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 0;zAs 2 5 -f- 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 



