26 MASS. EXPERIMENT STATION BULLETIN 379 



expelled, cool, and make to 2000 ml. at 25° C. One ml. contains 0.001 gram of 

 copper. 



Dilute copper solution. — To 20 ml. of the standard copper solution add 1000 ml. 

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

 contains 0.00001 gram of copper. 



Sodium diethyl dithiocarhamate solution. 2.5 grams per 1000 ml. 



/S-Na 



^N = (C2H5)2 



Ammonium citrate solution, Q.SM. (NH4)2HC6H507. 



Dissolve 226.2 grams of diammonium citrate or 210.1 grams of citric acid in water, 

 add ammonmm hydroxide until alkaline to litmus, and make to 2000 ml. Transfer 

 250 ml. to a 1000 ml. glass-stoppered pear-shaped separatory funnel, add an 

 excess of diphenylthiocarbazone (dithizone) solution (usually 3, 2, and 1 ml. 

 respectively), and shake out with three 25 ml. portions of carbon tetrachloride 

 to a green color. Discard the solvent laj^ers and filter the aqueous portions 

 through washed ashless paper. 



Ammonium hydroxide, concentrated. 



Nitric acid, concentrated. 



Carbon tetrachloride, reagent or redistilled. 



Method 



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

 0.00005 gram (0.01-0.30 mg.) of copper, 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, add concentrated ammonium hydroxide until 

 alkaline to litmus, and cool. Transfer the solution to a 500 ml. glass-stoppered 

 separatory funnel (pear-shaped with a short delivery tube), and wash to a volume 

 of about 85 ml. Add a few drops of ammonmm hydroxide in excess, 10 ml. of 

 ammonmm citrate, 5 ml. of carbamate solution, and mix. Add 10 ml. of carbon 

 tetrachloride (measured carefully), stopper the separatory funnel, shake for about 

 2 minutes, and allow to layer. Wipe the delivery tube with filter paper, rinse 

 with a few drops of the carbon tetrachloride solution containing the colored 

 copper salt (brownish-yellow), then draw off the remainder through a small 

 coarse filter (to remove traces of moisture) into a weighing bottle, and stopper to 

 prevent evaporation. 



Precautions. — If iron is precipitated on the addition of ammonium hydroxide, 

 which frequently occurs with soils, dried blood, etc., centrifuge the precipitate 

 and decant the clear solution into the separatory funnel. Wash the precipitate 

 twice with water containing a few drops of ammonia, centrifuge and decant, then 

 proceed as usual. The carbon tetrachloride seldom forms an emulsion on shaking 

 out the test, but this has been observed several times on the determination of 

 blanks. 



Compare the color in a Dubosq, using micro cups and a blue color filter (spectral 

 centroid about 435 millimicrons), against 5 ml. of dilute copper solution treated 

 in exactly the same manner. In hot weather the rapid evaporation of the carbon 

 tetrachloride in the cups necessitates a frequent change of standard. 



