Sciencemadness Discussion Board - unknown compound, arsenic trioxide?

unknown compound, arsenic trioxide?

Jor - 1-7-2009 at 09:34

My local supplier had an old bottle containing a bag with a red powder, labelled as arsenic trioxide.
It's bright red. I immediately this could not be arsenic trioxide, unless it was used as a pesticide and is dyed red for safety reasons, however arsenic trioxide is a 'Severely Restricted' pesticide, so this seems to be not an an option, although it is still sometimes used.

The powder does not dissolve in water, dilute NaOH or moderately concentrated HCl, even when heating (yes I know small amounts of AsCl3 could volatise). However, I did this heating only for one minute, so maybe it takes a long time to dissolve like As2O5. But it is probably not As2O3 I think.

Another possibility would be realgar, As4S4, with As(II) in it.
Maybe in combination wih an oxidiser the acid will dissolve this. I do not have the guts to smell for H2S (AsCl3 vapour).

However as I rather not waste too much of it to experiments on what it actually is, I first would like a second opinion and suggestions for experiments.

I also need to be sure it contains arsenic. I think I will try to dissolve it (not sure yet in what, I think HCl/H2O2, hypochlorite or nitric acid) and add some zinc, followed by heating walls of the DURAN test tube to decompose the AsH3. However, I am quite frightened of this, so I will make only a very small amount, and put the hood at full speed

I have played with SbH3, but this is easy to decompose before it can even come out of the the test-tube, and it is less toxic chronically (acutely it's the same).

But before I waste even more of it, any suggestions?

Oh and woelen, I'm sorry for this, I know you really wanted the rest of the As2O3 he had, but it probably isn't As2O3

[Edited on 1-7-2009 by Jor]

woelen - 1-7-2009 at 10:07

Hmmm... this does not sound good at all. I received an email that the material is sent to me, I expect to receive it tomorrow. I was told only a small amount was left over and because I also ordered some other things I got this small amount for free. But if it is a red powder, then I'm sure it is not As2O3.

JohnWW - 1-7-2009 at 10:30

A RED powder? That sounds more like Fe2O3. Or heavily contaminated with it

DJF90 - 1-7-2009 at 11:17

Jor says its bright red. Iron (III) oxide is more brown than bright

have you tried looking in brauer under arsenic compounds for substances that are red? This would likely give you the best start, unless someone else has other ideas.

not_important - 1-7-2009 at 12:38

Realgar is certainly strongly red, heated HNO3 will dissolve it with SO2 being given off, a few drops of HNO3 and a tad of the unknown is all that is needed - moisten blue litmus will turn red then bleach out.

Many decades ago I acquired my first sample of an arsenic compound by buying an insecticide that was mostly lead arsnate plus a red colouring agent. Boiling with NaOH then adding NaHCO3 extracted the arsenic leaving behind a basic lead carbonate, I think the colour came with the liquid so it may have been an organic dye. As I was out to make elemental arsenic, I just acidified the solution, let it evaporate, mixed with wood charcoal, and did the standard 'heat in crucible with a long sheet iron conical cap' to sublime the arsenic (outdoors to be sure). I think I was 13 at the time, certainly had no fume hood, and SFAIK am still alive, and whatever the red colour it dd not interfere with the reduction.

Paddywhacker - 1-7-2009 at 13:48

Same here. Arsenate of lead came to mind with the description of a red powder.

I found it was slightly soluble, and with a little H2S gave a black precipitate of PbS, and with more H2S the filtrate gave a bright yellow arsenic sulphide. All a long time ago.

Jor - 1-7-2009 at 14:06

Thank you for the tips all!

It is not Fe2O3. This is compound is BRIGHT red, a little but like mercuric iodide.

I will definately try your methods not_important, I can't wait!
Unfortunately I cannot experiment until about 14th of july!
Woelen, maybe you can do these experiments!

I will convert the arsenic into As2O3 and As anyway, if there is any As present. I couldn't do that before as i only had a small amount of As4O10. I only need to find a way to get As2O3 from it. Gonna be a nasty synthesis though.
And ofcourse I will keep a few grams of the 40 just for showing purposes, as the color is really beautiful bright red!

