Sciencemadness Discussion Board

Strengite from Mohr's salt and MAP

elementcollector1 - 25-1-2025 at 12:21

Man, it's been a while since I've been on here.

Recently decided to get back into crystal growing, and wanted to do something a little different - strengite (FePO4.2H2O).

Found this procedure off of a publication (DOI 10.1016/j.jcrysgro.2017.10.014 - Kinetics of solution crystal growth of strengite). Not entirely sure why the procedure is more complicated than iron salt + phosphate, but I'm guessing it has something to do with controlling oxidation in air or preventing formation of the dimorph phosphosiderite?

Procedure is as follows:
Quote:

To investigate the morphology of strengite crystals growing from aqueous solution at low temperature and in the absence of hydroxylamine and other special additives, a few experiments were made with air oxidation of Fe(II) at 35 °C. The reagents were0.1 M solutions of Fe(NH4)2(SO4)2.6H2O (Mohr’s salt) and NH4H2PO4, 10 mL of each added to a test tube, which was either left open to the air, stoppered with a stopper with a small hole or covered with Parafilm. The duration of these experiments was15 days.


After the aforementioned 15 days, this resulted in multiple small crystals of strengite, the biggest of which was about 50 μm in diameter. Hopefully I can get them just a touch bigger than that. Currently dealing with making Mohr's salt (which was a lot more sensitive to air than I was expecting!), got the monoammonium phosphate ready to go when that's recrystallized and re-dissolved.

Planning to up the concentration and volume by a lot (while still keeping that idea of paraffin wax over the top for slow air introduction), but does anyone have any thoughts on improving this? Obviously heat would help, but I'm trying to keep this a room-temperature endeavor.

Also, I have my own house now! No fume hood yet, unfortunately.

igxlabs - 25-1-2025 at 13:36

I went down the rabbit hole for a few minutes, and found some things you might find interesting (if you've not found them already.)

Some mineral information on strengite: https://www.mindat.org/min-3801.html

A PDF entitled THE CRYSTAL STRUCTURE OF METASTRENGITE AND ITS RELATIONSHIP TO STRENGITE AND PHOSPHOPHYLLITE

elementcollector1 - 25-1-2025 at 14:56

Ha. I use Mindat religiously, so I've already seen the page. Forgot phosphosiderite was once called metastrengite, though.

Also, figured I should report this, since reports of Mohr's salt synthesis seem to be somewhat lacking on the wiki - the solution was initially olive green (after clearing up the milky green Fe(OH)2 precipitate with additional sulfuric acid), but after two passes through a standard coffee filter, it's much clearer and much more pale in color. Put it in the fridge - not sure what the yield is going to be, since I'm having trouble finding data on solubility at temperatures other than 20 C. Worst-case scenario, I can crash it out with ethanol, I suppose...

elementcollector1 - 26-1-2025 at 20:34

Crashout with ethanol went well - it's still drying, so no estimate on yield just yet (theoretical limit would be 161.4g). Very nice, extremely pale green-white color.

Also found a bit more about why the runaround procedure for growing this: The mineral/salt is pretty close to insoluble (0.642g/100mL at 100 C according to Wikipedia). So while you could grow this the old-fashioned way, you'd need several gallons of water just for the few tens of grams I'm currently working towards.

elementcollector1 - 2-2-2025 at 16:53

15 days isn't up, but I'm pretty sure this isn't working - or at least, not in the way I'd like. I made two test tubes - one recreating the conditions of the original paper (0.1 M solutions), and one using 1 M solutions (or the closest equivalents) instead, to see if that resulted in larger crystals.



Bottom layer is undissolved Mohr's salt - I knew a 1 M solution would be over the solubility limit, but I was hoping the non-saturated MAP solution would provide enough extra water to dissolve the rest. No dice.

Top layer is, to the best of my knowledge, the desired FePO4.2H2O - however, it's pretty clearly not growing large crystals.

Middle layer... I'm unsure exactly what I've created here. The dark bits appear to be larger crystals, but they could be many things - vivianite, metavivianite (I didn't isolate this from light), strengite, etc. As for the gray stuff, I can only presume it's more of that same dark substance in smaller particle size.

