Marsh’s test for

Marsh s test for arsenic The arsenic-containing specimen is converted to volatile AsHs which is decomposed to a brown stain on heating. Estimation is by comparison of stains. Sb reacts similarly but the Sb stain is not soluble in NaOCl. 

Arsine, AsHs, is formed when many As-containing compounds are reduced with nascent hydrogen and its decomposition on a heated glass surface to form a metallic mirror formed the basis of Marsh s test for the element. The low-temperature reduction of AsCls with LiAlH4 in diethyl ether solution gives good yields of the gas as does the dilute acid hydrolysis of many arsenides of electropositive elements (Na, Mg, Zn, etc.). Similar reactions yield stibine, e.g. ... 

Small quantities of selenium in solution may be detected by the test described by Meunier,1 which is similar to Marsh s test for arsenic. If there is a relatively large amount of selenium present it is readily detected by its red colour, but if the amount of the element be small the deposit closely resembles that of arsenic, and in this case the following process may be adopted to detect the selenium. A current of hydrogen sulphide is passed into the hot solution containing a little sulphurous acid. The precipitate of finely divided sulphur carries down with it any selenium present and arsenic as arsenious sulphide, and it is clotted by stirring. In the presence of selenium the clots are brown they may be dried and the sulphur and selenium separated by careful sublimation in a closed tube. 

The bond lengths of the X—H bonds in NH3, PH3, and SbH3 are 1.02, 1.42, and 1.71 A, respectively. Estimate the length of the As—H bond in ASH3, the gaseous compound that decomposes on a heated glass surface in Marsh s test for arsenic. Which of these four hydrides has the weakest X-H bond.>... 

Arsenic present only in traces (in any form) can be detected by reducing it to arsine and then applying tests for the latter. In Marsh s test, dilute sulphuric acid is added dropwise through a thistle funnel to some arsenic-free zinc in a flask hydrogen is evolved and led out of the flask by a horizontal delivery tube. The arsenic-containing compound is then added to the zinc-acid solution, and the delivery tube heated in the middle. If arsenic is present, it is reduced to arsine by the zinc-acid reaction, for example ... 

Marsh s method for testing for arsenic is based on the fact that, under the action of nascent hydrogen, all arsenical compounds are transformed into hydrogen arsenide, which is decomposed with deposition of arsenic when heated. 

If made from arsenical phosphorus, and commercial phosphorus is usually arsenical, it is contaminated with arsenic acid, whose presence may be recognized by Marsh s test (q. v.). The acid should not respond to the indigo and ferrous sulfate tests for HNO,. 

Marsh s test this test is carried out exactly as described for arsenic. The stibine, SbH3 (mixed with hydrogen), which is evolved, burns with a faintly bluish-green flame and produces a dull black spot upon a cold porcelain dish held in the flame this deposit is insoluble in sodium hypochlorite or bleaching powder solution, but is dissolved by a solution of tartaric acid (difference from arsenic). 

Apparatus for Marsh s test. Sulfuric acid is added to zinc metal and a solution containing arsenicflll) oxide. 

Marsh et al. and Marsh and Evans utilized the DPC morphine-specific RIA to study the effectiveness of wash procedures and self-reporting of drug use. The assay was found to be highly specific, accurate, and precise. Evaluation of hair from drug-free controls produced results <0.3 ng/mg. In the study, the results of segmental hair tests for morphine for a period of 15 months were used to challenge a subject s declaration of heroin abstinence. In similar studies. Marsh and Evans evaluated the effectiveness of the DPC methadone RIA. The assay was also found to be highly specific, accurate, and precise. 

Arsine, ASH3, is much less stable than phosphine. For example, it readily decomposes upon heating to form metallic arsenic, which can be deposited as a mirror on hot surfaces. This is the basis of the once common criminological Marsh test for the presence of arsenic compounds. In practice, the contents of the victim s stomach are mixed with hydrochloric acid and zinc, producing hydrogen gas. The hydrogen in turn reacts with a variety of arsenic compounds to produce arsine that is thermally decomposed to the arsenic mirror. These processes are represented in... 

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