POISONOUS METALS ON SPRAYED FRUITS AND VEGETABLES. 17 



Arsenic. — Determine arsenic in an aliquot by the Gutzeit method (Bur. Chem. Circ. 

 102), modified as follows: The aliquot should contain less thanO.08 mg. arsenic. Dilute 

 to 50 CO. Add strong sulphuric acid so as to have 10 cc. present Add 1 gram sodium 

 chlorid to the aliquot in a small Erlenmeyer flask, heat on steam bath to about 90° C, 

 then add 1 cc. of a stannous chlorid solution containing 0.5 gram dissolved in hydro- 

 chloric acid, and leave on steam bath for about 5 minutes (temperature near 90° C). 

 Remove from steam bath, transfer to the 4-ounce generating bottle, dilute to 100 cc, 

 and cool to room temperature. This generating bottle is connected by a rubber 

 stopper with an upright tube 8 cm. long, 1 cm. diameter, containing lead acetate 

 paper. This tube is connected by a rubber stopper with a similar tube containing 

 cotton moistened with 5 per cent lead acetate solution. Connected by a rubber 

 stopper with this tube is a capillary tube 3 mm. in diameter, 12 cm. in length, carrying 

 the strip of mercuric bromid paper. Prepare these strips as follows: Cut heavy, close- 

 textured drafting paper into strips 2 mm. by 12 cm.; then soak them for an hour in 5 

 per.cent alcoholic mercuric bromid solution, take out, rapidly squeeze off excess of 

 solution, separate on glass rods, and allow to dry. Place three pieces of stick zinc 

 (about 10 grams) in the generating bottle and join it immediately to the apparatus tubes. 

 Allow the determination to run for IJ hours, keeping the temperature down to 

 room temperature by placing the bottle in cool water. From standards plot a 

 curve showing milligrams of arsenic to millimeters in length. As high as 0.08 milli- 

 gram of arsenic can be read on a paper. Determine the larger quantities of arsenic by 

 passing the arsine into a mercuric chlorid solution and either weigh the mercurous 

 chlorid or titrate the arsenious oxid. (Bur. Chem. Circ. 102, p. 5.) 



Copper. — Introduce an aliquot into a 100 cc. Erlenmeyer flask. Neutralize the 

 acid with ammonia, add 2 to 3 cc. hydrochloric acid for every 50 cc. of solution, and 

 saturate the solution with hydrogen sulphid. Stopper flask and let stand over night. 

 Filter off the copper sulphid and wash with hj^drogen sulphid water. Place the 

 filter paper containing the copper sulphid in a 50 cc. casserole, burn off the paper, 

 dissolve residue in 5 cc. (1: 1) nitric acid, evaporate to dryness, add water and 1 drop 

 ammonia, make faintly acid with acetic acid, and add a few drops of a 2 per cent 

 potassium ferrocyanide solution. Compare with standards. 



Lead. — Dissolve the sulphate precipitate, previously referred to, in hot 10 per cent 

 ammonium acetate solution, add 2 cc. (0.1 per cent solution) gum arabic, and make 

 to volume with hydrogen sulphid water in 50 cc. (or 100 cc.) Nessler tubes. Com- 

 pare the tubes thus prepared with standards made up similarly with gum arabic, 

 ammonium acetate, known amounts of lead, and hydrogen sulphid water. 



Where copper alone is to be determined, heat the dried sample cautiously over a 

 Bunsen burner and finally ash at the mouth of the electric-muffle furnace. Add 5 cc. 

 (1:1) nitric acid to the ash, evaporate almost to dryness on steam bath, dilute, and 

 make alkaline with ammonia. Filter off precipitate and wash. Dissolve precipitate, 

 reprecipitate with ammonia, and wash. Evaporate the united filtrates to dryness, 

 add water and one drop ammonia, make slightly acid with acetic acid, and add a few 

 drops 2 per cent potassium ferrocyanide solution. Compare with standards. 



The presence of between 0.02 and 0.24 milligram of copper can be 

 determined by this method. Larger amounts may be determined 

 by taking an aliquot, by comparing in ammoniacal solutions, or by 

 electrolysis. 



The presence of from 0.02 to 0.24 milligram of lead can be read in 

 the 50 cubic centimeter Nessler tubes, larger amounts by using 100 

 cubic centimeter Nessler tubes or by taking a smaller aliquot. 

 72638— 22— Bull. 1027 3 



