TRACE METALS IN FOODS 25 



Transfer to a 500 ml. volumetric flask, make to volume at 25° C, filter, and 

 determine the ash constituents in various aliquots. Such solutions will stand a 

 year without discoloration. 



With milk, fruit juices, and other liquid products, 100 grams are generally 

 employed. 



Determination of Iron 



Reagefits 



Standard iron solution. — Dry reagent iron wire free from surface oxidation i 

 and dust, transfer 2 grams (about 21.4 feet) to a volumetric flask, add about 

 200 ml. of water, 50 ml. of sulfuric acid (1-1), and 5 ml. of concentrated nitric 

 acid. Boil until solution is complete and the nitrous oxide expelled, cool, and 

 make to 2000 ml. at 25° C. One ml. contains 0.0010 gram of iron. 



Dilute iron solution. — To 20 ml. of the standard iron solution add 200 ml. of 

 sulfuric acid (1-1) and sufficient water at 25° C to make 2000 ml. One ml. contains 

 0.00001 gram of iron. 



Potassium thiocyanate solution, KCNS. 400 grams per 2000 ml. (Occasional 

 lots will not produce a stable color.) 



Nitric acid, 5%. — Boil 100 ml. of colorless concentrated nitric acid in 500 ml. 

 of water, cool, and make to 2000 ml. 



Method 



Transfer an aliquot (25 ml.) of the original ash solution containing about 

 0.0001 gram (0.02-0.60 mg.) of iron to a 50 ml. volumetric flask, add 5 ml. of 

 dilute nitric acid, mix, and allow to stand over night. Add 10 ml. of potassium 

 thioc>anate, make to volume, and mix thoroughly. Compare the color, Fe(CNS)3, 

 in a Duboscq, using a blue color filter (spectral centroid about 465 millimicrons), 

 against 10 ml. of dilute iron solution treated in exactly the same manner. 



sR 



% = X 200 for 0.50 gram (F) 



Ri 



0.02R 



p.p.m. 



Ri 



200R 



"rT 



s=grams ol standard employed. 

 R=^scale reading at which the standard was set. 

 R.=scale reading of the unknown. 

 F^lactor for converting the aliquot to percentage or p.p.m. 



The limit of error between parallel solutions should not exceed 0.00020% 

 or 2 p.p.m. 



Determination of Copper 



Reagents 



Standard copper solution. — Dry reagent electrolytic copper foil (0.002") free 

 from surface oxidation' and dust, transfer 2 grams (about 6 square inches) to a 

 volumetric flask, add 200 ml. of water, 50 ml. of sulfuric acid (1-1), and 10 ml. of 

 concentrated nitric acid. Boil until solution is complete and the nitrous oxide 



'Cleaned with fine crocus cloth. 



