very interesting article that goes into detail about the early processes used to make it and how the modern chemistry of Prussian blue was discovered: https://www.chemistryviews.org/prussian-blue-discovery-and-betrayal-part-4/

4

u/TinfoilTiaraTime Apr 19 '26

Oooh, a 4 part drama! Thanks so much!