66 METHODS OF ANALYSIS [Chap. 



3 amperes. After all the copper is deposited (requiring about 30 minutes), wash 

 the deposit with water bj^ siphoning, then rinse with alcohol, dry for a few min- 

 utes in an oven, weigh and calcula.e to per cent cupric oxid. 



14 Thiosulphate Method.^— Official. 



Determine copper in another aliquot of the nitric acid solution of copper oxid, 

 under 13, by titrating with N/20 thiosulphate solution, as directed under VIII, 29, 

 and calculate to per cent cupric oxiJ. 



LONDON PURPLE. 



15 MOISTURE.-TENTATIVE. 

 Determined as directed under 2. 



TOTAL ARSENIOUS OXID.«— OFFICIAL. 



16 REAGENTS. 



Described under 3. 



17 DETERMINATION. 



Dissolve 2 grams of the sample in a mixture of about 80 cc. of water and 20 cc. 

 of concentrated hydrochloric acid at a temperature of 60°-70°C.; filter and wash 

 until the combined filtrate and washings measure 250 cc. Treat 100 cc. of this solu- 

 tion with sodium bicarbonate in excess, transfer to a 500 cc. volumetric flask and 

 make up to the mark, adding a few drops of ether to destroy the bubbles. Mix 

 thoroughly and pass through a dry filter. Titrate 250 cc. of the filtrate as directed 

 under 3 (C) and calculate the per cent of arsenious oxid. 



TOTAL ARSENIC OXID.'— OFFICIAL. 



18 REAGENTS. 



The reagents and solutions used are described under 3. 



19 DETERMINATION. 



Boil, on a hot plate or over a low flame, 2 grams of the sample with 5 cc. of con- 

 centrated nitric acid and 20 cc. of concentrated sulphuric acid in a Kjeldahl diges- 

 tion flask or a covered casserole. After 10-15 minutes add fuming nitric acid or 

 powdered sodium nitrate, in small quantities at a time, until all organic matter 

 is destroyed and the solution is practically colorless. Cool, add about 50 cc. of 

 water ito decompose any nitro-sulphuric acid formed) and heat again until all 

 nitric acid fumes are expelled. Cool, transfer to a 250 cc. volumetric flask, make 

 up to the mark with water, mix thoroughly, and filter through a dry filter. 



Transfer 50 cc. of this filtrate to a 400 cc. Erlenmeyer flask, dilute with water 

 to 100 cc, add 1 gram of potassium iodid,' heat to boiling and evaporate to about 

 40 cc. not less). Cool, dilute to 150-200 cc, and remove the excess of iodin with 

 N/20 sodium thiosulphate. In case the solution is slightly colored from organic 

 matter or from any cause other than free iodin, add the thiosulphate until it is nearly 

 colorless, then a few drops of the starch indicator, and continue adding the thio- 

 sulphate slowly until the l)luo color just disappears. Continue at once as directed 

 under 3 C) beginning with "neutralize with sodium bicarbonate." Subtract from 

 this reading the number of cc. of the standard iodin solution corresponding to the 

 arsenious oxid obtained in 1 7. Calculate the per cent of arsenic oxid in the sample. 



