You could try the Marsh test, named for a murderer who used arsenic to do away with some people, and would not have been caught, but forsome clever chemistry which got her hanged in the end.

Digest the sample with nitric acid, to form arsenious acid, then CAREFULLY treat with zinc, this will liberate the extremely toxic gas arsine, hydrogen arsenide, analogous to phosphine, this is ignited as it passes through a tube from a closed vessel, and sublimed against a cold dish as arsenic in the elemental metallic state (as the black allotrope), or better, you could perhaps do even better with an improvised cold-finger apparratus.

The marsh test can detect as small a concentration as one 50th of a milligram of arsenic, so it is quite sensitive enough, I would think, for your purposes.

Good luck.

Savoy7, I think it could be done with reasonable safety, the soil samples treated could be very small, given the sensitivity of the test, and the arsine gas evolved is burned as soon as it exits the tube of the apparattus to allow the elemental arsenic to sublime upon the surface of the reciever.

I don't particularly care for either PH3 or its arsenical counterpart, but I would feel just a tiny bit safer with arsine, quite crudely put, from the perspective of one who does all his experiments in his garage arsenic is more of a slow insidious poison over multiple cumulative small exposures, and therefore SLIGHTLY more forgiving, where as phosphorus toxicity likes to grab you by the balls and let you know its just kicked your rear end

I know that for a fact, having experienced tempory near-paralysis of my arm, following exposure of it to a tiny amount of white phosphorus, never experienced arsenic poisoning though thank the Goddess, I am bloody well careful with arsenic and its compounds, I guess, perhaps, that familiarity breeds contempt, or more accurately, occasional lack of care.

At most I was exposed to a few mg of white phosphorus, from the edge of a boiling tube used for distillation of the phosphorus, a tiny blob got on my arm, and was scraped off without burning myself severely, (the WP, not my arm ) and a small amount got under my fingernail, and smoked for a second or so, before I dipped the finger in CuSO4 solution prepared beforehand for just such an accident.

I noticed in two days or so, that my arm that recieved the exposure became very weak and experienced tremors in my hand, making fine motor skills such as writing or typing almost impossible for nearly three weeks.


All I am trying to say, is that both the white allotrope of phosphorus, and arsenic compounds, deserve extreme respect, but you do have a slightly better margin of safety with the equivalent toxic amount of arsenic than you do with phosphorus, as arsenic, in tiny quantities is a slow and cumulative poison, so you have the chance to realise that you have been exposed, and take action to avoid further exposure (and the fact that a treatment, British Anti Lewisite, coupled with chelation therapy to detoxify and aid excretion of the arsenic is available)

Perhaps a setup could be improvised to burn the gas in the same exit vent as it comes from, in a wider chamber, supplied with oxygen to allow combustion just before sublimation of the arsenic.




I am going to be slightly impolite here, and hijack the thread just a little, but I have an online source for an arsenic sample, but it costs roughly £30 for a few hundred milligrams in a sealed ampoule, so I am thinking of burning a pressure-treated log that has been treated with a common arsenical fungicide.

A 10 foot 2x4 that has been so treated actually contains upwards of 20 grams of elemental arsenic, so I intend to aqquire one or two such planks, burn them (carefully, as ingestion of only 5 or 6 grams of the ash would prove fatal due to arsenic content), then perform the same reaction of the Marsh test, only on a greater scale, acidify, treat with zinc, then burn the arsine gas to condense crystalline elemental arsenic of good purity in a glass tube surrounded with a jacket full of ice/salt/dichloromethane to act as a cold finger.

Just a question on this reaction though, I apologise, as I stated, as I know now to be in error, that the gray allotrope of arsenic would be deposited on the reciever, not the black form, as I erroneously stated in my previous post.

How would I go about converting the gray allotrope, to both the black form, and the slightly unstable yellow arsenic allotrope, as I wish to have a good sealed sample of each allotrope for an element collection?

and also, can other oxidizing acids be used, other than HNO3, as I wish to waste no precious nitric acid, perhaps, concentrated H2SO4, or an oxidizing halogen acid such as hypochlorous acid or another halogen oxyacid to liberate the arsine?

For Arsenic determination, please search: ASTM D2972 - method A- Silver diethyldithiocarbamate colorimetric. This method need  Arsine generator and the method detection range from 5 to 250 ug/L.

Just a question on the Marsh test.

Does the arsine actually have to be ignited with a flame? or does it disasociate on being passed through a glass tube heated to red heat?

I have read accounts of the test that say both are true.

I would far prefer just to be able to pass the arsine through a red-hot pyrex glass tube, and distill off the elemental arsenic in my scale-up of the process, than have to burn it, and collect the fumes, just for safety reasons.

So can arsine be disassociated by heat alone, or is actual combustion nescessary?

The method for determination of As by the use of Atomic Absorption Spectrometer (AAS), the generated arsine is purged with nitrogen / argon gas to the open ended quratz tube to the burner of AAS and burn in the AA flame. That is to say the arsine is oxidized to As atom when burn in air.

Bellow is the arsine generator