METH0D8 OF ANALYSIS 9 



acetate, boil for about a minute, then cool until quite cold and di- 

 lute with 50 ec. cold water. Add about three or four grama of 



KI. and shako about until dissolved. Cuprous iodide will be pre- 

 cipitated and iodine liberated. 



2 On (C Jf ; 2 ) 2 +4 KL=Cu,:C I KC_n,0_ I. 



The free iodine is soluble in KI and colors the solution brown. 

 Titrate at once with the hyposulphite solution until the brown 

 tinge has become weak, then add about 5 cc. of starch solution, or 

 a sufficient amount to produce a good blue color. Continue the 

 titration until the blue solution has entirely disappeared, leaving 

 the solution milky and somewhat yellow. 



Calculate the standard as before. 



f>. Ammonium Molybdate. — Weigh into a No. 2 beaker .200 

 grams of pure lead foil and dissolve in 5 cc. nitric acid and 5 cc. 

 water by boiling for a few minutes. Remove from the heat and 

 add 20 cc. dilute sulphuric acid, mix well with a stirring rod and 

 allow to settle. Filter through a 9 cm. filter, wash the precipitate 

 into the filter with dilute sulphuric acid, and with two or three 

 washings with water. 



Put the filter paper and contents into another clean No. 2 

 beaker and add grams of ammonium acetate and fill the beaker 

 half-way with boiling water. Heat to boiling and titrate the con- 

 tents just as they are. using a solution of tannic acid as an indi- 

 cator on the spot plate, the end reaction being a light orange 

 color. Great care must be used to avoid running o 



