Sciencemadness Discussion Board

Solvent for realgar?

woelen - 16-8-2023 at 04:04

I purchased some natural realgar from AliExpress. This is the material I received (the red variation, the other one is orpiment, which I already had ordered a few years ago).

https://nl.aliexpress.com/item/1005005505996737.html

This is the real stuff for a decent price and it is quite pure! It dissolves in alkali solutions, giving a colorless solution and a precipitate of elemental arsenic. It is possible to scrape powder from these 'stones', but it is a very tedious job. With KClO3 it forms amazingly powerful mixes, which are very easy to ignite. I only tried this in mg quantities and ignited it outside, with a fair breeze from behind to avoid breathing the smoke, which may contain As2O3 or potassium arsenite. The result, however, is impressive.

I want to have a decent amount of powdered or at least much more finely divided realgar. Crunching and grinding is difficult and scraping powder from the 'stones' is OK for a few mg, but not for a large batch. Besides that, crunching a toxic substance like this, with the risk of bringing a lot of fine very toxic dust into the air is also not a very pleasant idea.

I also ordered some powdered realgar: https://nl.aliexpress.com/item/1005004618172370.html
The stones are the real stuff, the powder, however, is fake. It just is some white powder (an insoluble mixed oxide/carbonate, which easily dissolves in dilute acids, but not at all in alkalies), which is colored with an orange dye. I let the seller know about this and told him about my tests with alkalies, acids and KClO3 and showing him the difference with the real stuff. He will refund me and contact his supplier.

Now I was thinking of dissolving the stones and then let the solvent evaporate. This also could serve as an extra purification step, because it leaves behind insoluble silica-based impurities if they are present. Realgar is a molecular covalent compound, which has discrete As4S4 molecules in its crystal lattice. I tried a few apolar solvents, but found no solvent, which dissolves a decent amount of it. A search on internet does not yield interesting results.

Is there a solvent for realgar, which does not chemically destroy it, but just dissolves it, and which can be evaporated to leave behind the pure solid?




[Edited on 16-8-23 by woelen]

charley1957 - 16-8-2023 at 05:36

Realgar is insoluble in water and most organic solvents, resulting in poor bioavailability, and so its wide use in clinical situations has encountered many difficulties, such as lack of significant efficacy, need for high doses, and poor patient compliance.11 In order to improve the poor bioavailability of realgar caused by its limited solubility, some means of increasing its solubility is needed. Since realgar is nonionizable, its solubility cannot simply be increased by converting it into a salt.12 However, it has been suggested that a particle-size reduction might significantly accelerate its rate and extent of absorption.13 Transforming poorly water-soluble drugs into nanosize crystals will dramatically enhance their bioavailability and extend their clinical use.14 Colloidal gold and iron oxide nanocrystals are examples of nanoparticles (NPs) that are widely used in biology and medicine.15 To date, realgar NPs have been prepared by physical and chemical methods. Physical methods include high-energy milling2,16–18 and air-current grinding,19 while chemical methods include solvent-relay technology,20 chemical precipitation,21 and coordination chemistry.22 When chemical methods are used to prepare realgar NPs, some problems arise, such as removal of organic solvent residues and the larger particle size. High-energy milling, which was used in this study to prepare realgar NPs, is a suitable tool for preparing particles in the nanosize region by a simple solid-state approach and a production process that is also simple and suitable for production on a large scale.


Realgar is a poorly water-soluble compound that exhibits poor bioavailability. To improve this, the authors reduced the particle size of realgar to nanoscale by high-energy ball milling and optimized the preparation process under which (realgar weight 40 g, milling time 9 hours, milling speed 38 Hz, milling temperature −20°C) realgar nanoparticles (NPs) with an average size of 78±8.3 nm were prepared.

Tian Y, Wang X, Xi R, Pan W, Jiang S, Li Z, Zhao Y, Gao G, Liu D. Enhanced antitumor activity of realgar mediated by milling it to nanosize. Int J Nanomedicine. 2014 Jan 31;9:745-57. doi: 10.2147/IJN.S56391. PMID: 24516332; PMCID: PMC3916444.


chornedsnorkack - 16-8-2023 at 05:49

This
https://library.sciencemadness.org/library/books/Mellor_ACTI...
page 270 also does not find any solvents for "decent amount": A. Schiiller said that realgar is slightly
soluble in carbon disulphide, and benzene, especially at a high temp. A. Schüller, Ber. Math. Naturw. Ungarn, 12.
265, 1894;

Texium - 16-8-2023 at 06:03

Admittedly, it’s a wild-ass guess, but I feel like carbon disulfide would be the most promising solvent. Other solvents that tend to dissolve lots of things like DMSO and DMF also unfortunately tend to have very low volatility, and thus wouldn’t be applicable to your situation, even if they dissolved it.

