VII] INSECTICIDES AND FUNGICIDES 75 



tin supposed to be present. Stir thoroughly and let stand overnight. Before filter- 

 ing, stir the precipitate to see that it settles quickly and is in crystalline form; then 

 filter on an ashless filter paper, and wash with cold dilute hydrochloric acid (1 to 

 1000). Transfer the paper and precipitate to a weighed platinum crucible, dry 

 carefully, and ignite until all carbon is destroyed. Finally heat over a Teclu or 

 Meker burner for not more than 10 minutes. The weight of the residue multiplied 

 by 0.114 gives the weight of nicotin present in the aliquot. 



FORMALDEHYDE SOLUTIONS. 



FORMALDEHYDE. 



Hydrogen Peroxid Method.^^ — Official. 



64 REAGENTS. 



(a) N/1 sulphuric acid. 



(b) N/1 sodium hydroxid. — One cc. is equivalent to 30.02 mg. of formaldehyde. 

 (C) Hydrogen peroxid. — An approximately 3% solution. If the hydrogen per- 

 oxid solution is acid, neutralize with (b), using litmus solution as indicator. 



(d) Litmus solution. — A solution of purified litmus. 



65 DETERMINATION. 



Measure 50 cc. of N/1 sodium hydroxid into a 500 cc. Erlenmeyer flask and add 

 50 cc. of the hydrogen peroxid. Then add 3 grams of the formaldehyde solution un- 

 der examination, allowing the point of the pipette to reach nearly to the liquid in the 

 flask. Place a funnel in the neck of the flask and heat on the steam bath for 5 min- 

 utes, shaking occasionally. Remove from the steam bath, wash the funnel with 

 water, cool the flask to about room temperature, and titrate with N/1 acid, using 

 the litmus solution as indicator. It is necessary to cool the flask before titration 

 with the acid to get a sharp end point with the litmus. Calculate the per cent of 

 formaldehyde. 



Cyanid Method.^"— Official. 



66 REAGENTS. 



(a) N^/IO silver nitrate. 



(b) N/10 ammonium sulphocyanate. 



(C) Potassium cyanid solution. — Dissolve 3.1 grams of potassium cyanid in 500 

 cc. of water. 



(d) 50% nitric acid. 



67 DETERMINATION. 



Treat 15 cc. of the N/10 silver nitrate with 6 drops of the 50% nitric acid in a 

 50 cc. volumetric flask; add 10 cc. of the potassium cj-^anid solution, dilute to the 

 mark, shake well, filter through a dry filter and titrate 25 cc. of the filtrate with 

 N/10 ammonium sulphocyanate as directed under III, 15. Acidify another 15 cc. 

 portion of the N/10 silver nitrate with 6 drops of the 50% nitric acid and treat with 

 10 cc. of the potassium cyanid solution to which has been added a measured quan- 

 tity (the weight of which must be calculated from the specific gravity) of the for- 

 maldehyde solution containing not over 2.5 grams of a 1% solution or the equiva- 

 lent. Make up to 50 cc, filter and titrate a 25 cc. aliquot with the N/10 ammonium 

 sulphocyanate for the excess of silver as before. The difference between the num- 

 ber of cc. of N/10 ammonium sulphocyanate used in these 2 titrations, multiplied 



