ASSAY OF ARSENICAL DIPPING FLUIDS. 9 
METHOD FOR " TOTAL ARSENIC." 
Strictly speaking the oxidized form of arsenic no longer contains 
arsenious oxid as such (As 2 3 ) but only arsenic oxid (As 2 5 ). Never- 
theless, for the purpose of making simple and convenient compari- 
sons, it is necessary to refer all quantitative statements regarding 
arsenic compounds to the common basis of their equivalent in arseni- 
ous oxid. Therefore, the term "total arsenic" is employed to signify 
all arsenic present in any form of combination or degree of oxidation, 
expressed in terms of arsenious oxid. 
The method to be described is a standard method based on the 
well-known fact that the reaction As 2 3 + 41 + 2H 2 O^As 2 5 + 4HI is 
reversible, going from left to right in neutral or alkaline solutions, 
and from right to left in solutions which are freely acidified with a 
strong mineral acid. The reaction from left to right is the basis for 
the previously described methods for the determination of actual 
arsenious oxid. To determine total arsenic it is only necessary to 
allow the reaction to progress to completion from right to left, then 
to determine the resulting arsenious oxid in a manner similar to that 
already described. 
The solutions and reagents necessary, in addition to the same stand- 
ard iodin solution, starch solution, dilute sulphuric acid, and solid 
sodium bicarbonate, are anhydrous powdered sodium carbonate, solid 
potassium iodid, a one-tenth per cent solution of methyl orange, 
possibly concentrated sulphuric acid, and lastly a solution of sodium 
thiosulphate containing about 25 grams per liter. 
The first step is to clarify and decolorize the solution. Hence 
proceed exactly as in method "B" for actual arsenious oxid (p. 8) 
until the filtered solution has been obtained. When both actual and 
total arsenious oxid are to be determined it is most convenient to 
double the quantities of both acid and charcoal, and to filter into a 
200 c. c. volumetric flask. After thorough washing and cooling, the 
contents of the flask are made to the mark, mixed, and divided into 
two equal portions, one for actual arsenious oxid, the other for total 
arsenic. In either case transfer the solution containing the equiva- 
lent of 25 c. c. of the original bath to a 200 c. c. beaker, add 4 c. c. 
of concentrated sulphuric acid and about 2 grams of potassium 
iodid, boil gently until the volume of liquid is reduced to 50 c. c, then 
cool to room temperature. The next step is to remove free iodin from 
the solution. This may be done while the solution is still in the 
beaker, by adding the solution of sodium thiosulphate drop by drop 
from a burette until the yellow color of free iodin just disappears. 
Any excess of thiosulphate must be carefully avoided. A safer 
procedure for one not experienced with the method is to wash the 
contents of the beaker into a flask of 600 to 800 c.c. capacity, dilute 
29207°— 14 2 
