298 Mr Fenton, 



An Indicator for Strong Acids and Bases. By H. J. H. 

 Fenton, Sc.D., F.R.S., Christ's College. 



[Head 26 February 1906.] 



In previous communications to the Society reference has been 

 made to a new condensation-product, derived from bromomethyl- 

 furfural, which has the molecular formula C u H 8 4 and to which 

 the constitution 



CH 3 C 4 H 2 . CO . C 4 H 2 . CHO or C 4 H 3 . CO . CO . C 4 H 2 . CH 3 



has been provisionally assigned. 



It was mentioned that this substance may have useful appli- 

 cations in organic analysis since it gives highly characteristic 

 colour-reactions with certain classes of compounds such as amines 

 and ureas. 



Later experiments have shewn that this reagent may con- 

 veniently be used in the form of test papers and in this way the 

 application of the tests is greatly simplified although some of the 

 reactions are more sensitive when brought about in solution in 

 the manner previously suggested. 



Test papers prepared from an aqueous-alcoholic solution of the 

 reagent give, immediately, an intense green colour with primary 

 amines in acetic acid solution ; this colour is at once destroyed by 

 strong mineral acids or by alkalis. 



With urea in presence of strong hydrochloric acid a very 

 brilliant blue colour is slowly developed ; it appears more quickly 

 in presence of dehydrating substances such as acetyl chloride or 

 phosphorus oxychloride. 



With caustic alkalis a beautiful violet-blue colour is imme- 

 diately produced which is at once discharged by acids. The 

 colour given with normal caustic soda or potash is very intense; 

 as the concentration of the solution is diminished the colour 

 becomes paler and is only just perceptible in centi-normal 

 solutions. The gradual variation of the colour-intensity with 

 concentration is so well marked that it affords at once a rough 

 indication of the degree of alkalinity or concentration of hy- 

 droxyl ions. 



The limit at which the colour is just barely visible varies, as 

 might be expected, greatly with different bases. Rough pre- 

 liminary observations indicated that the limit of concentration 

 for sodium hydroxide, potassium hydroxide, or tetramethyl- 



