Notes on scientific research. 



125 



neutral sodium sulphite solution (about 20 p. c.) is added and the whole titrated with 

 seminormal sulphuric acid until the red colour has disappeared. At the same time a 

 blank test should be made and the acid used up deducted from the quantity of acid 

 which is required for the determination of the aldehyde. The quantity of alkali con- 

 sumed in neutralising the aldehyde is converted into the corresponding quantity of 

 benzoic acid and the latter deducted from the quantity of benzaldehyde which has 

 been used. This method gave a result of about 99 p. c. benzaldehyde. The method 

 is however a complete failure with salicylic aldehyde and vanillin, with anisic aldehyde the 

 results obtained were too low. The calculation is made according to the following 

 equation : R • CHO + Na 2 S0 3 + H 2 O = R • C (OH) Na HS0 3 + NaOH. 



The method proposed by ). Hanus for estimating vanillin, by precipitating as 

 jp-bromophenylhydrazone, was also found to answer with anisic aldehyde. 



Feinberg obtained good results when estimating the aldehydes as />-nitrophenyl- 

 hydrazone by the method of W. Alberda van Ekenstein and J. J. Blanksma 1 ). In the 

 case of benzaldehyde he proceeds as follows: 25 cc. of a 1 p. c. benzaldehyde solution 

 (in 12 p. c. acetic acid), is diluted with 50 cc. water and mixed with 30 cc. of 30 p. c. 

 acetic acid containing double the calculated quantity of _p-nitrophenylhydrazine. After 

 5 hours the precipitate is filtered in a Gooch crucible and washed out with 10 p. c. 

 acetic acid until a well-defined coloration has ceased to ensue upon the addition of alkali. 

 The precipitate is then dried at 105 to 110° and weighed. The benzaldehyde-content 

 is found by multiplying with 0,44. Here also it is necessary to ascertain by titration 

 the quantity of benzoic acid which is present in the benzaldehyde, and to deduct it 

 from the quantity of benzaldehyde used. The average proportion of benzaldehyde 

 found was about 99 p. c. The same prescription applies to salicylic aldehyde, but in 

 this case filtration may be commenced after 1 hour (factor 0,4747). 



For vanillin and anisic aldehyde Feinberg gives the following prescription: Dissolve 

 0,5 gram aldehyde, if necessary in a little alcohol and acetic acid, and after diluting 

 with 75 cc. water and warming, mix the solution drop by drop with constant stirring 

 with a solution of ^-nitrophenylhydrazine in double-normal hydrochloric acid. Filter 

 after 30 minutes in a Gooch crucible and wash out with double-normal hydrochloric 

 acid and afterwards with water until only a faint opalescence is produced by silver 

 nitrate. For the estimation of the anisic aldehyde content use as factor 0,50188, for 

 that of vanillin, 0,5353. 



The results obtained by the various methods are summarised in the table below: 



Bisulphite 

 p.c. 



Neutral sulphite 

 p.c. 



2>-bromophenyl- 



hydrazine 



p.c. 



p-nitrophenyl- 



hydrazine 



p.c. 



Benzaldehyde . . . 

 Salicylic aldehyde 

 Anisic aldehyde . 

 Vanillin 



92,02 to 95,19 

 80,04 „ 82,23 

 89,90 „ 90,40 

 94,81 „ 95,77 



97,31 to 99,09 

 indeterminable 

 92,33 to 94,90 

 indeterminable 



91,12 to 93,55 

 93,04 „ 94,35 

 99,09 „ 99,50 

 98,50 „ 99,37 



98,12 to 99,42 

 94,40 „ 95,42 

 99,70 „ 100,25 

 99,77 „ 100,52 



The constants of the aldehydes examined were as follow: 



Benzaldehyde ... B. p. 112,5 to 113° (100 mm.), 



Salicylic aldehyde „ 166 to 166,5° (360 mm.), 139 to 139,5° (100 mm.), 



Anisic aldehyde . „ 195 to 195,5° (205 mm.). 



!) Recueil trav. chim. des P.-B. 24 (1905), 33; Chem. Zentralbl. 1905, I. 1277. 



