﻿Manchester Memoirs, Vol. xlix. (1905), No. 10. 3 



In a search for a reaction capable of detecting 

 quantities less than the lower limit of the above reaction, 

 I found that the ordinary Nessler solution reacted with 

 formaldehyde. 



(2) With Nessler Solution. 



If to a solution of formaldehyde in a Nessler glass 

 the usual quantity of Nessler reagent (2 ccs.) is added a 

 reaction takes place dependent on the strength of the 

 formaldehyde solution. If the solution is fairly strong an 

 immediate blackish-brown precipitate is produced due to 

 the reduction of the Nessler solution. 



In weaker solutions the reaction proceeds more slowly, 

 a canary-yellow colouration being produced at first, then 

 the solution becomes slightly opalescent and of a greenish- 

 yellow colour and finally darkens, and on allowing to 

 stand a precipitate falls to the bottom, the colour of which 

 depends on the concentration of the formaldehyde solu- 

 tion. 



In a dilution of 1 part HCHO in 1,000,000 of water 

 the change from the canary-yellow to the dark brown 

 takes about one hour to complete. The reaction is quite 

 apparent at a dilution of 1 part HCHO in 2,000,000, and 

 the suspended matter settles down to the bottom in 24 

 hours. 



This reaction is therefore of great delicacy, and can 

 be used for detecting minute traces of HCHO. 



(1) Zeits. /. analyt. Chemie, vol. 36, pp. 18-24, 1897. 



