652 poisons: their effects and detection. [§§ 830 , 831 . 
nitrate, 13-14 c.c. nitric acid (1-4 specific gravity), and the solution 
made up to 100 with water. The temperature should be from 60°-70°,. 
the current from 3-7-4-0 volts and 11-13 amperes, and it is best to 
use a rotating cathode ; 1 under these conditions the lead is thrown 
down mainly as dioxide within twenty minutes. The film is washed 
with water without interrupting the current, and then with alcohol and 
ether. The pole with deposit should be dried for half an hour at a 
temperature of 200°-230°, and then weighed. It is usual to multiply 
the weight found by 0-8643 ; but according to Hollard {Bull. Soc. 
Chi?n., 1904), if a platinum anode roughened by the sand-blast be used, 
the ratio of Pb : PbO takes a constant value represented by the factor 
0-855. 
After weighing the lead dioxide, the identity of the substance may 
be confirmed by the tetra-methyl-phenyl test. 
§ 830. Estimation of Lead. —By far the greater number of estima¬ 
tions of lead are made by weighing as lead sulphate, first precipitating as 
sulphide ; careful oxidation with nitric acid of the latter converts it 
rapidly into sulphate. Recently, ammonium persulphate has been 
recommended 2 as a precipitant with a view to estimation. Ammonium 
persulphate added to an acid solution of a lead salt, precipitates the lead 
quantitatively. 
The best precipitant is a 2 per cent, ammonium persulphate solution 
containing a trace of silver nitrate ; this is heated to 80°, and the lead 
solution added to it drop by drop. The solution should be kept 
at 80° for three hours, then filtered and washed with a 3 per cent, 
solution of ammonium sulphate ; finally, the precipitate, consisting of 
lead oxide, lead peroxide, and lead sulphate, is converted into lead 
sulphate by igniting with a drop of sulphuric acid. Every 100 parts of 
lead sulphate equal 73-6 PbO. 
Estimation as binoxide and the colorimetric method have already 
been described. 
Lead is also estimated as chloride, as chromate, and as sulphide, but 
the processes detailed are sufficient for the toxicologist. 
♦ 
2. COPPER. 
§ 831. Copper, Cu = 63-5 ; specific gravity, from 8-921 to 8-952 ; 
fusing-point, 1091° (1996° F.). Copper in analysis occurs either as a 
film or coating on such metals as platinum, iron, etc., or in a state of 
fine division, or, finally, as a bead. In thin films, copper has a yellowish 
or a yellowisli-red colour ; it dissolves readily in nitric, slowly in 
hydrochloric acid. If air be excluded, hydrochloric acid fails to dissolve 
copper, and the same remark applies to ammonia ; but, if there be free 
1 Ralph O. Smith, Journ. Amer. Chem. Soc., 1905. 
2 Max Dittrich and A. Reise, Ber. d. deutsch. chem. Ges., 1905. 
