SOME EFFECTS OF SODIUM AESEXITE 7 
cause AsEL can not be formed in the presence of oxidizing agents. 
The solution was then cooled and diluted to 200 cubic centimeters 
and filtered, if necessary. 
PROCEDURE OF ANALYSIS 
To 40 cubic centimeters of the above solution in the generating 
bottle, 4 cubic centimeters of 20 per cent solution of KI were added. 
This was then heated to 90° C. and four or five drops of SnCl 2 solu- 
tion (40 grams SnCl 2 2H 2 crystals made to 100 cubic centimeters 
with concentrated HC1) were added. The solution was kept at 
90° for 10 minutes, then cooled, and pieces of As-free stick zinc, 
amounting to 5 to 8 grams, were dropped in and the bottle was con- 
nected to "the apparatus. The KI and SnCl 2 were added to change 
the arsenic into the arsenious form; nascent hydrogen reduces 
arsenates with much more difficulty than arsenites. The SnCl 2 also 
takes up the liberated free iodine and, according to Parsons and 
Stewart (36) and Harkins (£5), prevents the retention of arsenic 
by iron. According to these authors, retained arsenic was carried by 
hydrated iron oxide in solution when no SnCl 2 or' salts of Bi, Cd, 
etc., were used. Fractions of a percentage of iron are found in the 
purest obtainable zinc. 
Into the receiving bottle of the apparatus were poured 5 cubic* 
centimeters of 5 per cent HgCl 2 , and water was added to make the 
volume about 60 cubic centimeters. This solution was to receive and 
absorb the AsH, coming from the generating bottle. TThen arsine 
was passed in, the solution assumed a yellowish white tinge. Two 
hours were allowed for the reaction. Toward the end of the 2-hour 
period a little dilute H,S0 4 (1:3) was added to insure a complete 
reaction. 
The apparatus was disconnected and 5 cubic centimeters of KI 
and 5 cubic centimeters of N/10 iodine solution were added to the re- 
ceiving bottle. It sometimes was necessary to use a rubber policeman 
to break up the precipitate. To obtain accurate results the iodine 
should be in excess, as the KI dissolves the precipitate formed when 
AsH 3 is passed into a HgCl 2 solution and probably forms a double 
salt, K 2 HgI 4 . The iodine also oxidizes AsH 3 to As 2 5 , according to 
the familiar equations : 
2AsH 3 +6I 2 +3H 2 0=A&0 3 +12HL 
As 2 8 +2I 2 "+2H 2 "0=As 2 0;+ 4HI. 
By titrating the excess of iodine with N/100 Na 2 S 2 3 , using starch 
as an indicator, the quantity of arsenic was calculated indirectly 
from the iodine absorbed (8 parts of iodine to 1 of arsenic). Quan- 
tities of arsenic as small as 0.00005 gram were determined easily by 
this method. The accuracy of the method was tested with Na 3 As0 4 
in a solution by itself and also when mixed with arsenic-free bar- 
berry tissue and alfalfa tissue. The results agreed with the calcu- 
lations. 
EXPERIMENTAL RESULTS 
KILLING CONCENTRATIONS OF ARSENICAL SOLUTIONS 
Several different commercial brands of sodium-arsenite solutions 
especially prepared for use in killing weeds and trees are on the 
