ARSENIC. 
§§ > 6 9> 77°-] 
595 
The copper is washed with alcohol and then with water, and, lastly, with 
absolute alcohol, and dried at a very gentle heat. It is then dropped 
into a small tube and gently heated to a temperature just below a red 
heat. If arsenic is present the metal sublimes in the characteristic 
crystals of arsenious acid. 
Dr John Clark (Journ . Chem. Soc., 1893) has proposed dissolving off 
the black film by potash and hydrogen peroxide ; the solution is boiled 
and any copper hydrate filtered off. Should arsenic be present, it now 
exists as potassic arsenate ; if antimony be present, it exists as potassic 
antimonate. If both arsenic and antimony are present, the potassic salts 
of both are formed. The arsenate may be decomposed and reduced by 
ferrous chloride and strong hydrochloric acid and distilled into water, 
the arsenic being recognised in the distillate by sulphuretted hydrogen. 
Any antimony left in the flask is precipitated by SH 2 ; should a dark 
black precipitate form, this means contamination by copper ; on treat¬ 
ment with caustic soda and boiling, copper sulphide may be filtered off 
and pure antimony sulphide obtained by acidifying the solution and 
again treating with SH 2 . 
§ 769. Arsenic in Glycerin. —Arsenic has been frequently found in 
commercial glycerin, the quantity varying from 0-1 to 1 mgrm. in 100 c.c. 
The B.P. method for detection and estimation of arsenic in glycerin is as 
follows :—2*5 grms. of glycerin are diluted in 50 c.c. of hot water, 10 c.c. 
of stannated hydrochloric acid added, and submitted to the Gutzeit 
test (p. 589). Permissible limit of impurity, 4 parts per million. 
§ 770. Detection of Arsenic in Organic Matters and in Organo- 
Arsenical Compounds. —Preparation of the substance previous to apply¬ 
ing tests for the detection of arsenic varies much according to whether the 
substance tested is simply mixed with organic matter or is an organo- 
arsenic compound, and therefore the arsenic in a non-ionic condition, 
not responding to the usual tests until the molecule is broken up by 
oxidation, or whether the substance is more or less organic. Mineral 
substances generally are easily dealt with by grinding up to a minute 
stafce of division and employing suitable solvents and reducing agents. 
The organo-arsenical compounds require quite special treatment, and 
such matters as the tissues and the organs of the body must be so 
oxidised as to be practically free from carbon; otherwise success is uncer¬ 
tain, unless considerable quantities of arsenic are present. 
Basic Methods. —Thorpe [Journ. Chem. Soc., T., 1906) devised in 1906 
what has been called the basic method of destroying organic matters, for 
the determination of arsenic. He applied it to the determination of 
arsenic inwall-paper. It is a method of general utility,but not applicable 
to all organic arsenical compounds. The weighed sample of, say, 2 grms. 
is moistened in a platinum dish with hot water ; next, 20 c.c. of arsenic- 
free lime water and 0-5 grm. of calcined pure magnesia are added. The 
