FUNGICIDES 651 



Find the percentage of formaldehyde by multiplying by two 

 the number of cubic centimeters of soda solution used, when 

 three grams of substance are examined. 



Acetaldehyde reacts much more slowly with the reagent than 

 formaldehyde, and it is doubtful whether the reaction is quanti- 

 tative. 



Paraldehyde reacts still more slowly with hydrogen peroxid, 

 even in the presence of a trace of ferrous salt. Benzaldehyde is 

 acted upon more quickly, particularly in presence of ferrous sul- 

 fate, and a considerable evolution of gas takes place, but a long 

 time is needed to complete reaction. 



Formaldehyde. Method III. 35 This method is to be used es- 

 pecially in solutions containing a small amount of formaldehyde. 



Solutions Required. (a) Silver nitrate. A tenth-normal so- 

 lution. 



(b) Ammonium sulfocyanate. A tenth-normal solution. 



(c) Potassium cyanid. A solution containing 3.1 grams to 

 500 cubic centimeters of water. 



(d) Nitric acid. A 50 per cent, solution. 



Determination. Treat 15 cubic centimeters of the silver ni- 

 trate with six drops of 50 per cent, nitric acid in a 50 cubic cen- 

 timeter flask; add 10 cubic centimeters of the solution of potas- 

 sium cyanid and shake well. Then make the solution up to the 

 mark and titrate an aliquot of the filtrate (say 25 cubic centi- 

 meters) with tenth-normal solution of ammonium sulfocyanate 

 for the excess of silver. Acidify another 15 cubic centimeter 

 portion of tenth-normal silver nitrate with six drops of 50 per 

 cent, nitric acid and treat with 10 cubic centimeters cf the potas- 

 sium cyanid solution to which has been added a weighed quan- 

 tity of the dilute formaldehyde solution. Make up the whole 

 to 50 cubic centimeters and titrate a 25 cubic centimeter filtrate 

 from it with tenth-normal ammonium sulfocyanate for the excess 

 of silver as before. The difference between these results multi- 

 plied by two gives the amount of potassium cyanid that has been 

 used by the formaldehyde in terms of tenth-normal ammonium 

 sulfocyanate. Each cubic centimeter of this is equivalent to 

 three milligrams of formaldehyde. 



85 Romijn, Zeitschrift fiir analytische Chemie, 1897, 36 : 18. 



