ALDEHYDES. KETONES. 491 



Formaldehyde gas is now very generally used for disinfecting rooms, etc., 

 and has practically displaced the method of burning sulphur to obtain sulphur 

 dioxide. The simplest method of filling a closed space with the gas is to pour 

 the commercial solution of formaldehyde upon small crystals of potassium per- 

 manganate, contained in a spacious metallic vessel. A vigorous reaction takes 

 place, with destruction of a portion of the formaldehyde, approximately 

 according to this reaction : 



4KMn0 4 + 3HCOH + H 2 = 4MnO(OH) 2 + 2K 2 C0 3 + CO 2 . 



The great heat produced causes nearly all the remaining solution to vaporize 

 and fill the space with formaldehyde gas and water vapor, which latter is an 

 essential factor in the disinfection. The temperature of the room should be 

 not less than 10 C. (50 F.), but a higher temperature is better. The propor- 

 tions adopted by some Boards of Health are 500 c.c. of formaldehyde solution 

 and 237 grammes of potassium permanganate per 1000 cubic feet of space. It 

 is well known that formaldehyde is mainly a surface disinfectant, having very 

 little power to penetrate objects, as clothing, etc. 



The formaldehyde odor clinging for days to rooms which have been disin- 

 fected by it may be quickly removed by evaporation of some ammonia water, 

 hexamethylene tetramin, (CH 2 ) 6 N 4 , being formed. 



Acetic aldehyde, C 2 H 4 O or CH 3 .C (Ethyl aldehyde). Alcohol 



may be converted into aldehyde by the action of various oxidizing 

 agents ; the one generally used is potassium dichromate in the pres- 

 ence of sulphuric acid, which oxidizes two hydrogen atoms of the 

 alcohol molecule, converting it into aldehyde : 



C 2 H 6 + O = C 2 H 4 + H 2 0. 



Experiment 56. Place in a 500 c.c. flask, provided with a funnel-tube and 

 connected with a Liebig's condenser, 6 grammes of potassium dichromate. 

 Pour upon this salt through the funnel-tube, very slowly, a previously pre- 

 pared and cooled mixture of 5 c.c. of sulphuric acid, 24 c.c. of water and 6 c.c. 

 of alcohol. Chemical action begins generally without application of heat, and 

 often becomes so violent that the liquid boils up, for which reason a large flask 

 is used. The escaping vapors, which are a mixture of aldehyde, alcohol, and 

 water, are collected in a receiver kept cold by ice. From this mixture pure 

 aldehyde may be obtained by repeated distillation. Use the distillate for 

 silvering a test-tube by adding some ammoniated silver nitrate. How much 

 potassium dichromate is needed for the conversion of 5 grammes of pure 

 alcohol into aldehyde? 



Aldehyde is a neutral, colorless liquid, having a strong and charac- 

 teristic odor ; it mixes with water and alcohol in all proportions and 

 boils at 21 C. (69.8 F.). The most characteristic chemical property 

 of aldehyde is its tendency to combine directly with a great number 

 of substances; thus it combines with hydrogen to form alcohol, with 

 oxygen to form acetic acid, with ammonia to form aldehyde-ammonia, 



