XII] METALS IN FOODS 173 



generator of the apparatus described in 2 and add 20 cc. of the dilute sulphuric acid. 

 If the total volume is less than 40 cc, dilute to that volume with water and add 



4 cc. of the 20% potassium iodid solution. Heat to about OO^C, add 3 drops of 

 the stannous chlorid solution and heat for 10 minutes. Cool the generator and its 

 contents in a pan containing water and ice; when cold add about 15 grams of the 

 stick zinc and connect the entire apparatus as described in 2. Keep the bottles in 

 ice water for 15 minutes, then remove from the bath and allow the evolution of gas 

 to proceed for an hour longer. Remove the sensitized paper and compare the stain 

 with similar ones produced under like conditions with known amounts of arsenic, 

 using portions of the standard arsenic solution, containing 0.001, 0.002, 0.005, 0.010, 

 0.015, 0.025 and 0.030 mg. of arsenious oxid (AS2O3), and adding such quantities of 

 water and sulphuric acid that the same volume and acid strength are maintained as 

 above. 



TIN^ 



5 Gravimetric Method. — Tentative. 



Weigh 50-100 grams of the sample (depending upon the amount of dry substance 

 present and the relative ease with which the organic matter is o.xidized) into an 800 

 cc. Kjeldahl flask and add 100 cc. of concentrated nitric acid. Allow to stand over- 

 night (this procedure being preferred if much fat or sugar is present) or else place 

 the flask on a wire gauze over a free flame and heat until the contents boil quietly. 

 Add 25-50 cc. of concentrated sulphuric acid (depending upon the amount of dry 

 substance present in the sample), and heat until white fumes are generated, cool 

 somewhat, then add 5-10 cc. of concentrated nitric acid and continue heating as 

 before. Repeat the addition of nitric acid until the solution remains clear after 

 boiling off the nitric acid and fumes of sulphur trioxid appear. 



Add 200 cc. of water to the digested sample, prepared as directed above, and pour 

 into a 600 cc. beaker. Rinse out the Kjeldahl flask with 3 portions of boiling water 

 so that the total volume of the solution is about 400 cc. Cool, add concentrated 

 ammonia until just alkaline and then hydrochloric or sulphuric acid until the acidity 

 is about 2%. Place the beaker, covered, on a hot plate, heat to about 95°C. and pass 

 in a slow stream of hydrogen sulphid for another hour. Digest on the hot plate for an 

 hour and allow to stand 1-2 hours longer. 



Filter the tin sulphid on an 11 cm. filter, similar in quality to No. 590, white ribbon, 

 S. & S. Wash alternately with 3 portions each of wash solution (100 cc. of saturated 

 ammonium acetate solution, 50 cc. of glacial acetic acid and 850 cc. of water) and 

 hot water. Digest the filter and precipitate in a 50 cc. beaker with 3 successive 

 portions of ammonium polysulphid, heat to boiling each time and filter through a 

 9 cm. filter. Wash the precipitate on the filter with hot water. Acidify the filtrate 

 with acetic acid, digest on a hot plate for an hour, allow to stand overnight and 

 filter through a double 11 cm. filter. Wash alternately with 2 portions each of the 

 wash solution and hot water and dry thoroughly in a weighed porcelain crucible. 

 Ignite over a Bunsen flame, very gently at first and later at full heat. The cru- 

 cible, partly covered, is then heated strongly with a large or Meker burner. Stan- 

 nic sulphid must be roasted gently to the oxid, which may be heated strongly with- 

 out loss by volatilization. Weigh as stannic oxid and calculate to metallic tin. 



Volumetric Method^. — Tentative. 



6 RE.\GENTS. 



(a) Air-free wash solution. — Dissolve 20 grams of sodium bicarbonate in 2 liters 

 of boiled water and add 40 cc. of concentrated hydrochloric acid. This solution 

 should be freshly prepared before use. 



