400 Palmer — Application of Potassium Ferricyanide 



was made up by dissolving 55 grams of the crystallized magne- 

 sium chloride and 29 grams of purified ammonium chloride in 

 a liter of water, with the addition of about 5 cubic centimeters 

 of concentrated ammonium hydroxide. The potassium ferri- 

 cyanide used was purified by recrystallization, but even then, 

 as in the previous work, it was necessary to apply a correction 

 to the determinations for the number of cubic centimeters of 

 permanganate required to give the pink coloration to the ferri- 

 cyanide alone, generally amounting to about one-tenth of one 

 cubic centimeter. A solution of potassium ferricyanide of 

 convenient strength for use in the determination of all three 

 elements contained 20 grams to 100 cubic centimeters. 



In the preliminary experiments, recorded in Table I, low 

 results were obtained, apparently due to the incomplete oxida- 

 tion of the arsenic by the amounts of ferricyanide and potas- 

 sium hydroxide used. If at least ten times the theoretical 

 amount of ferricyanide, with a rather dilute solution of potas- 

 sium hydroxide, is used, as in experiments (1) to (12) of Table 

 II, the oxidation is complete; or, if a more concentrated solu- 

 tion of potassium hydroxide is used, as in experiments (13) 

 and (14) of Table II, less ferricyanide is necessary. In any 

 case, it is advantageous that the total volume of the solution 

 be rather small, preferably less than 100 cubic centimeters, 

 since, as shown by experiments (4), (5), and (6) of Table I, if 

 the volume is greater than that, low results are obtained. 









Table 



I. 



Vol. 









As 2 3 







of 



As 2 3 







taken 



K 3 FeC 6 N 6 



KOH 



sol. 



found 



Error 





grm. 



grm. 



grm. 



cm 3 . 



grm. 



grm. 



(1) 



0-0997 



2 



1-25 



100 



0-0966 



— 0.0031 



(2) 



0*0997 



4 



1-25 



75 



0-0987 



—o-ooio 



(3) 



0-1496 



8 



1-25 



100 



0-1486 



—o-ooio 



(4) 



0-0997 



8 



125 



150 



0-0982 



— 0-00i5 



(5) 



0-0997 



4 



2-5 



175 



0-0978 



— 00019 



(6) 



0-1001 



4 



4- 



150 



0-0984 



— 0-0017 



The procedure to be recommended, therefore, is as follows: 

 To the solution containing the arsenic in the arsenious condi- 

 tion is added an amount of potassium ferricyanide equal to at 

 least five times the amount theoretically required to oxidize 

 the arsenic to the higher condition of oxidation, and about 25 

 cubic centimeters of a 20 per cent solution of potassium 

 hydroxide, keeping the volume of the solution less than 100 

 cubic centimeters. After standing a few minutes, the solution 

 is made ammoniacal by dissolving in it about 10 grams of am- 

 monium sulphate, which acts on the potassium hydroxide, set- 



