METHODS OF ANALYSIS. 59 



3 to 5 cc of concentrated arsenic-free sulphuric acid, and evaporate on the steam 

 bath to a small bulk. Remove from the steam bath and treat on a hot plate till, 

 dense white fumes arise. Cool, dilute, boil, and filter into a 100 cc flask. Make up 

 to the mark and use aliquot portions for determining arsenic by means of standard 

 mirrors, or weighing the mirrors, in the Marsh-Berzelius apparatus, which method is 

 described under the determination of arsenic in cattle-food materials. Test all 

 reagents and apparatus by running a blank before using to be sure of absence of traces 

 of arsenic . 



Total copper. 



Digest 10 grams of soil on the steam bath for four hours with iOO cc of 25 per cent 

 nitric acid, using a porcelain evaporating dish with a watch-glass cover. Dilute, 

 filter into a 500 cc flask, cool, and make up to the mark. Take an aliquot of 200 cc for 

 analysis, nearly neutralize with ammonia, heat, and pass hydrogen sulphid through 

 for about half an hour. Filter off the precipitate, wash, dry, and burn in a porcelain 

 crucible. Add nitric acid to dissolve the precipitate and heat till the copper is dis- 

 solved. Filter through the smallest sized filter and determine copper by either of two 

 methods according to the amount of this element present. 



If copper is present in excess of 2 or 3 milligrams, proceed as follows: Neutralize the 

 copper solution with sodium carbonate, adding a trifling excess of the latter. Add 

 1 cc of ammonia (sp. gr. 0.960) and titrate the dark-blue solution to the disappearance 

 of the blue color with tenth-normal potassium cyanid which has been standardized 

 against a known amount of copper by exactly the same method as that just described 

 for the determination of the copper present. 



If the copper is present in very minute quantities, proceed as follows: Add ammo- 

 nium hydroxid to the copper solution until slightly in excess, heat on the steam bath 

 a few minutes, filter on the smallest sized filter, wash, and evaporate the filtrate in a 

 porcelain dish to 2 or 3 cc, cool, add acetic acid a drop at a time until the solution is 

 just acid, and then add 5 drops of 4 per cent potassium ferrocyanid. Compare this 

 color with the color produced by treating a known amount of copper dissolved in a 

 similar amount of water in a porcelain dish with acetic acid and potassium ferrocyanid 

 in exactly the same way as just described. 



Water-soluble copper. 



Weigh out 100 grams of soil, transfer to a flask, and treat with 1,000 cc of water. 

 Shake the flask occasionally during eight hours, then let stand till the supernatant 

 liquid is practically clear. Siphon off the clear liquid and filter. Evaporate a 

 750 cc aliquot of the liquid to dryness, take up the residue with 1 or 2 cc of nitric acid 

 by the aid of heat, dilute, and filter. Heat the filtrate, pass hydrogen sulphid through 

 for half an hour, filter, wash, dry, and burn the filter, and precipitate in a porcelain 

 crucible. Add nitric acid to dissolve the precipitate and heat till the copper is dis- 

 solved. Filter through the smallest sized filter and determine copper by one of the 

 methods described above under "Total copper." 



Soluble salts. 



Weigh out 50 grams of soil, transfer to a flask and treat with 1,000 cc of water, shake 

 the flask occasionally during eight hours, and let stand till the supernatant liquid is 

 practically clear. Siphon off the clear liquid and filter. Use aliquots of the above 

 liquid for determining carbonates, bicarbonates, chlorids, sulphates, calcium, magne- 

 sium, and sodium. 



Follow the methods given for determining these constituents in waters in Bulletin 

 91, Bureau of Chemistry, U. S. Department of Agriculture, except in one particular, 

 which is as follows : 



