Catharine Hawks on Arsenic Testing

Cathy Hawks generously gave me permission to post the version of the arsenic test she uses.  

ARSENIC TESTING

 

An improvement on the Gutzeit test for arsenic is a modification of the Merckoquant (EM Quant) Arsenic Test kit, which was formerly supplied by EM Science. The kit was designed to detect the presence of arsenic (3+ or 5+ valance states) in water, soil extracts, pharmaceuticals, prepared biological materials, and liquid foods. Unfortunately, the kit is no longer available. 

Kits can be created using chemicals commonly found in labs (1M or 1N KOH [potassium hydroxide] solution, zinc dust, small quantity of concentrated HCl [hydrochloric acid]) and by purchasing test strips impregnated with mercury bromide to detect the arsine gas evolved in the process. 

The test strips are available from:

http://www.sigmaaldrich.com/catalog/search/ProductDetail/FLUKA/83544 or http://www.ctlscientific.com/cgi/display.cgi?item_num=90762 and probably from other suppliers. The procedure given below could then be used for testing. 

Always wear a lab coat, a rubber lab apron, safety goggles, and nitrile gloves while doing the tests. Conduct the tests in a chemical vapor hood or in a well-ventilated area. Never attempt to test more than 10 samples at a time. 

Take samples using cotton-tipped swabs dampened very lightly with deionized water. Roll one side of the swab gently on the surface of specimens/objects, over as much area as possible. When dealing with taxidermy mounts and animal skins, concentrate on the areas around the eyes, nose, mouth, ears, ventral suture (if present), base of tail, and bottom of feet. Collect samples from each object/specimen on at least 3 swabs. 

Always conduct the tests using a negative control—a fresh cotton swab tip, dampened with deionized water. As long as there is no discoloration of the test strip over this swab, the results for the actual samples should be valid. 

The test:

  • Use disposable styrene vials with snap caps as test vials (5 or 7 dram vials are available from Lab Safety Supply at www.lss.com). The snap caps are slit to accommodate the test strips, so that the test area of the strips extends into the headspace of the vials.
  • Cut the slits and insert the test strips in the caps, but do not yet place the caps on the vials
    • Cut the tips of the swabs and place the tips in individual vials.
    • Add a few drops of the 1M or 1N potassium hydroxide (KOH) solution to each vial, including the negative control. Premixed solutions are available from Carolina Biological Supply at www.carolina.com. The KOH is used to help dissolve the material that has been picked up on the swabs.
    • Add a small amount of zinc dust to each vial, including the negative control.
    • Add a 2-3 drops of the concentrated HCl to a vial and then immediately cap the vial with the test strip extending into the vial (strip should not be in direct contact with the swab).
    • Repeat the addition of HCl for the remaining vials, including the negative control. The contents of the vials should appear to be effervescing and producing a gas when the HCL is added to the zinc dust.

 

The test is designed to detect tri- or pentavalent arsenic by converting the arsenic to arsine gas. This is accomplished by adding zinc dust to the samples dissolved in KOH, then adding hydrochloric acid. The reaction between the metal dust and the acid generates hydrogen gas that combines with arsenic in the samples to produce arsenic trihydride (arsine, AsH3), a very toxic, colorless gas with an odor of garlic (consequently, it is important to do the test with closed caps and to do it in a well ventilated area). 

The arsine gas reacts with the treated potion of the test strip to produce a color change that is indicative of the presence of arsenic in the sample. The treated portion of the strip contains mercury bromide, which reacts with the arsine gas to form a colored compound. The test should be considered to qualitative (i.e., a positive or negative test). Used in the way described, it is not reliable for quantitative measurements.  

While the strips purportedly give a quantitative indication, the concentration is largely irrelevant for most museum objects because arsenic salts were never applied evenly, and the concentration in the sample is an artifact of the sampling procedure, rather than a reflection of the overall concentration of arsenic on the objects. Consequently, the test results should be regarded as simply positive or negative for arsenic. If test results are equivocal (it is uncertain whether there has been a color change on the test strips for at least 2 of the 3 test samples), repeat the tests with fresh samples. 

When the tests are completed, push the test strips down into the vials, collect the used vials in a sealed, heavy-weight, ziptop, plastic bag. The bag can be disposed of in ordinary trash because the amount of arsenic involved in 10 or less samples does not constitute hazardous waste (at least, based on testing to date), and because the potassium hydroxide helps to neutralize the acid.

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