TRACE METALS IN FOODS 31 



the so-called single color method. An excess dithizone produces a mixed color 

 and is preferred by some analysts. 



Shake out for at least 2 minutes to extract the colored zinc salt, allow to sep- 

 arate, rinse the delivery tube with a few drops of the solvent layer, and draw 

 off the remainder through a dry ashless filter (to remove traces of moisture) 

 into a weighing bottle and stopper to prevent evaporation. 



Compare the color in a comparator or spectrophotometer using a narrow 

 band green color filter (spectral centroid 515 millimicrons) against 5 ml. of dilute 

 zinc solution treated in exactly the same manner. 



sR 



%= x 250 for 0.40 gram (F) 



Ri 



0.0025R 



Ri 



25R 



p. p.m. = 



Ri 



s=grams of the standard employed. 

 R=scale reading at which the standard was set. 

 Ri^scale reading of the unknown. 

 F=factor for converting the aliquot to percentage or p. p.m. 



Since a small amount of zinc will frequently be found in the reagents after 



., . (s + B)R 

 careful purification, should be substituted in the calculations and the 



Ri 



blank deducted. 



R R S R 



Example— — = R(s + B)=RiB; — = B=2.50 



^ Ri s-l-B 25 10-fB 



The limit of error between parallel solutions should not exceed 0.0003% or 3 

 p.p.m. 



If due care is exercised in the manipulation, no modifications will be found 

 necessary such as a second shaking out with carbon tetrachloride in the three 

 extractions, etc. With proper color filters the mixed color method is easier to 

 handle. 



An aliquot of a sulfuric solution prepared by wet combustion may be used for 

 the determination of zinc and has been found to yield similar results. 



Lead may be determined in a similar manner by adding 10 ml. of potassium 

 cyanide solution instead of carbamate in the third extraction to inhibit the zinc. 



Publication of this Document Approved by Commission on Administration and Finance 

 5m-8-41-7017. 



