30 MASS. EXPERIMENT STATION BULLETIN 379 



layers through dry ashless paper and dilute to 200 ml. with carbon tetrachloride. 

 Reasonably stable in a brown bottle. 



H H 



I I 

 ^^^--N-N-CeHs 



\N = N— CeHs 



Ammonium citrate solution, O.SM. (NH4)2HC6H507' 



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

 water, add ammonium 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 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 layers and filter the aqueous portions through washed ashless 

 paper. 



Carbon tetrachloride, reagent or redistilled. 



Potassium cyanide solution, 10%. 50 grams per 500 ml. 



Method — Dry Combustion 



Transfer 2 or 4 grams of finely ground (1 mm.) air-dry material to a flat- 

 bottomed platinum ash dish, incinerate carefully in an electric mufifle (with the 

 door open), continue the heating (door closed) at 500° C (visible redness) until 

 a white or gray ash is obtained. If carbon persists, moisten the ash with water 

 to bring the particles to the surface, evaporate to dryness on a steam bath and 

 reheat. Transfer the ash with small portions of hot water, distilled hydrochloric 

 acid, and a rubber policeman to a 150 ml. Griffin beaker, add an excess of distilled 

 hydrochloric acid, and evaporate to dryness on a steam bath. Take up with 

 20 ml. of 0.20 N hydrochloric acid, heat to boiling, and transfer to a 100 ml. 

 volumetric flask, cool,, make to volume at 25° C. and pass through a dry filter. 



Transfer 10 ml. of the ash solution containing about 0.00001 gram (0.002 to 

 0.025 mg.) of zinc to a 500 ml. glass-stoppered separatory funnel (pear-shaped 

 with a short delivery tube). Add 23 ml. of water, 7 ml. of 0.20 N ammonium 

 hydroxide, 10 ml. of ammonium citrate solution, 15 ml. of carbon tetrachloride, 

 and dithizone solution in small portions until a yellow color is imparted to the 

 ammoniacal aqueous solution after shaking. Shake for at least 2 minutes to 

 extract copper, lead, and zinc (also cobalt, cadmium, and mercury (ic) when 

 present). Allow the mixture to separate and draw off the solvent layer into a 

 second separatory funnel. Discard the ammoniacal aqueous layer, which should 

 be yellow and contain the non-reacting bases and acids. 



To the carbon tetrachloride layer, add 45 ml. of water and 5 ml. of 0.20 N 

 hydrochloric acid and shake out to extract the lead and zinc as chlorideo in the 

 acid aqueous solution, which should be colorless, leaving the copper iu the solvent 

 layer which may be used for the determination of copper if desired although a 

 larger aliquot of the original ash solution is preferable. 



To the acid aqueous solution aad 20 ml. of water, 15 ml. of 0.20 N ammonium 

 hydroxide, 10 ml. of ammonium citrate solution, 5 ml. of carbamate solution, 

 10 ml. of carbon tetrachloride, a measured quantity of dithizone solution (in 

 small portions) sufficient to impart a yellowish tint to the ammoniacal aqueous 

 solution after shaking, then additional carbon tetrachloride to a total volume of 

 15 ml. including that from the dithizone. A greater dilution may be necessary 

 for some samples when a spectrophotometer is employed. This procedure is 



