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Topic: Testing for lead & arsenic  (Read 12162 times)

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savoy7

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Testing for lead & arsenic
« on: February 06, 2005, 02:25:26 PM »
I've been searching around for ways to test for lead in soil and arsenic in groundwater.  I would like it to be quantitative.  I would also like it to be spectrophometric (don't have my hands on mass spec, etc).  I do have specs in the visible light region and colorimeters.  It could also be a titration.  We have been looking at doing labs where the community could drop off some samples and students could do the tests.  Arsenic is a huge problem in this area of the country.

Any thoughts?  

savoy out

Offline limpet chicken

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Re:Testing for lead & arsenic
« Reply #1 on: February 06, 2005, 06:39:50 PM »
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.
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savoy7

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Re:Testing for lead & arsenic
« Reply #2 on: February 06, 2005, 09:31:25 PM »
Thanks L Chicken,

I've looked at that technique - I'm scared of the gas.  We are dealing with students here.

The federal stds for arsenic in drinking water went down from 50 ppb to 10 ppb.  So, those are the units I'm looking for.

thanks again,
savoy

Offline limpet chicken

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Re:Testing for lead & arsenic
« Reply #3 on: February 06, 2005, 11:27:06 PM »
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 :o

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 ;D) 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?
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Re:Testing for lead & arsenic
« Reply #4 on: February 07, 2005, 07:42:59 AM »
We performed the Marsh Test in my college Forensic Toxicology Lab on spiked blood samples.  It is IMPRESSIVELY sensitive, and the positives easily showed up as a black smear on the copper condensor we were using.  The entire test is really pretty tame, and providing that these are college level, or higher-level high-school students there shouldn't be a problem.  You'll just need to impress upon them the respect that arsenic deserves and remind them to do EVERYTHING in a fume hood.  In reality, the nitric acid is more of a danger than anything else here.  With the arsine, your nose is an incredible instrument.  Arsine has a very easily detectable garlic-odor that is completely unmistakeable.  If there is even the tiniest amount of arsine gas in the air, somebody should be able to sense it.  

For the quantitative aspect, you should weight the condensation "thing" that you're using before the lab to get a tare weight.  Then collect the As on it and allow it to completely dry in a dessicator so that all the water is removed.  Then weigh it again and you'll have the As weight.
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Offline kevins

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Re:Testing for lead & arsenic
« Reply #5 on: February 08, 2005, 12:00:03 PM »
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.


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Re:Testing for lead & arsenic
« Reply #6 on: February 08, 2005, 07:30:54 PM »
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?
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Offline kevins

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Re:Testing for lead & arsenic
« Reply #7 on: February 08, 2005, 08:42:22 PM »
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
« Last Edit: February 08, 2005, 08:49:09 PM by kevins »

Offline kevins

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Re:Testing for lead & arsenic
« Reply #8 on: February 10, 2005, 05:32:32 AM »
For lead determaination:
1. Principle: lead is extracted form alkaline solution by dithizone in chloroform solution. The excess dithizone is removed by treatment with sodium cyanide and the intensity of the red colour of the lead dithizonate is measured at 515nm. Hydroxylamine hydrochloride is added to prevent oxidation of the dithizone by any iron (III) present.

2. Reagents
2a. conc ammonia solution.
2b. Citric acid: 5gm in 100mL water.
2c. Dithizone solution: 4mg dithizone in 100mL chloroform.(Freshly prepare)
2d. Sodium cyanide: 10g in 100mL water.
2e. Chloroform.
2f. Bromothymol blue: 0.04% indicator solution.
2g. Hydroxlamine hydrochloride: 20gm in 100mL water.

3. Procedure

3a Measure 25mL sample to a measuring cylinder.
3b. Add 2mL citric acid solution, 0.1 mL bromothymol blue indicator, and add conc ammonia solution drop by drop until the mixture is just blue.
3c. Transfer the solution to a 50-mL separating funnel which containing 1 mL of dithizone solution, 1mL of sodium cyanide solution (CARE!!!!), and 1mL of hydroxylamine hydrochloride solution.
3d. Shake, until the red colour in the chloroform reaches a maximum.
3e. The chloroform layer should then be run off and the extraction repeated with further 1 mL of reagent 2c.
3f. Combine the chloroform extracts to other clean separating funnel.
3g. Add 10 mL of water, 0.1 mL Sodium cyanide solution and 0.1 mL conc ammonia solution. Shake.
3h. Transfer the chloroform layer to a graduated vessel and make up to 5 mL of chloroform.
3i. measure the abs at 515nm.
3j. construct the calibration curve.

Please try!

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