The sodium thiosulphate is run in a little at a time, and occasionally a drop of 

 tin- solution is withdrawn and added to a drop of starch paste. This will, of 

 course, give a blue color with the starch, which becomes fainter as the iodin is 

 used up. Finally, when a drop of the solution gives only the slightest blue color 

 with the starch, a little starch paste is added directly to the whole solution and 

 the blue color dissipated with a few drops of thiosulphate. With a little prac- 

 tice the chemist can in this way get the exact end point. The solution is 

 immediately made alkaline with solid sodium carbonate. It is again made 

 slightly acid with hydrochloric acid, taking care that all of the solid particles of 

 the sodium carbonate <m the bottom are neiitrali/ed by the acid, and finally made 

 alkaline with sodium bicarbonate. Starcli paste is now added ami t \\entieth- 

 normal iodin until the blue color appears. This end point is easily read if the 

 beaker is placed on a white surface between the eye and the light and the iodin 

 solution run in until a distinct purple color appears. The figure thus obtained 

 gives the number of cubic centimeters of iodin corresponding to the total amount 

 of arsenic in the solution as arsenious oxid. Subtracting from this the number 

 of cubic centimeters of iodin corresponding to the arsenious oxid in the previous 



?thod gives the number of cubic centimeters of iodin corresponding to the 

 ai -'-nic oxid in ' am of the sample. 



TOTAL ARSKMC OXID, MKTIIOD II (PROVISIONAL). 

 1 >r>i^ned to eliminate part of coloring matter.) 



SOLUTIONS REQUIRED. 



The same as in preceding method. 



METHOD. 



One hundred cubic centimeters of the hydrochloric acid solution of london 

 purple prepared under the determination of total arsenious oxid is placed in a 

 heaki-r and made alkaline with a saturated solution of sodium carbonate. It is 

 then transferred to a 200 cc flask, filled to the mark, and 100 cc filtered off 

 thnniL'h a dry filter for analysis. This is evaporated to about 50 cc on a hot 

 plate, allowed to cool to about 80 C and 50 cc concentrated hydrochloric acid 

 added, followed by 3 grams of potassium iodid. The mixture is allowed to stand 

 for 1 "i minutes, the " ic " arsenic being reduced to "ous" arsenic, and the 

 iodin set free. The solution is then rinsed out into a large beaker and the 

 iodin exactly used up with tenth-normal sodium thiosulphate. To get the 

 exact end point it may sometimes be necessary to proceed as in the previous 

 method, but usually there is no trouble. The solution is immediately made 

 alkaline with >olid sodium carbonate, again made slightly acid with hydrochloric 

 acid, taking care that all solid particles of sodium carbonate are acted on, and 

 finally made alkaline with sodium bicarbonate. Starch paste is now added and 

 the solution titrated to a blue color. The calculation is the same as that 

 bed under Method I for total arsenious oxid. 





SOLUBLE ARSENIOUS OXID (PROVISIONAL). 



SOLUTIONS REQUIRED. 



The same as those used in the preceding method. 



HOD. 



:tract 1 gram of london purple in a stoppered fl as fc with ~>(K)cc of cold carbon- 

 dioxid-free water for 7 days, shaking eight times each day. Filter off 100 cc of 

 the solution through a dry filter, add sodium bicarbonate, and titrate with 

 standard iodin, using starch as indicator. 



