130 EXPEULMENT STATION. [Jan. 



Make up to about 150 cubic centimeters, add excess of sodium 

 bicarbonate and titrate as usual with 'N/20 iodine, rei3orting as 

 arsenic oxide. The residue in the graduated flask is brought 

 onto the Alter, washed, calcined and weighed as insoluble matter. 



Care should be taken to have sufficient sulfuric acid to cover 

 the bottom of the beaker when heated on asbestos. A decided 

 excess of acid is also necessary when boiling with potassium 

 iodide to insure vigorous action and rapid volatilization of 

 iodine. Undue concentration should be avoided. If free iodine 

 persists add more water and continue the boiling. The use of 

 caustic soda is permissible under the conditions described. The 

 livdrate is a much more convenient and rapid agent than the 

 carbonate. Practically no difference was noted in the titration 

 when the lead sulfate was allowed to remain, but the data at 

 hand do not cover a sufficient number of samples to warrant a 

 statement that this will always hold true. 



The iodine method, as modified for arsenitcs and arsenates, 

 has been given a careful study, and proved repeatedly, in the 

 work at the Massachusetts station, to yield excellent results in the 

 analysis of the insecticides mentioned, if reasonable attention is 

 paid in following the details. While no radical changes in the 

 method have been recommended, this article is offered in hopes 

 that some of the points noted may prove of assistance to other 

 analysts working along similar lines. 



