Apr. 7, X923 
Action of Soap upon Lead Arsenates 
89 
between soap and the lead hydroxy arsenate; hence treating that com¬ 
pound with soap should not bring arsenic into solution. 
McDonnell and Smith {2) state that triplumbic arsenate is formed under 
certain conditions, but that it has a very limited range of existence and is 
readily changed over to the more basic lead arsenate (lead hydroxy arse¬ 
nate) by treatment with ammonia. By that change 10 per cent of its 
arsenic should go into solution; hence that should be the maximum pos¬ 
sible to obtain by treatment with soap if the end product is in fact lead 
hydroxy arsenate. 
On the other hand, if the reaction between soap and lead arsenate is 
of the simple double decomposition type, it might be possible for diplum- 
bic arsenate to yield more than the 40 per cent above quoted, 
and it might be that the lead hydroxy arsenate would yield soluble 
arsenic. Furthermore, whichever the type of the reaction, i^ts complete¬ 
ness might be different with soaps of different fatty acids. With these 
points in view, the writer^s materials were selected as follows: 
THE SOAPS 
The soaps were prepared in the laboratory from c, p. sodium hydroxid 
and the best grade of the free fatty acids, stearic and oleic. Palmitic was 
omitted, as no satisfactory palmitic acid was at hand. After the soaps 
had been prepared they were analyzed for free alkali and for moisture. 
The stearic soap contained 83.1 per cent moisture and 0.85 per cent free 
alkali, computed as sodium carbonate. It was weighed in this form for 
the first series of trials but is recorded on the dry basis. For the second 
series this soap was dried to 4.3 per cent moisture and weighed in that 
form. 
The oleic soap contained 83.5 per cent moisture and no free alkali. 
Though it was not found practicable to dry the oleic soap, on the tables it 
is reported on the dry basis. . 
THE LEAD ARSENATES 
The lead arsenates were selected from a large number of samples that 
had been finmished by manufacturers for comparison and are desig¬ 
nated in this paper as ‘‘M’' and “T"'. “M'" was labeled by the manu¬ 
facturer “Monoplumbic Lead Arsenate'" and contained at the time of 
the first series of tests 41.9 per cent water. When dry, it contained 
arsenic equivalent to 31.53 per cent AS2O5, and lead to 65.4 per cent 
PbO. When received, it was analyzed for water soluble arsenic and 
found to contain, on the dry basis, 0.85 per cent AS2O5. This was 
taken to indicate that this arsenical was a practically pure diplumbic 
hydrogen arsenate. A later test on the dried material with 4 per cent 
ammonium hydroxid, in the manner described by Tartar and Robinson 
(4, 7), dissolved 8.85 per cent AS2O5, indicating ^at only about 67 per 
cent of the material was really diplumbic arsenate (PbHAsOJ and that 
30 per cent of the arsenic present was in some other form. 
“ M " was used in paste form for the first series and in dry form for the 
second. 
“T" was. labeled “Triplumbic Ortho Lead Arsenate" and contained 
46 per cent water, and in the dry material arsenic to the amount of 
25*75 per cent AsjOg and lead to 72.10 per cent PbO. The water- 
soluble arsenic when received was equivalent to 0.43 per cent AS2O4, on 
the dry basis. This was supposed to indicate that the paste was as 