Anyway, while I wait for this to maybe eventually grow into something I can measure, I've found a new synthesis method to potentially try - direct recrystallization. Following this paper (DOI 10.1016/j.cjche.2016.06.009):
Quote:
The solubilities of iron phosphate dihydrate in deionized water and phosphoric acid solutions at high concentrations from (1.13 to 10.7) wt% were systematically measured at temperature from (298.15 to 363.15)K and atmosphere pressure. The solubility was found to increase 5 orders of magnitude or more with increasing the concentration of phosphoric acid, and decrease 1 to 2 orders of magnitude with increasing the equilibrium temperature. The phosphoric acid addition and temperature was found to affect the solubility of iron phosphate dihydrate by the formation or dissociation of coordination species, which could further accelerate the phase transformation from the amorphous iron phosphate dihydrate to orthorhombic iron phosphate dehydrate by dissolution-recrystallization mechanism. The high dependences of the solubility of iron phosphate materials on H3PO4 concentration and temperature could be well predicted by calibration equations and effectively exploited to regulate the precipitation, crystallization and growth processes between iron (III) and phosphate ions in aqueous solutions for synthesizing iron phosphate materials with diversified crystalline structure and morphology.


Basically, precipitated FePO4.2H2O powder can be re-dissolved in a solution of phosphoric acid in water (10.7 wt% was the limit of the study, but higher would presumably also work) and then slowly precipitated back out in the traditional manner of crystal growth. The solubility at 25 C and 10.7 wt% H3PO4 (the maximum solubility in the study) isn't too high - about 7.398 g FePO4.2H2O / kg solvent - but it's not anywhere near the milligrams to micrograms per kilogram I'd otherwise be dealing with.

However, this runs into two main problems:

1) Evaporating the water over time increases the concentration of phosphoric acid, which is very non-volatile. This should increase the solubility of the FePO4, according to the paper.

2) Cooling the solution also increases the solubility of the FePO4 (not by nearly as much as increasing the H3PO4 content), meaning initial heating of a saturated solution and subsequent slow cooling to room temperature would accomplish exactly the opposite of what I want. I could then do the opposite - an initial cooling, followed by warming back to room temperature - but the temperature differential I'd be able to get before freezing the solution solid is unclear, and I'm not entirely convinced the solubility vs temperature remains inverted at lower-than-room temperatures (these weren't part of the study).

Does anyone know of any other possible solvents for FePO4.2H2O? I thought about MEK, but there's pretty much no data on the salt's solubility outside of water and phosphoric acid solutions, and I'm hesitant to throw away perfectly good MEK if I don't have to.

DraconicAcid - 2-2-2025 at 17:55

There's no way MEK or any organic solvent is going to dissolve an ionic compound with high charges like FePO4.

If you half-filled a test tube with a concentrated solution in 10% phosphoric acid, then put it in a sealed jar with pure water, vapour diffusion of the water would dilute the acid and cause some of the product to slowly precipitate.

elementcollector1 - 2-2-2025 at 18:36

Quote: Originally posted by DraconicAcid  
There's no way MEK or any organic solvent is going to dissolve an ionic compound with high charges like FePO4.

If you half-filled a test tube with a concentrated solution in 10% phosphoric acid, then put it in a sealed jar with pure water, vapour diffusion of the water would dilute the acid and cause some of the product to slowly precipitate.



Huh. Thought one of the old 'standard' practices for isolating Nd from Nd-Fe-B magnets after HCl leaching was MEK uptake of iron. Been a while, though, so I could be wrong.

Vapor diffusion's an interesting idea, I'll have to look into that. I eventually want to scale this up a little bit, but the first step is producing visible, purple crystals (and right now, that's challenge enough).

DraconicAcid - 2-2-2025 at 22:39

Quote: Originally posted by elementcollector1  

Huh. Thought one of the old 'standard' practices for isolating Nd from Nd-Fe-B magnets after HCl leaching was MEK uptake of iron. Been a while, though, so I could be wrong.


FeCl3, maybe, but definitely not phosphate.