Special tests for small amounts of arsenic

These tests are applicable to all arsenic compounds, and play an important role in forensic analysis. 

Marsh s test 1. Marsh s test this test, which must be carried out in the fume cupboard, is 

On igniting the mixed gases, composed of hydrogen and arsine (after all the air has been expelled from the apparatus), they burn with a dark blue flame and white fumes of arsenic(III) oxide are evolved if the inside of a small porcelain dish is pressed down upon the flame, a black deposit of arsenic is obtained on the cool surface, and the deposit is readily soluble in sodium hypochlorite or bleaching powder solution (distinction from antimony). 

The following reactions take place during these operations  

The solution containing the arsenic compound is then added in small amounts at a time to the contents of the flask. If much arsenic is present, there will be an almost immediate blackening of the silver nitrate solution. 

The solution containing the arsenic compound is then added in small amounts at a time to the contents of the flask. If much arsenic is present, there will be an almost immediate blackening of the silver nitrate solution. Disconnect the rubber tube at C. Heat the tube at A to just below the softening point a mirror of arsenic is deposited in the cooler, less constricted portion of the tube. A second flame may be applied at B to ensure complete decomposition (arsine is extremely poisonous). When a satisfactory mirror has been obtained, remove the flames at A and B and apply a light at C. Hold a cold porcelain dish in the flame, and test the solubility of the black or brownish deposit in sodium hypochlorite solution. 

Smaller amounts of arsenic may be present in the silver nitrate solution as arsenious acid and can be detected by the usual tests, e.g. by hydrogen sulphide after removing the excess of silver nitrate with dilute hydrochloric acid, or by neutralizing and adding further silver nitrate solution, if necessary. 

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