90 
Journal of Agricultural Research 
Voi. XXIV, No. I 
labeled, a pure triplumbic arsenate; but since the authorities already 
noted cast considerable doubt upon the existence of this salt in com¬ 
mercial lead arsenates, it might have been a mixture. The dry matter 
when treated with 4 per cent ammonium hydroxid, )delded but 0.22 per 
cent of soluble ASjOg, indicating but 1.70 per cent PbHAs04. 
BRINGING THE MATERIALS IN CONTACT 
Preliminary work indicates that when the soap solution is mixed with 
the lead arsenate, either as paste or dry, it is difficult to get concordant 
results for the quantity of arsenic made soluble. It was also noted that 
when finely divided lead arsenates were shaken with soap solutions the 
resulting precipitates were bulky and the individual curds frequently 
large. Besides, as would be expected of either lead soaps or free fatty 
acids, they were water-repellent. It seemed probable, therefore, that 
the particles of lead arsenate, when brought into contact with the soap 
solution, were at once acted upon at the surface and surrounded with an 
envelope of the water-repellant reaction product, which in turn pro¬ 
tected the lead arsenate at the interior of the curd from further action. 
This reasoning led to the following procedure. 
In each case, the treatment was carried out with 2 liters of soap solution 
in a 2X“hter acid bottle. The soap to be used for each sample was 
dissolved in i liter of distilled water, to be later mixed with a second liter 
of distilled water, as follows: The water was placed in the acid bottle 
and the weighed sample of lead arsenate was placed in a porcelain 
mortar where it was ground with successive portions of the soap solution. 
The soap and finely divided material was poured into the liter of water, 
taking care that the coarser particles were left in the mortar to be ground 
with the next portion of the soap solution. When all the lead arsenate 
had become so fine as to pass easily into the water with the soap solution, 
the remainder of the liter of soap solution was poured into the bottle, 
making 2 liters. The bottle was stoppered and shaken at intervals for 
five days, then filtered through paper, using suction. One half the 
samples were treated as stated; the other half were ground in the water 
and poured into the soap solution, the only difference being the fluid 
used for the grinding. 
The lead arsenates in the first series were weighed in paste form, 
2 gm. of “M*’ amounting to 0.346 gm. of AS2O5 and to 0.707 gm. PbO, 
and 2.15 gm. of “T'* amounting to 0.289 gm. of ASjOg and to 0.805 gm. 
PbO. 
The soaps were used in two concentrations, 0.6 gm. and 1.8 gm. (dry 
basis) per 2 liters of solution. 
The filtrate thus secured was analyzed for arsenic, while the insoluble 
material was dried, removed from the filter, and preserved for analysis 
for lead and for total arsenic. 
It was found advisable to remove the soap that remained dissolved in 
the filtrate. This was accomplished as follows: The measured aliquot 
to be analyzed was placed in a beaker and a few drops of barium chlorid 
solution were added, just enough being used to clear the solution. The 
insoluble barium soap separated at once and was removed by filtering, 
leaving a clear solution that could be reduced and titrated with iodin 
in the usual Gooch and Browning method ( 8 , p. 2jp). 
The quantity of arsenic rendered soluble and found in the filtrate is 
given in Table I. 
