Sciencemadness Discussion Board

Some arsenic chemistry

woelen - 22-12-2020 at 13:08

Arsenic is an element, which is not used very much in home experiments, for perfectly understandable reasons. However, the element has some interesting chemistry, and this is demonstrated in this series of experiments:

The chemistry of the arsenic sulfides is quite colorful and interesting.

valeg96 - 22-12-2020 at 15:52

What I find amusing about arsenic is that its compounds were the late 1700s-mid 1800 equivalent of today's essential oils.

Quacks would grind any arsenic sulfide or oxide or salt with all kinds of oils and scented powders and sell them to cure any illness. I really should get a sample as well...

Lion850 - 23-12-2020 at 01:38

Interesting! I was thinking to make some arsenic triiodide but never acquired any elemental arsenic.

Heptylene - 23-12-2020 at 01:56

Another great article woelen! And those mineral samples are gorgeous!

valeg96 - 27-12-2020 at 03:40

Also, orpiment is supposedly soluble in solutions of ammonium sulfide or ammonium carbonate. I'm trying to figure out if there's a feasible way to isolate some As compound from the orpiment solution without dragging along any sulfur. Have you tried reacting those solutions with salts that give poorly soluble compounds, like calcium arseniate, magnesium-ammonium arseniate, lead arseniate (though I suppose you'd get a lot of black PbS in this case)? I wonder what happens if you try to carry out the preparation for Schweinfurt's green with an alcaline solution of As2S3 instead of an alcaline solution of As2O3; I wonder if CuS will crash out by itself or along As.

All this stems from bib research from old books, haven't tried anything yet.

woelen - 27-12-2020 at 04:19

if you want As without sulfur, then it is easier to use realgar. One third of the arsenic in that compound can be precipitates as elemental arsenic. The powder you get from this is nearly black. I now have a tiny amount of elemental powdered arsenic (it's a dull nearly black powder). The purity of the sample probably is not that high. The surface of silvery metallic looking arsenic quickly tarnishes in normal water-containing air (it becomes brown and then it becomes dark grey). Given the fact that I have a fine powder, it must have been oxidized quite a lot.

I can imagine precipitating some arsenic from orpiment as well, by first oxidizing it with bleach (all he way to sulfate and arsenate) and then the arsenate must be reduced again. Sulfate is quite inert and is not easily reduced. In the past I have done experiments with that, but it is not easy to reduce arsenate to elemental arsenic in a pure way. With tin(II) chloride, arsenate can be reduced and a brown precipitate is formed, but I have read that this arsenic is very impure and a lot of tin is coprecipitated with it. It should be possible to make purer arsenic with borohydride, but this introduces the risk of formation of some AsH3 as well and I don't want to do that experiment. AsH3 is not only highly acutely toxic, but it also is a strong carcinogen, not something I want to have around me in some experiments.

valeg96 - 27-12-2020 at 06:35

The neutralized solid mixture of Na3AsO4, Na2SO4 and NaCl you probably get by adding NaOH to the orpiment/acid/bleach solution (after filtration and evaporation) could be a decent starting material for the preparation of solid and non-volatile organoAs compounds by the Bart reaction.

Which is, basically, an azo coupling with an As(III) or As(V) salt to produce the As equivalent of a phosphonic acid. I somewhat want to try that, there seem to be a lot of procedures around. I believe the nitrite takes care of the reduction to As(III) and the products are plain white solids that can be easily extracted with ethanol and crystallized.

[Edited on 27-12-2020 by valeg96]

valeg96 - 23-3-2021 at 00:48

On another note, Woelen, where did you find the reactions from your article? I found some books that mention the AsOS salts formed by dissolving orpiment in alkali, but the stoichiometry is different than yours. They claim the main species produced are AsS4 (3-), As2S7 (2-), AsS3 (-), AsO3S (3-), AsO2S2 (3-) and AsOS3 (3-), and they can supposedly be fractioned by selective precipitation with earth alkali metals. I can't find any reference to the AsOS2 (3-) and AsO2S (3-) species you mention.