XII] METALS IN FOODS 175 



atmosphere of carbon dioxid. Lower the tubes in the cylinder. When cool, dis- 

 connect the flasks one at a time, putting a glass plug into the carbon dioxid inflow. 

 Wash the tubes, rubber stopper and sides of the flask with the air-free wash solution, 

 add starch paste and titrate at once with the N/100 iodin. 



If it is desired to titrate by the excess method, run an excess of the N/100 iodin 

 into the flask while it is still connected with the carbon dioxid stream. Then wash 

 out the tubes and titrate the excess of iodin with the N/100 sodium thiosulphate. 



The rubber connections should be washed with water after each determination. 



8 COPPER.— TENTATIVE. 



Destroy organic matter as directed in 5. Concentrate the sulphuric acid residue 

 by continued digestion to a volume of 10-15 cc, cool, dilute with a little water, trans- 

 fer to a 400 cc. beaker, rinse the Kjeldahl flask with water, adding the rinsings to the 

 contents of the beaker, dilute to about 200 cc. and boil to expel nitrous fumes. Cool, 

 render the solution slightly alkaline with ammonium hydroxid and boil to expel the 

 excess of ammonia. Add 5 cc. of concentrated hydrochloric acid for each 100 cc. 

 of solution, heat to incipient boiling and saturate the solution with hydrogen sul- 

 phid. Allow to stand on a steam bath for a few minutes until the sulphid flocculates, 

 filter and wash the precipitate with hydrogen sulphid water. Protect the precipi- 

 tate from contact with air as much as possible, use only hydrogen sulphid water for 

 washing and carry out this operation without interruption. Reserve the filtrate 

 for the determination of zinc, if necessary. Place the filter containing the copper 

 sulphid precipitate in a small flask, add 4-5 cc. of concentrated sulphuric acid and 

 the same amount of nitric acid and heat until white fumes appear. Continue the 

 oxidation, adding a little nitric acid from time to time, until the liquid remains 

 colorless upon heating to the appearance of white fumes. Cool, dilute with about 

 30 cc. of water, add an excess of bromin water and boil until all bromin is expelled. 

 Determine the copper as directed in VIII, 29, using N/100 sodium thiosulphate for 

 the titration. 



9 ZINC— TENTATIVE. 



Proceed as directed in 8 to the point indicated by the sentence "Reserve the fil- 

 trate for the determination of zinc, if necessary". Boil the filtrate, containing the 

 zinc, to expel hydrogen sulphid and to reduce the volume to about 250-300 cc, add 

 a drop of methyl orange and 5 grams of ammonium chlorid and make alkaline with 

 ammonium hydroxid. Add dilute hydrochloric acid, drop by drop, until the re- 

 action is faintly acid, then add 10-15 cc. of 50% sodium or ammonium acetate solu- 

 tion and pass in hydrogen sulphid for a few minutes until precipitation is complete. 

 Allow the precipitate to settle, filter, refilter, if necessary, until the filtrate is clear, 

 and wash the precipitate twice with hydrogen sulphid water. Dissolve the precipi- 

 tate on the filter with a little hydrochloric acid (1 to 3), wash the filter with water, 

 boil the filtrate and washings to expel hj'drogen sulphid, cool and add a distinct 

 excess of bromin water. Then add 5 grams of ammonium chlorid and ammonium 

 hydroxid until the color, caused by free bromin, disappears. Add hydrochloric 

 acid (1 to 3), drop by drop, until the bromin color just reappears, then add 10-15 cc. 

 of 50% sodium or ammonium acetate solution and 0.5 cc. of 10% ferric chlorid solu- 

 tion, or enough to precipitate all the phosphates. Boil until all the iron is precipi- 

 tated. Filter while hot and wash the precipitate with water containing a little 

 sodium acetate. Pass hydrogen sulphid into the combined filtrate and washings 



