638 AGRICULTURAL ANALYSIS 



oxids have been carefully tested by the Association of Official 

 Agricultural Chemists and have usually given good results. 



534. Lead Arsenate. 10 Lead arsenate is usually prepared by the 

 action of either lead acetate, or lead nitrate on crystallized di- 

 sodium hydrogen arsenate and usually comes on the market in 

 the form of a thick paste. In making an analysis of this com- 

 pound, the following determinations are usually made: Mois- 

 ture, total arsenic oxid, soluble arsenic oxid, total lead oxid, sol- 

 uble lead oxid, and soluble impurities (exclusive of soluble lead 

 oxid and arsenic oxid). 



General Direction. In case the sample is in the form of a 

 paste, as it usually is, dry the whole of it to constant weight at 

 the temperature of boiling water and calculate the results as 

 total moisture. Grind the dry sample (which will gain a small 

 amount of moisture by so doing) to a fine powder and deter- 

 mine the various constituents as follows : 



Moisture. Heat two grams of the sample in the water bath 

 for eight hours or in the hot air bath at 110 for five to six 

 hours or till constant weight is obtained. 



Total lead oxid. Dissolve two grams of the sample in about 

 80 cubic centimeters of water and 15 cubic centimeters of con- 

 centrated nitric acid on the steam bath : transfer the solution to 

 a 250 cubic centimeter flask and make up to the mark. To 50 

 cubic centimeters of the solution add three cubic centimeters of 

 concentrated sulfuric acid, evaporate on the steam bath to a sirupy 

 consistency and then on a hot plate till white fumes appear and 

 all nitric acid has been given off. Add 50 cubic centimeters of 

 water and 100 cubic centimeters of 95 per cent, alcohol, let stand 

 for several hours and filter off the supernatant liquid, wash about 

 ten times with acidified alcohol (water 100 parts, 95 per cent, al- 

 cohol 200 parts, and concentrated sulfuric acid three parts) and 

 then with 95 per cent, alcohol till free of sulfuric acid. Dry, 

 remove as much as possible of the precipitate from the paper 

 into a weighed crucible, and ignite at a low red heat. Burn the 

 paper in a separate porcelain crucible and treat the residue first 

 with a little nitric acid, which is afterwards evaporated off, and 



18 Haywood, Bureau of Chemistry, Bulletin 105, 1907 : 165. 



