ARSENIC. 355 



compounds themselves are used, only very dilute solutions should be 

 tested, in order to appreciate the delicacy of the tests. The arsenic 

 should be in the form of an arsenows compound for the above test, as 

 in this condition it is more readily reduced to arsine. This is insured 

 by adding to 5 c.c. of a 10 per cent, aqueous solution of the chemical 

 to be tested (in some instances special previous treatment is neces- 

 sary, which may be seen in the U. S. P.) 1 c.c. of a mixture of equal 

 volumes of concentrated sulphuric acid and water, and 10 c.c. of 

 fresh saturated solution of sulphur dioxide. The liquid is evaporated 

 over boiling water until it is free from sulphur dioxide and has been 

 reduced to 5 c.c. in volume. It is then introduced into a 75 c.c. flask 

 containing 2 or 3 grammes of granular zinc and 20 c.c. of 8 per cent, 

 hydrochloric acid, a small wad of clean dry gauze is inserted into the 

 lower end of the neck of the flask, followed by a wad moistened with 

 lead acetate solution. The mouth of the flask is then covered by 

 folding over it a filter-paper, the center of which has previously been 

 three times successively wet with a saturated alcoholic solution of 

 mercuric chloride and dried. After one-half to one hour, the paper cap 

 is examined for a yellow stain, which indicates arsenic. The presence 

 of arsenic much in excess of the permissible limit of the U. S. P. 

 (1 in 100,000) is shown by a distinct yellow to orange spot. All the 

 reagents used must be free from arsenic, which is determined by 

 making a blank test, omitting the chemical to be tested. The stu- 

 dent should carry out the test on 2 or 3 c.c. of a -\-$ per cent, solu- 

 tion of arsenic trioxide, which need not be submitted to the reduc- 

 tion with sulphur dioxide. Antimony gives a dark coloration. 



11. Fleitmann's test. This is similar to the Gutzeit's test, the 

 chief difference being that hydrogen is evolved in alkaline solution, 

 which has the advantage that the presence of antimony does not 

 interfere, because this metal does not form antimonetted hydrogen in 

 alkaline solutions. 



Place about 1 gramme of pure zinc in a test-tube, add about 5 c.c. 

 of potassium hydroxide solution and a few drops of the arsenic solu- 

 tion, which should not be acid. Provide paper cap as described in 

 Gutzeit's test, and set the test-tube in a box containing sand heated 

 to about 90 C. (194 F.). A brown or black stain of metallic silver 

 will appear upon the paper. 



12. Marsh's test. While this test is not used now for qualitative 

 determinations as much as formerly, it is of great value because it 

 may serve for collecting the total amount of arsenic present in a 

 specimen, thus permitting quantitative estimation. The apparatus 



