58 ALDEHYDES 



Similarly H. and A. Euler* have shown that when a 2 

 per cent solution of formaldehyde is heated for some hours 

 with calcium carbonate, a pentose sugar arabinoketose is 

 produced ; in addition to this substance, glycollic aldehyde and 

 dihydroxyacetone are produced, but in smaller quantity. 



FORMALDEHYDE. 



From the point of view of photosynthesis formaldehyde 

 is of outstanding interest ; as is well known, it is at ordinary 

 temperatures a colourless gas with a pungent odour; when 

 cooled to 21 it condenses to a liquid. It is usually met with 

 in the form of an aqueous solution, commercial formalin, 

 which contains about 40 per cent of the gas dissolved in water 

 and is used as a disinfectant or as a hardening medium for 

 pathological and other specimens and occasionally as a pre- 

 servative for milk. It undergoes most of the general reactions 

 for aldehydes which have been mentioned above. 



Its peculiar behaviour towards ammonia, resulting in the 

 formation of hexamethylene tetramine, has already been 

 mentioned ; this substance, which is used under the name of 

 urotropine, is a crystalline base which dissolves in hot or cold 

 water ; with bromine it forms an additive compound tetra- 

 bromo-hexamethylene tetramine (CH 2 ) 6 N 4 Br 4 which has been 

 used for detecting small quantities of formaldehyde in solu- 

 tion. 



Formaldehyde also reacts with ammonium salts as well as 

 with free ammonia, as follows : 



6CH 2 + 4 NH 4 C1 = (CH 2 ) 6 N 4 + 6H 2 O + 4 HCI 

 Hexamethylene tetramine 



This reaction has been made use of as a means of estimating 

 ammonium salts in solution by titrating the amount of free 

 acid liberated according to the above equation on adding suffi- 

 cient formaldehyde to a solution containing ammonium salts. 

 For this purpose both the formaldehyde solution and the solu- 

 tion to be analysed must be previously neutralized, if necessary. 

 An excess of the neutralized formaldehyde solution is then 

 added to a known volume of the solution containing the 

 ammonium salts, and after thoroughly shaking for one or two 

 * Euler, H. and A. : " Ber. deut. chem. Gesells.," 1906, 39, 36, 39. 