Edit: didn’t see chornedsnorkack’s post when I replied, but nice to see my wild-ass guess is somewhat supported by evidence!

[Edited on 8-16-2023 by Texium]

chornedsnorkack - 16-8-2023 at 10:20

Quote: Originally posted by Texium  
Admittedly, it’s a wild-ass guess, but I feel like carbon disulfide would be the most promising solvent. Other solvents that tend to dissolve lots of things like DMSO and DMF also unfortunately tend to have very low volatility, and thus wouldn’t be applicable to your situation, even if they dissolved it.

One limitation of carbon disulphide is high volatility, at 1 bar. Another is toxicity.
For DMSO, see the example of sulphur:
https://www.sciencedirect.com/science/article/abs/pii/S01677...
Sulphur is poorly soluble in DMSO at 25 C - reported 1 g/l is close to experiment - but the solubility increases a lot on heating, reaches 100 g/l at 175 C.
If cold DMSO is not a good solvent for realgar, but hot DMSO is, then realgar might be dissolved in hot DMSO and recovered on cooling?

Fery - 16-8-2023 at 10:29

Cannot it be just melted at 320 C in an inert atmosphere (maybe it does not decompose significantly???), let the impurities to settle down or float on top, let it crystallize and finally mechanically grind away the contaminated surfaces?

Texium - 16-8-2023 at 10:39

That's true, if it was possible to recyrstallize it from DMSO, that would be a good solution. Then excess DMSO could be easily washed off with water. Or the DMSO solution could be diluted with water to force precipitation of the realgar. All of this of course hinges on whether realgar is appreciably soluble in DMSO at any temperature...

B(a)P - 16-8-2023 at 12:51

I had a look through some of my old geology text books, but sadly nothing of interest. I do think that heating in a closed vessel or under an inert gas might be an option worth further investigation.

From https://www.mindat.org/show.php?id=3375

Quote:

Thermal Behaviour: Heated in a closed tube, it melts, volatilizes, and gives a transparent red sublimate. Heating slowly in an open tube, it gives sulphurous fumes and a white crystalline sublimate.


F. Habashi, in Encyclopedia of Materials: Science and Technology, 2001
Quote:

(a) Arsenic sulfides
Arsenic sulfides occurring in nature are realgar, As4S4; orpiment, As2S3; arsenopyrite, FeAsS; and enargite, Cu3AsS4. Realgar can be distilled without decomposition while orpiment melts at 310 °C and boils at 707 °C without decomposition.


Boffis - 16-8-2023 at 13:42

At one time very high refractive index liquids were prepared by dissolving arsenic tri-iodide and realgar in methylene diiodide. I found the "receipts" in various old texts on measuring high refractive indices of minerals. If you are interested I can dig out the references. Another solvent I came across is phenyl-diiodoarsine but this sounds like a death wish :D.

Carbon disulphide, suggested above, sound like the best lab possibility but I couldn't find any data on its ability to dissolve As4S4.

woelen - 16-8-2023 at 23:19

I'll try dissolving As4S4 in CS2. Not difficult to do that at a test tube scale, I have a fair amount of CS2. CS2 is somewhat toxic, but is manageable quite well. Unfortunately, CS2 does not leave much room for heating, it is very volatile, so if it does not work well at room temperature, then it does not work for me. I do not have equipment for working with sealed and pressurized containers.

Using CH2I2 as solvent does not sound like a practical and affordable thing. CH2I2 is not very stable, and I expect it to be outrageously expensive, certainly not suitable as a bulk solvent.

teodor - 17-8-2023 at 23:27

Also realgar melts at 320C.

