Reduction of Arsenic Acid in Analysis. 



69 



reached and passed a little by the addition of the acid potassium 

 carbonate. After cooling completely a definite amount of 

 starch solution was added and the titration of the arsenious 

 acid was proceeded with as usual, due correction being made for 

 the amount of iodine necessary to produce the end color into 

 the volume of liquid and starch solution employed. 



The value of the standard solution of the arseniate was fixed 

 by two series of determinations. One series was made accord- 

 ing to Levol's method of precipitating the ammonium magne- 

 sium arseniate and weighing as the pyroarseniate, modified, 

 however, in that the precipitate was collected on asbestus in a 

 perforated crucible and ignited after moistening with ammo- 

 nium nitrate. In the second series McCay's modification* of 

 Reich's method was followed, excepting that the silver arseni- 

 ate was collected, dried and weighed on asbestus in a perforated 

 crucible. The mean of several closely agreeing determinations 

 gave for the contents of 50 grins, of the solution in arsenic 

 pentoxide as 0*3824: grm. by Levol's method and 0*3830 grm. 

 by McCay's modification of Reich's process. We took the 

 mean of these figures 0*3827 grm. as the standard of the solu- 

 tion. 



The details of the experiments with this solution are recorded 

 in the following table. 



KI 

 taken. 



1*5 grm. 

 1*5 " 

 1*5 " 

 ]*5 " 

 1*5 " 



1*5 " 



1 " 



1 " 



1 " 



1 " 



1 " 



1 " 



HoS0 4 [1 : 

 taken. 



10 cm s 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 

 10 " 



1] 



As 2 5 

 taken. 



0*3861 grm. 



0*3862 " 



0*3861 " 



0*3860 " 



0*3863 " 



0*3862 " 



0*1927 " 



0*1928 " 

 0*1930" " 



0*1930 " 



0*1936 " 



0*1929 " 



As 2 5 



found. 



0*3862 grm. 



0*3856 " 



0*3862 " 



0*3862 " 



0*3862 " 



0*3862 " 



0*1922 " 



0*1922 " 



0*1925 " 



0*1927 " 



0*1929 " 



0*1928 " 



+ 



Error. 

 0*0001 grm. + 

 0-0006 " - 

 0*0001 " + 

 0*0002 

 0*0001 

 0*0000 

 0*0005 

 0*0006 

 0*0005 

 0*0003 

 0*0007 

 0*0001 



Experiments in which smaller quantities of arsenic were 

 handled were made similarly, excepting that the standard solu- 

 tion, from which portions for the tests were measured, was 

 made by diluting the former standard ten times, and centinor- 

 mal iodine was used in the titration. 



When the amount of hydriodic acid in solution is small, 

 correspondingly small amounts of iodine are retained after 



*Am. Chem. Jour., vol. viii, p. 77. 



