TRACE METALS IN FOODS 29 



to about 70 ml., add 10 ml. of ammonium molybdate solution, mix, add 5 ml. of 

 amino-naphthol-sulfonlc acid solution, make to volume, mix thoroughly, and 

 allow to stand about 10 minutes to develop its maximum color (molybdenum 

 blue, MoaOe). Cold or strongly acid solutions react slowly. 



Compare the color in a Dubosq, using an orange color filter (spectral centroid 

 about 645 millimicrons), against 5 ml. of standard phosphate solution treated in 

 exactly the same manner. 



^ sR 



%=-- x 500 for 0.20 gram (F) 

 Ri 



0.25R 

 Ri 



2500R 

 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^actor for converting the ahquot to percentage or p.p.m. 



The limit of error between parallel solutions should not exceed 0.005% or 

 50 p.p.m. 



Synopsis of Reaction 

 8(NH4)6Mo7024.4H20+2H3P04+21H2S04 = 2(NH4)3P04.12Mo03 + 



32H2Mo04+21(NH4)2S04+24H20 

 3(12Mo03) +4NH2 = I2M03O8+4NO2+4H2O 



Determination of Zinc 



Reagents 



Standard zinc solution. — Dry reagent zinc foil (0.002") free from surface oxida- 

 tion ^ and dust, transfer 2 grams (about 8.43 square inches) to a volumetric flask, 

 add about 200 ml. of water and an excess of distilled hydrochloric acid. Boil 

 until solution is complete, cool, and make to 2000 ml. at 25° C. One ml. contains 

 0.0010 grams of zinc. 



Dilute zinc solution. — To 4 ml. of the standard zinc solution add sufficient water 

 at 25° C to make 2000 ml. One ml. contains 0.000002 gram of zinc. 



Hydrochloric acid, 0.20 iV.--Distill the hydrochloric acid into cold "metal 

 free" water by allowing concentrated sulfuric acid to drip from a separatory 

 funnel into concentrated hydrochloric acid below the surface, and dilute to the 

 required strength at 25° C. 



Ammonium hydroxide, 0.20 N. — Distill the ammonium hydroxide to about 

 70° C into cold "metal free" water and dilute to the required strength at 25° C. 



Sodium diethyl dithiocarbamate (carbamate) solution. — 2.5 grams per 1000 ml. 

 Preserve in a brown bottle. 



Diphenylthiocarbazone (dithtzotie) solution. — Dissolve 0.03 gram of dithizone 

 in 20 ml. of 0.20 N ammonium hydroxide, crushing the aggregates to facilitate 

 solution, and transfer to a 500 ml. pear-shaped separatory funnel with 180 ml. 

 of water. Shake out with 25 ml. portions of carbon tetrachloride to a green color 

 and discard the solvent layers. Add 45 ml. of 0.20 N hydrochloric acid, 5 ml. cf 

 water, and shake out with 50 ml. portions of carbon tetrachloride, filter the solvent 



"Cleaned with fine crocus cloth. 



