TRACE METALS IN FOODS 27 



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. 



The limit of error between parallel solutions should not exceed 0.00010% 



or 1 p. p.m. 



Determination of Manganese 



Reagents 



Standard sodium oxalate solution. — -Dry sodium oxalate (Soerensen's), transfer 

 0.3050 gram to a volumetric flask, dissolve in water, and make to 1000 ml. at 

 25° C. Unstable, use a fresh solution. 



Potassium permanganate solution. — Dry reagent potassium permanganate 

 0.2877 grami per 2000 ml. at 25° C. One ml. contains 0.00005 gram of manganese. 

 Potassium periodate, KIO^ (G. Frederick Smith Chem. Co.). 

 Nitric acid, concentrated. 



Sulfuric acid, 2 N, 9.26%. 110 ml. per 2000 ml. 



Sulfuric acid, 1-1. 



Standardization of Manganese Solution 



5Na2C2O4+2KMnO4+8H2SO4 = 2MnSO4+5Na2SO4+K2SO4 + 10CO2+8H2O 



5 X 134.014 : 2 x 158.026 :: 0.30497 : 0.14385 potassmm permanganate 



5 X 134.014 : 2 x 54.93 :: 0.30497 : 0.05 manganese in 1000 ml. 



Titrate the potassium permanganate against 10 ml. of standard sodium oxalate 

 solution to which has been added 5 ml. of 2 N sulfuric acid in a 150 ml. Erlenmeyer 

 flask at 80° C to a faint pink that will persist for about 1 minute. 



KMn04 titer : 10 :: 0.05 : X actual manganese in 1000 ml. 



Method 



Evaporate an aliquot (100 ml.) of the original ash solution containing about 

 0.00025 gram (0.04-1.5 mg.) of manganese, together with 10 ml. of concentrated 

 nitric acid, in a 150 ml. Griffin beaker on a steam bath. Wash down the sides 

 of the beaker with 10 ml. of water, cover with a watch glass, and heat on a steam 

 bath. Add from 0.2 to 0.5 gram of potassium periodate and continue the heating 

 until the pink color develops (2 hours). ^ 



2MnS04+5KI04+3H20 = 2HMn04+5KI03+2H2S04. 



Cool, filter through a sugar tube with a glass wool and asbestos felt into a 

 50 ml. volumetric flask (glass-stoppered) containing a small amount of periodate, 

 and wash nearly to volume. Stopper the flask and again heat on the steam bath 

 to develop the maximum color (2 hours). Cool, make to volume, and mix. 



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

 about 515 millimicrons) when the amount of manganese is appreciable, against 



'Preferably 0..^0 gram to allow for impurities and slight decomposition. Filter through porous 

 glass or asbestos to remove insoluble matter. 



^The presence of hydrochloric acid vitiates the results. 



