VII] INSECTICIDES AND FUNGICIDES 67 



20 WATER-SOLUBLE ARSENIOUS OXID.— TENTATIVE. 



Proceed as directed under 12, slightly acidifying the aliquot employed with 

 hydrochloric acid before adding the excess of sodium bicarbonate. 



WATER-SOLUBLE ARSENIC OXID.-TENTATIVE. 



21 REAGENTS. 



The solutions and reagents used are described under 3. 



22 DETERMINATION. 



Transfer an aliquot, 250 cc, of the water extract, from 20, to a casserole, add 5 

 cc. of concentrated sulphuric acid, evaporate to a small volume and heat on a hot 

 plate till white fumes of sulphuric acid appear. Cover the casserole and add 1-2 

 cc. of fuming nitric acid and again heat till the appearance of white fumes. Cool, 

 add a little water and, in order to expel the last traces of nitric acid, once more 

 evaporate till white fumes appear. Cool, dilute to about 100 cc. with water, add 

 1 gram of potassium iodid^ and sufficient sulphuric acid to make the total amount 

 present about 5 cc. Boil until the vo'ume is reduced to about 40 cc. Cool, dilute 

 to about 200 cc, remove the excess iodin with N/20 sodium thiosulphate and pro- 

 ceed as directed under 3 (C) beginning with "neutralize with sodium bicarbonate." 

 Correct for the amount of the standard iodin solution necessary to produce the 

 same color, using the same reagents and volume. Subtract from the corrected titra- 

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

 arsenious oxid, obtained in 20. Calculate the per cent of arsenic oxid present. 



LEAD ARSENATE. 



23 MOISTURE.-TENTATIVE. 



(a) Powder. — Dry 2 grams to constant weight at lOS^-llO" C. and report the 

 loss in weight as moisture. 



(b) Paste. — Proceed as under (a), using 50 grams. 



Grind the dry sample to a fine powder, mix well, transfer a small portion to a 

 sample bottle and again dry for 1-2 hours at 105°-110°C., and use this anhydrous 

 material for the determination of total lead oxid and total arsenic. 



TOTAL LEAD OXID. 



24 Method I. ^—Official. 



Heat, on a hot plate, 0.6906 gram of the dry powdered sample with about 25 cc. 

 of dilute nitric acid (1 to 4) in a 600 cc. beaker. If necessarj^ remove any insoluble 

 residue by filtration. Dilute to at least 400 cc, heat nearly to boiling, add am- 

 monium hydroxid to incipient precipitation, then dilute nitric acid ,1 to 10) to re- 

 dissolve the precipitate, adding 1-2 cc. in excess. Pipette into this solution, kept 

 almost boiling, 50 cc. of a hot 10% potassium chromate solution, stirring constantly. 

 Decant while hot through a weighed Gooch, previously heated at 140°-150°C., wash 

 several times by decantation and then on the filter with boiling water until the 

 washings are colorless. Dry the lead chromate at UO'-LoO^C. to constant weight. 

 The weight of lead chromate multiplied by 100 gives the per cent of lead monoxid 

 (PbO) in the dried sample. 



The lead chromate precipitate maj' contain a small amount of lead arsenate which 

 causes slightly high results. This error rarely amounts to more than 0.1-0.2%. 