Ozone - 1-7-2009 at 15:35

Realgar is a-As4S4 (when I was a kid, this was simply AsS). Also, when I was a young mineral collector, we thought that exposure to light converted it into the yellow mineral, orpiment. Apparently, that was wrong, and the light-triggered conversion yields the yellow b-As4S4. If your material is realgar, exposure to light should make it turn bright yellow.

I thought lead arsenate was a white powder?

Cheers,

O3

not_important - 1-7-2009 at 17:17

Quote:

I thought lead arsenate was a white powder?

It is, back when you could get it at a hardware store for pest control, it was coloured to make it obviously not be flour, sugar, or other common foodstuff, and I assume to make it stand out if mixed into food (except for paprika and other chili powders or tomato paste)

While am I ever saw was red stained, a friend elsewhere bought some lead arsenate that was dyed bright blue, at that time an even less likely food shade.

woelen - 2-7-2009 at 03:56

I also received the material now and indeed it is bright red, not at all like Fe2O3. Right now I am not in the position of being able to experiment, but tonight or maybe tomorrow I'll try some experiments, such as adding HNO3 to it. If it contains As2O3, then quite some NOx should be formed.

woelen - 2-7-2009 at 11:15

I now did some simple tests with the red material.

When it is added to 65% HNO3, then it does not dissolve, nor is there any change of color. I obtained a red suspension in the HNO3.

When the suspension is heated, then at first nothing happens, but when the acid is quite hot (near boiling), suddenly a reaction sets in. A small amount of gas is produced, the air above the liquid becomes pale brown. The red color changes to a deep yellow/orange color. A pale yellow precipitate is formed, in a deep golden yellow solution when the liquid is allowed to stand.

The golden yellow solution is decanted and the pale yellow solid is rinsed with water. This solid does not dissolve. When it is shaken with water, a pale yellow liquid is obtained (remains of the golden yellow acidic liquid) and the pale yellow solid settles again at the bottom.

The water was decanted, new water was added and then quite some NaOH was added. The NaOH dissolved, but the pale yellow solid did not dissolve. This shows that the material is not As2O3. According to literature, As2O3 easily dissolves in a solution of NaOH, giving arsenite anion in solution. After this, I decante the NaOH again and rinsed with water. Now I still have some of the pale yellow/brown precipitate.

I have no clue what it can be, but it definitely is not As2O3. It does not even react with conc. hot HNO3, except that the color changes from red to yellow. Apparently the red indeed is a dye, added to some other compound.

Maybe, indeed it is lead arsenite and did I leach out the arsenic with the NaOH and the pale yellow brown material might be lead oxide. But wouldn't lead arsenite dissolve completely in conc. HNO3, giving Pb(NO3)2 and arsenic acid?

[Edited on 3-7-09 by woelen]

UnintentionalChaos - 2-7-2009 at 16:00

Quote: Originally posted by woelen

Maybe, indeed it is lead arsenite and did I each out the arsenic with the NaOH and the pale yellow brown material might be lead oxide. But wouldn't lead arsenite dissolve completely in conc. HNO3, giving Pb(NO3)2 and arsenic acid?

No.

The Ksp of Pb3(AsO4)2 is 4.0 x 10^-36

pKa 1, 2, and 3 of arsenic acid are 2.19, 6.94, and 11.5 respectively.

[xy] <- this notation indicates concentration]

azeotropic nitric acid is 15.8M, which we will pretend is the same as 15.8M H3O+ (even though it isn't)

Ksp = [cation][anion]

4.0*10^-36 = [Pb+2][AsO4-3]

since the ratio of Pb+2 to AsO4-3 is fixed (no common ion effect), we can rewrite this as

4.0*10^-36=[3x][2x]

Since the presence of [AsO4-3] is pH dependent, this isn't quite correct either. We need to figure out what fraction of solvated [AsO4-3] remains unprotonated.