Fery - 18-8-2023 at 08:35

CH2I2 is quite expensive, costs more than 50 EUR per 100 g https://sklep-chemland.pl/en/dijodometan-98-loba-op-100g.htm... which is only circa 30 ml due to its high density.
But could be prepared reacting CH2Cl2 + 2 NaI in acetone solvent.
Maybe you can sublimate it as B(a)P wrote. In high vacuum, so use at least 2 stage oil rotary vane vacuum pump. B.p. at atm. pressure is 565 C. https://www.chemicalbook.com/ChemicalProductProperty_US_CB01...

woelen - 21-8-2023 at 10:52

I tried the experiment with CS2. It does not work at all. I added 100 mg or so of finely crunched As4S4 to 2 ml of CS2. The material is wetted immediately and sinks to the bottom. This is different from adding it to water. In water it is not that easily wetted. I swirled the test tube with the CS2 and As4S4 in it, but nothing dissolves. When I stop swirling the test tube, then the solid settles at the bottom again and the liquid above it remains perfectly colorless. Not even the faintest yellow or orange color can be observed.
Next, I left the test tube stoppered with the CS2 and As4S4 for a few hours. But even then, still no visible color is given to the CS2. On evaporation of all CS2 the powdered As4S4 is not altered at all. Outside temperature was approximately 25 C.

[Edited on 21-8-23 by woelen]

Bezaleel - 27-8-2023 at 12:59

Maybe there is a deep eutectic solvent for this. I remember dissolving soft calcined RE oxides in these solvents, so maybe there is a DES for realgar as well.

averageaussie - 27-8-2023 at 16:32

just did a google search, and the first result for "realgar solubility" is snippets from a bunch of papers regarding realgar.
https://www.sciencedirect.com/topics/earth-and-planetary-sci...
right below figure 26.4, "The solubility of these minerals is a function of pH. At a pH value of 12, orpiment, stibnite, and realgar dissolve significantly"
the solvent is likely a sodium cyanide solution, because that is what is used in gold leaching, the topic of the paper being gold leaching.
I have done no real searching on this site, so there may very well be more useful information there.

teodor - 28-8-2023 at 05:51

But why Woelen do you need a slovent for such low-melting salt? Do you think it can produce too much toxic fumes in the state of liquid?

I believe you can try to use molten sulfur as a solvent to keep the temp 150C lower.

So you either dissolve realgar in sulfur at ~150C or you mix two liquids at ~350-380C. After solidification you just leach sulfur with, let say, toluene.
Or may be boiling toluene is a potent solvent for realgar also?

[Edited on 28-8-2023 by teodor]

[Edited on 28-8-2023 by teodor]

chornedsnorkack - 28-8-2023 at 11:54

Quote: Originally posted by teodor  
But why Woelen do you need a slovent for such low-melting salt? Do you think it can produce too much toxic fumes in the state of liquid?

I believe you can try to use molten sulfur as a solvent to keep the temp 150C lower.

So you either dissolve realgar in sulfur at ~150C or you mix two liquids at ~350-380C. After solidification you just leach sulfur with, let say, toluene.
Or may be boiling toluene is a potent solvent for realgar also?

[Edited on 28-8-2023 by teodor]

[Edited on 28-8-2023 by teodor]

The problem is that sulphur may react with realgar. What is wanted is a decent low-polarity solvent which is tolerant of high temperatures.
Yes, realgar melts at 320 Celsius. The thing is that it is mixed with an excess of inert rocks. Which means that though it would be melted at 320 C, surface tension would stop it from simply draining.

woelen - 28-8-2023 at 13:02

I do not feel comfortable heating realgar to 300 C or so. It is not a salt, but a covalent compound, which is somewhat volatile at 300 C.
My realgar is quite pure, it hardly contains insoluble rocky material. It was sold to me as natural realgar, but maybe it is artificially made stuff. It is a set of 'stones' of varying size, which are fairly soft. The 'stones' are covered by an orange powder.

@averageaussie: The material indeed dissolves in alkaline solutions, but that is a chemical reaction, which destroys the realgar. No cyanide is needed for that, plain NaOH does the job. Realgar dissolves, giving a deep red solution, which quickly decomposes, precipitating arsenic and keeping mixed arsenite/thioarsenite in solution.

averageaussie - 28-8-2023 at 15:06

Quote: Originally posted by woelen  


@averageaussie: The material indeed dissolves in alkaline solutions, but that is a chemical reaction, which destroys the realgar. No cyanide is needed for that, plain NaOH does the job. Realgar dissolves, giving a deep red solution, which quickly decomposes, precipitating arsenic and keeping mixed arsenite/thioarsenite in solution.


sorry about that, didnt read the original post properly :/

you can also apparently buy some realgar powder from dye shops, 10 grams for $30.80; https://www.naturalpigments.com/realgar-pigment.html

I know you had some woes about ordering from aliexpress, and while this source is more expensive, it does look legit.