628 AGRICULTURAL ANALYSIS 



timeters by boiling in a 250 cubic centimeter flask to half its 

 volume, and allow to cool to about 80. Add an equal volume 

 of concentrated hydrochloric acid and three grams of potassium 

 iodid, mix, and allow the whole to stand for 10 minutes to re- 

 duce the arsenic oxid formed by boiling the alkaline arsenite 

 to arsenious oxid. Dilute the solution with cold water and add 

 an approximately tenth-normal solution of sodium thiosulfate, 

 drop by drop, until the solution becomes exactly colorless. This 

 end point is easy to read without the aid of starch. Make this 

 solution slightly alkaline with dry sodium carbonate, using a 

 drop of methyl orange to read the change, and then make slight- 

 ly acid with hydrochloric acid, taking care that all lumps of 

 sodium carbonate on the bottom are acted on by the hydrochloric 

 acid. Add sodium bicarbonate in excess and run in the solu- 

 tion of iodin drop by drop, using starch water to read the end 

 reaction. Sometimes the solution becomes dark toward the end 

 of the titration. This change must not be confused with the 

 final dark-blue color given by the iodin-starch. 



From the number of cubic centimeters of iodin solution and 

 the weight of arsenious oxid used determine the value of each 

 cubic centimeter of iodin in terms of arsenious oxid. 



Determination. Transfer two grams of paris green to a 250 

 cubic centimeter flask by means of about 100 cubic centimeters 

 of a two per cent, sodium-hydroxid solution. Boil this mixture 

 for five to 10 minutes, or until all the green particles have 

 changed to red cuprous oxid, then cool it to room temperature 

 and make the volume up to 250 cubic centimeters. Filter the 

 well-shaken liquid through a dry filter and use 50 cubic centi- 

 meter portions for analysis. Conduct the analysis from this 

 point as when standardizing the iodin. solution. 



Total Arsenious Oxid. Method 7/. 98 Solutions Required. (a) 

 Standard iodin solution. Prepare an iodin solution as in Method 

 I. To standardize this solution place one gram of dry, chemical- 

 ly pure arsenious oxid in a 250 cubic centimeter flask, and dis- 

 solve by boiling about 20 minutes with five grams of sodium bi- 



98 Hay wood, Bureau of Chemistry, Bulletin 81, 1904 : 195. 

 Journal of the American Chemical Society, 1903, 25 : 963. 

 Bureau of Chemistry, Bulletin 107, 1907 : 26. 



