6 N. J. Agricultural Experiment Stations, Bulletin 355 



with various forms of aphids to determine the relation of the 

 actual amount of HCN gas present in greenhouse fumigation 

 to the amount necessary to kill the insects. 



The first step in solving the problem w^as the finding of an 

 indicator to denote quantitatively the presence of the gas. The 

 ideal indicator for this purpose is one w^hich, on the addition of 

 a definite concentration of HCN, would indicate clearly by a 

 color change or a precipitation that this concentration had been 

 reached, thus obviating the necessity for a titration of the HCN. 

 Tests were made with many substances, but all failed to reach 

 this ideal. Among the substances used were Fehling's solu- 

 tion, which reacted by changing from deep blue to colorless, but 

 lacked delicacy ; the Prussian blue reaction which was suffi- 

 ciently delicate, but was too complicated for practical use ; 

 phenolphthalein, which had to be abandoned because of the in- 

 terference of the CO2* in the air, necessitating a cjualitative sepa- 

 ration of the CO3 and the HCN, an impracticable operation. 

 The iodine test and the silver nitrate test both necessitate the 

 use of titration methods, and as the AgNOgS test gave a sharper 

 reaction with the minute quantities of HCN which were ab- 

 sorbed in the tests, it was decided to use it as indicator, titrating 

 the HCN absorbed in a dilute solution of NaOIi.$ 



Description of the Apparatus 



The apparatus in its final form consisted of a set of four 8- 

 liter aspirator jars, which draw air from the greenhouse through 

 a o.oiA^ II solution of NaOH in a test-tube by means of a modi- 

 fied Folin-Denis absorption tube. The gas was drawn from 

 various parts of the greenhouse to the absorption tubes through 

 rubber tubing. The absorption of the HCN in the aspirated air 

 was complete, as shown by testing the air for HCN by passing 

 it through silver nitrate solution after it had passed through 

 the absorber. 



Operation of the Apparatus 



In operation, a liter" of air, as measured by the amount of 

 water run out of the aspirator jar, was drawn through the rubber 

 tube leading from the greenhouse, to clean the tube preparatory 

 to absorbing the HCN in a second liter of air. This second 

 liter of air was run through the absorber, the test-tube contain- 

 ing the solution then being removed, and another put in its 

 place. Before taking a sample of air, a liter of air was arawn 

 through the rubber tubing each time. The absorptions could be 

 repeated indefinitely by filling the aspirator bottles as soon as 

 emptied. The titration of the absorbed HCN gas in the form 

 of NaCNlf was carried out in the test tubes of the absorbing 

 apparatus, v/hich were numbered and removed after a liter of 



* CO., — carbon dioxide. || O.OIA-— 1/lOOth normal. 



§ AgNO^ — silver nitrate. ° Liter — 61.023 cu. in., or 1.05071 qt. 



t NaOH — sodium hydroxide. H NaCN — sodium cyanide. 



