514 Heath — Determination of Arsenic and Antimony. 



reduces the higher salts slowly. Tartaric acid is somewhat 

 irregular in action and tends to cause an interfering precipita- 

 tion of acid potassium tartrate. The action of citric acid is 

 more favorable. The amount of iodine set free, by the combined 

 action of citric acid and potassium iodide upon an arsenic com- 

 pound iu the higher condition of oxidation is appreciable if 

 the solution is allowed to stand, while in the case of an anti- 

 monic salt the action is not nearly so rapid. 



A few determinations were carried out to test the action of 

 citric acid quantitatively. Portions of tartar emetic were oxi- 

 dized to the higher condition by iodine in presence of sodium 

 bicarbonate, the solution was then acidified with citric acid, 

 weighed amounts of citric acid and potassium iodide were added, 

 and the solution was allowed to stand for a definite time to find 

 whether there was any reduction. When the solution was 

 acidified, after the oxidation with iodine solution, a little iodine 

 was set free aud this was bleached with a drop of sodium thio- 

 sulphate before continuing the experiment. For the test with 

 arsenic a standardized solution of potassium arseniate was used. 

 The results of these experiments are shown in Table I. 









Table I. 









Effects 



of Citric 



Acid and 

 and 



Potassium 

 Antimonic 



Iodide 

 Salts. 



in reducing 



Arsenic 



Sb 



As 



Citric 



KI 



Vol. 



Time 



I 



taken 



taken as 



acid 



used 



of 



of 



found 





KH 2 As0 4 



nsed 





sol. 



standing 





grm. 



grm. 



grrn. 



grm. 



cm 3 . 



min. 



grm. 













no color 



•1209 







3 



4 



60 



10 



with 

 starch 



•0797 







3 



2 



50 



20 



a 



•1057 







3 



3 



60 



20 

 -^ 60 



a 



trace 





•1238 



2 



3 



50 



10 



•0009 







•1238 



o 



4 



60 



10 



•0012 







•1238 



3 



3 



50 



10 



•0019 







•1238 



3 



5 



75 



10 



•0017 



•1211 



•1238 



3 



5 



100 



10 



•0010 



•1211 



•1238 



3 



5 



100 



10 



•0017 



From these results it appears that in the iodometric deter- 

 mination of copper associated with arsenic there must be no 

 delay in titrating the iodine even when the free acid is citric 

 acid ; otherwise, some reduction of the arsenic acid and libera- 

 tion of iodine may take place, thus affecting the result of the 

 copper determination. Antimonic acid is not reduced appre- 

 ciably, in a reasonable time, under similar conditions. 



