XII. METALS IN FOODS. 

 ARSENIC^.— TENTATIVE. 



1 REAGENTS. 



(a) Nitric and sulphuric acids, arsenic-free. — Specific gravities 1.42 and 1.S4, 

 respectively. 



(b) Stdphuric acid (1 to 2). 



(C) Zinc, arsenic-free. — Stick zinc broken into pieces approximately 1 cm. in 

 length. 



(d) Lead acetate paper. — Heavy filter paper soaked in 20% lead acetate solution, 

 dried and cut into pieces about 4.5 by 16 cm. 



(e) Lead acetate cotton. — Absorbent cotton soaked in 5% lead acetate solution. 



(f) Mercuric bromid paper. — Cut heavy, close-textured drafting paper (similar 

 to Whatman's cold pressed) into strips exactly 2.5 mm. wide and about 12 cm. long. 

 Soak for an hour in a 5% solution of mercuric bromid in 95% alcohol, squeeze out 

 the excess of solution and dry on glass rods. Cut off the ends of the strips before 

 using. 



(g) 20% potassium iodid solution. 



(h) Stannous chlorid solution. — Forty grams of stannous chlorid crystals made up 

 to 100 cc. with concentrated hydrochloric acid. 



(i) Standard arsenic solution. — Dissolve 1 gram of arsenious oxid in 25 cc. of 20% 

 sodium hydroxid solution, neutralize with dilute sulphuric acid, add 10 cc. of the 

 concentrated sulphuric acid and dilute to 1 liter with recently boiled water. One 

 cc. of this solution contains 1 mg. of arsenious oxid (AS2O3). 



Dilute 20 cc. of this solution to 1 liter. Fifty cc. of the latter solution when 

 diluted to 1 liter give a dilute standard solution containing 0.001 mg. of arsenious 

 o.xid (AS2O3) per cc. which is used to prepare the standard stains. The dilute solu- 

 tions must be freshly prepared immediatel}' before use. 



2 APPAR.\TUS. 



Use a 2 ounce wide-mouthed bottle as a generator. Fit this by means of a per- 

 forated rubber stopper with a glass tube, diameter 1 cm. and 6 cm. long, containing 

 a piece of the lead acetate paper rolled into a cylinder. Connect this tube by means 

 of a perforated rubber stopper with a similar tube filled with the lead acetate cotton, 

 squeezed to remove excess of the solution. The cotton in all tubes used should be 

 uniformly moist to obtain comparative stains. Connect the second tube by means 

 of a perforated rubber stopper with a narrow glass tube, internal diameter 3 mm. 

 and 12 cm. long, containing a strip of the mercuric bromid paper. See Fig. 7. Rubber 

 stoppers used for connections must be free from any white coating. 



3 PREPARATION OF SOLUTION. 



Weigh 5-50 grams of the finely divided and well mixed sample into a porcelain 

 casserole, the amount selected depending upon the character of the material and the 

 ease with which it is oxidized. With dry, highly nitrogenous substances employ 

 5 grams; pulped vegetables, 25 grams; liquids with low solid content like beer or 



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