592 POISONS : THEIR EFFECTS AND DETECTION. [§ 768 . 
To 1500 c.c. of commercial hydrochloric acid slightly above IT sp. gr. 
about 40 c.c. of redistilled commercial wood spirit are added. Th6 
mixture is contained in a Wurtz flask of two litres capacity. About 5 
to 10 grms. of arsenic-free granulated zinc are then added. The flask 
is connected with a reflux condenser by an ordinary cork, in which is 
fixed a glass rod supporting a coil of electrolytic copper foil, having 
a surface of about 120 square inches. The side tube of the Wurtz 
flask having been plugged, the condenser is connected with an ex¬ 
haust-pump, and the boiling commenced. The acid is digested for 
about three hours, the copper being withdrawn and cleaned at least 
once during that period. During the digestion a black, tarry, fuming 
liquid distils over, and the greater part of this is caught in a vessel 
between the condenser and the pump. Another vessel containing water 
is interposed to catch any hydrogen chloride which passes off. 
Thorne and Jeffers’ Method A—Redistilled hydrochloric acid is 
diluted to a sp. gr. of a little under IT, and poured on to 2 to 3 grms. 
of a copper-tin couple prepared as follows :— 
Cuprous chloride is dissolved in excess of HC1, and a little granu¬ 
lated tin added ; when the tin is dissolved, zinc dust is added, and the 
copper and tin, which are precipitated as a grey spongy mass, washed 
by decantation. The acid and couple are gradually heated, and boiled 
gently for half an hour. The acid is then at once distilled from a flask 
containing a small quantity of the couple and a little piece of 100-mesh 
copper gauze. The distilled acid is free from arsenic. 
Sulphuric Acid. —This acid may now be obtained free from arsenic, 
but if it is found to be impure it may be freed from arsenic by diluting 
with four volumes of water, adding a little sodium chloride, and distilling. 
The first y^-th of the distillate contains all the arsenic, and is rejected. 
The precautions to be observed in Marsh’s or Marsh-Berzelius process 
are :— 
1. Absolute freedom of the reagents used from arsenic, antimony, 
and other impurities. 
2. The sulphuric acid or hydrochloric acid should be diluted with 
from four to five times its weight of water, and if freshly prepared 
should be cooled before use. Strong acid must not be employed. 
3. The fluid to be tested should be poured in little by little. 
4. Nitrous compounds, nitric acid, chlorides, are all more or less 
prejudicial. 
5. The gas should come off regularly in not too strong a stream, 
nor out of too small an opening. 
6. The gas should pass through the red-hot tube at least half an 
hour before adding the substance to be tested ; if there is then no 
1 L. T. Thorne and E. H. Jeffers, “ The Purification of Zinc and Hydrochloric 
Acid from Arsenic,” Analyst, April 1906, p. 102. 