[AsO4-3]=(K1K2K3)/([H]^3+[H]^2K1+[H]K1K2+K1K2K3)

[AsO4-3]=((10^-2.19)(10^-6.94)(10^-11.5))/([15.8]^3+[15.8]^2(10^-2.19)+[15.8](10^-2.19)(10^-6.94)+(10^-2.19)(10^-6.94)(10^-11.5))

[AsO4-3]=5.94089*10^-25*[2x]

4.0*10^-36=[3x][2x](5.94089*10^-25)

x=1.12217*10^-12

so, molar mass of Pb3(AsO4)2=899.44g/mol

1.12217*10^-12 = molarity or resulting lead arsenate solution

solubility of lead arsenate in concentrated nitric acid = (1.12217*10^-12)*899.44 = 1.00932*10^-9 grams/liter

This is still virtually insoluble. (assuming I did that math right, which may not be the case)

[Edited on 7-3-09 by UnintentionalChaos]

Rowald - 3-7-2009 at 11:24

Is it possible that the yellow/brown precipitate, that Woelen had formed, might be As2S3 (also called orpiment)? That was the only yellow/brown arsenic compound I could find so far.
There's As2S5, that's also a yellow arsenic compound. But, according to wikipedia, this will dissolve in nitric acid.

I don't know how you can make a sulphide by reacting a compound with nitric acid, but it is an option.

DJF90 - 3-7-2009 at 13:23

I really dont think its an option... However I have noticed a mistake with UC's working. Woelen is suggesting that it might be lead arsenIte, whereas UC has done the calculations for lead arsenAte. Not sure how much of a difference it will make though,

Ben Messaoud.F - 4-7-2009 at 03:20

Why you do not affect measurements of X-ray and scanning electron microscopy for your compound I think they will give you great information about your compound.

woelen - 4-7-2009 at 10:34

X-ray and electron microscopy in a home lab? You must be kidding

UnintentionalChaos - 4-7-2009 at 11:12

Quote: Originally posted by DJF90

I really dont think its an option... However I have noticed a mistake with UC's working. Woelen is suggesting that it might be lead arsenIte, whereas UC has done the calculations for lead arsenAte. Not sure how much of a difference it will make though,

Well, arsenite, even if somewhat soluble in nitric acid, would be oxidized to arsenate with heating. Reread the post by woelen before mine as to why

I suspect that the pale yellow precipitate is lead arsenate with the yellow compound (oxidized dye?) trapped in the structure.

It's Ksp is on par with lead sulfide, so there is virtually no hope for metathesis to free the arsenate. On the other hand, it's a very safe form of both lead and arsenic due to it's ridiculous insolubility and inertness.

Jor - 4-8-2009 at 08:31

Ok, I am back to experiment again.

I just did one experiment so far.

When the red powder in a test tube is heated with a propane burner immediately a red compound sublimes and settles on the test tube walls, and some brown vapour is vormed (probably the same compound in the gas phase) . The remaining powder turns light grey finally, first going through brown/grey (hard to see in my fume hood TL-light, and I certainly don't want to take it out of the fume hood).
I did not do any experiments on this powder yet. First I will drive of all of the red compound followed by analysing it. Any suggestions? I was thinking of not_importants method, but any other suggestions are very welcome.

I think the red compounds could indeed be a dye, and because it is so volatile it should be organic. I will see if it dissolves in a organic solvent like boiling ethanol or DCM (or maybe you suggest another solvent, I have more, also benzene, CCl4, chloroform).

After trying not importants method, I will see if any basic lead carbonate is left behind. I try to dissolve it in acetic acid and try to add some KI.

The light grey powder is very unlikely to be As2O3 (or As4O6

), as this should sublime at the temperatures I heated with the torch. Second as woelen demostrated it does not react with nitric acid.

So if we have an As-compound here, it will probably be PbHAsO4, the insecticide (it is not lead arsenite or lead arsenate, but lead hydrogen arsenate).

Jor - 4-8-2009 at 09:19

Well I did the following experiment on the light grey-white powder after driving off all of the red substance.

-boil in some 20% NaOH. This gives a white suspension, and also some brown insolble material floats in it, but there is little of it.
-I added 2 pinches of NaHCO3 and boiled again.

Next the liquid was decanted and some white precitipate remained. This was washed with some 20% NaOH.
Next an excess of 50% acetic acid was added. There was just some bubbling but this was most likely due to traces of remaining carbonate.
A white suspension was obtained. Addition of KI to this suspension gave no yellow color, and adding some KI to the decanted liquids also gave no change.

Does this mean there is no lead arsenate in there?