Marsh test for arsenic

The hydrides of arsenic, antimony, and bismuth are unstable at elevated temperature. The Marsh test for arsenic depends on this instability when an arsenic mirror forms as arsine is passed through a heated tube ... 

Marsh test for arsenic [ARSENIC AND ARSENIC ALLOYS] (Vol 3)... 

FIG. 21-5. The Marsh test for arsenic. The sample is introduced through the thistle tube. Both arsenic and antimony produce a mirror in this test the chemical properties of the deposit permit a distinction to be made between an arsenic mirror and an antimony mirror. 

Ores of arsenic orpiment, realgar, arsenolite, arsenopyrite. Compounds of arsenic arsine, arsenic trioxide, arsenious acid, cupric hydrogen arsenite, arsenic pentoxide, arsenic acid, sodium arsenate. The Marsh test for arsenic. U.ses of arsenic and its compounds lead shot, insecticides, weed killers, chemotherapy. 

The Marsh test for arsenic involves the following steps (a) The generation of hydrogen gas when sulfuric acid is added to zinc, (b) The reaction of hydrogen with As(III) oxide to produce arsine, (c) Conversion of arsine to arsenic by heating. Write equations representing these steps and identify the type of the reaction in each step. 

Here then, we offer much of the descriptive aqueous chemistry in the older sources, with corrections, interpreted with the added insights and improved symbolism of recent decades, plus new information, including reactions of the recently discovered elements, but without many of the older strictly analytical techniques. Still, we mention the Marsh test for arsenic, for example, because not all laboratories around the world have the instmments that give quicker results. 

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. 

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. 

Arsenic.—50 grams of the minced sample are weighed into a round-bottomed flask and heated over a naked flame with 10 c.c. of concentrated sulphuric acid when the mass becomes dense, 30 c.c. of the same acid are added, the heating being continued and further small quantities of acid added until the liquid is completely decolorised. When cold, the solution is poured carefully into 150 c.c. of cold water, the resulting liquid being filtered and the filtrate tested for arsenic in the Marsh apparatus see later, 5, b) and also for any other metals (zinc, nickel, etc.). 

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. 

Arsine (AsH3) is extremely poisonous. Its ready thermal decomposition to arsenic, which is deposited on hot surfaces as a mirror, is utilized in tests for arsenic, for example, the well-known Marsh test, where arsenic compounds are first reduced by zinc in HC1 solution. 

It also is formed by reduction of soluble arsenic compounds by zinc in acidic solution. This reaction is the basis of an important and sensitive test for arsenic, the Marsh test (Fig. 21-5). The arsenic is deposited as a steel-gray or black mirror from the burning gas onto a cold glazed porcelain dish held in the flame. Antimony produces a velvety brown or black deposit, which is not soluble in sodium hypochlorite solution, whereas the arsenic deposit is. The antimony deposit, but not that of... 

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.>... 

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... 

A survey of the hydrides, oxides, hydroxides, and halides highlights the network components. The hydrides of nitrogen and phosphorus emphasize the uniqueness of the lightest element. Unlike the polar ammonia, the nonpolar phosphine is a poor base and not capable of forming hydrogen bonds. Arsine is less stable than phosphine, and its decomposition is the basis of the criminological Marsh test for the presence of arsenic. 

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 test analy chem A test for the presence of arsenic in a compound the substance to be tested is mixed with granular zinc, and dilute hydrochloric acid is added to the mixture gaseous arsine forms, which decomposes to a black deposit of arsenic, when the gas is passed through a heated glass tube. Also known as Marsh-Berzelius test. marsh, test ... 

Most of the compounds of arsenic are toxic when in contact with the skin, when inhaled, or when ingested. As with arsenics cousin phosphorus above it in group 15 of the periodic table, care must be taken when using arsenic. The compound arsenic trioxide (As O ), an excellent weed-killer, is also carcinogenic. Copper acetoarsenite, known as Paris green, is used to spray cotton for boll weevils. A poisonous dose of arsenic as small as 60 miUigrams can be detected within the body by using the Marsh test. 

The concept of traces in chemistry probably originates from descriptions of the purity of chemical reagents, which in the nineteenth century included a statement such as trace for minor components when a more accurate description was not needed or not possible. One early example of determination of traces was the Marsh test, developed as early as 1836 for the determination of very small amounts of arsenic, which was of primary importance in toxicology. 

Arsenic is readily detected the Marsh test was devised in 1836 and was used for over a hundred years. More sophisticated and very sensitive methods are now available, using X-ray analysis or a technique known as mass spectrometry which detects the arsenic atoms. Even before the Marsh test a careful scientific process of experiment would sometimes be successful, as demonstrated by the doctor in the case of Mary Blandy in 1752 (see case notes p. 223). Arsenic remains in tissues of the body for a long time, hence analysis of Napoleon s hair was possible long after his death (see case notes p. 222). The sensitive analysis of hair and nails from the body of American president Zachary Taylor, exhumed in 1991, also established that he was not poisoned in 1850. 

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