650 AGRICULTURAL ANALYSIS 



For determining formaldehyde in solutions of this substance 

 the following methods are commonly used: 



Formaldehyde. Method 7. 33 Solutions Required. A normal 

 solution of sulfuric acid, a normal solution of sodium hydroxid 

 and a solution of purified litmus. 



Determination. Place 50 cubic centimeters of normal sodium 

 hydroxid in a 500 cubic centimeter erlenmeyer flask and add 50 

 cubic centimeters of hydrogen dioxid. Then add three cubic 

 centimeters of the formaldehyde solution under examination (the 

 specific gravity of which has been previously determined), allow- 

 ing the point of the pipette to almost reach the liquid in the flask. 

 Place a funnel in the neck of the flask and put on the steam bath 

 for five minutes, shaking occasionally during this time. Remove 

 from the steam bath, wash the funnel with distilled water, cool 

 the flask to about room temperature, and titrate the excess of 

 sodium hydroxid with normal acid, using litmus as indicator. 

 This cooling of the flask before titration with acid is necessary 

 in order to get a sharp end reading with the litmus. From the 

 volume of formaldehyde used and the specific gravity determine 

 the per cent, of formaldehyde. 



Formaldehyde. Method II. 34 Solutions Required. Double 

 normal sodium hydroxid and double normal sulfuric acid. 



Determination. Place three grams of the solution in a tall 

 erlenmeyer flask containing 25 to 30 cubic centimeters of double 

 normal sodium hydroxid. Then gradually add 50 cubic cen- 

 timeters of pure 2.5 to three per cent, hydrogen peroxid during 

 a space of three minutes through a funnel placed in the neck of 

 the flask to prevent spurting. After standing two or three min- 

 utes, wash the funnel with water and titrate the unused sodium 

 hydroxid with double normal sulfuric acid, using litmus as in- 

 dicator. 



When analyzing solutions containing less than 30 per cent, of 

 formaldehyde, the mixture must be allowed to stand 10 minutes 

 after adding the hydrogen peroxid to complete the reaction. 



J3 Haywood and Smith, Journal of the American Chemical Society, 

 1905, 27 : 1183. 



84 Bureau of Chemistry, Bulletin 107, 1907 : 33. 



