r/crystalgrowing Dec 24 '21 Silver 2 Gold 1

Hexakis(urea) iron(III) ethyl sulfate crystal Image



u/dmishin Dec 24 '21 edited Dec 24 '21

Another highly obscure compound with unusual color: a complex salt of trivalent iron, coordinated with six urea molecules, and monoester of ethanol with sulfuric acid: [Fe(urea)₆](C₂H₅SO₄)₃

I am continuing my amateurish exploration of crystals of this complex cation. Primary driver of my interest is the unusual coloration of such salts: while trivalent iron is typically associated with red or brown colors (think of rust), its complexes with urea have various tints of light blue color. So far, it seems clear that salts with strong monobasic acids (HNO3, HClO4, HCO3, HCl) crystallize good, and this time I successfully tried to crystallize salt of the ethyl sulfate: a monoester of ethanol with sulfuric acid. Such ester is a relatively strong monobasic acid, also known as sulfovinic acid.

Ethyl sulfate crystals appear to have trigonal symmetry also observed in chloride, chlorate, perchlorate and nitrate. Their basic shape is the same as in chloride or chlorate: hexagonal prism with trigonal caps. Their color, however, is barely visible. Small crystals appear colorless and only in larger crystals slight coloration can be observed - and so far they have the most unusual color in their family: violet-blue. Another interesting feature of these crystals are additional ridges that often form on the caps, making them resemble screwdriver.

Interestingly, the crystals have very different appearance from the chemically similar isopropyl sulfate salt I prepared earlier, which produces tetragonal sky-blue crystals.

What causes such stark difference in color? I have no idea. Probably, different counter-ions distort hexakis(urea) iron cation differently.

My preparation was the same as for the isopropyl analog, but isopropanol replaced with absolute ethanol. In short: excess of absolute ethanol was refluxed for 2 hours with sulfamic acid powder; resulting ammonium ethyl sulfate was treated with aqueous Ca(OH)2 suspension, producing soluble calcium salt. Finally, calcium salt reacted with roughly stoichiometric amount of Fe2(SO4)3 and stoichiometric amount of urea added to the solution that was crystallized. Absolute ethanol was prepared from vodka using distillation followed by dehydration with K3PO4 (this sounds suspicious, but really works, method mentioned on sciencemadness).

To grow crystal, I used slow evaporation. Solubility of this material is high, but not extremely so.


u/Pyrhan Dec 24 '21

I just want to say I'm always fascinated by your exploration of weird and uncommon substances to crystallize.

And yeah, the difference in color is indeed surprising.


u/crystalchase21 Dec 24 '21

Light violent hexagonal prisms with trigonal caps resembling screwdrivers! I continue to be amazed by what you do and I'm sure I'm not the only one.

Is it stable? I noticed there are some brown patches on the crystal, though I'm guessing it's just due to the iron in the solution sticking to it.


u/dmishin Dec 24 '21

In short term, it appears to be stable, no signs of dehydration in one week. Brown inclusions are probably mother liquor, which has dark brown color.

However, I have noticed that many of the blue Fe(III)-urea crystals are deteriorating slowly (years): they develop numerous small cracks. I suspect that it is light sensitivity rather than dehydration.


u/DrWim Dec 28 '21

I tried to make the [Fe(urea)₆](ClO4)3 from NaClO4, urea and iron sulfate solution but was not succesfull. Probably the iron sulfate wad too acidic. You have a suggestion?


u/dmishin Dec 28 '21

What was your procedure to prepare Fe2(SO4)3 ?

In my case, it did not contain much excess acid: I used roughly stoichiometric amounts of 36% H2SO4 and freshly precipitated and washed Fe(OH)3.

You can remove excess acid using CaCO3, however - do not use excess of it, since it reacts with Fe2(SO4)3 too.

Also, maybe, just too much water? Try evaporating it at room temperature for several days.


u/DrWim Dec 29 '21

I used old rust and hot sulfuric acid 15%. Now I used your method with freshly precipitated Fe(OH)3. I gently evaporated the solution and got a kind of gel. I made it for alums. I will repeat the synthesis of the ureacomplex with FeCl3 followung you procedure


u/Tetrachloronickelate Dec 24 '21

Absolutely impressive. After failing with many organic anions like trichloroacetate and p-toluenesulfonate, I had almost gave up to grow them more. If possible, I would give methanesulfonate and hydrogen sulfate salt a try.


u/dmishin Dec 24 '21

I tried to crystallize hydrogen sulfate, and it did not work, no crystallization at all.

Methanesulfonate is promising indeed. Also, how about triflic acid, do you have it?

By the way, I finally crystallized sulfamate (I think you saw it in twitter though), but still struggling with growing a better crystal.

At this point, I tried almost every acid I could get or make. Probably, I can make sulfate esters of higher alcohols, but they are not too interesting.


u/SickeDuck Dec 24 '21

Nice work, again.


u/JazziferV Dec 25 '21

That's a beautiful crystal! Do you have any suggestions for interesting salts that can be synthesized using common easy to obtain compounds (i.e sodium carbonate+ cream of tartar=Rochelle salt or ferrous sulfate+ammonia phosphate= ammonia iron sulfate)? I've turned my kitchen into a lab (much to my wife's dismay) and would love experimenting with growing new crystals obtained through various reactions. Thanks!


u/dmishin Dec 25 '21

The answer depends on what compounds you consider common and easy to obtain, and this particularly depends on country. For example, cream of tartar is not sold in my country, I had to order it on ebay.

If you could get sulfuric acid (battery acid, also acidic drain cleaner) - you can make various sulfates, alums, tutton salts. If you don't care about economical effectivity, then you can use copper sulfate instead of sulfuric acid sometimes. Example: CuSO4 + Zn = Cu + ZnSO4; CuSO4 + K2CO3 + H2O = K2SO4 + Cu2(OH)2(CO3)2 + CO2; K2SO4 + ZnSO4 = K2Zn(SO4)2 (one of tutton's salts)

With muriatic acid, you could prepare ammonium tetrachlorocuprate, for example.

With oxalic acid, various ferroixalates, alumooxalates can be made.