An Indicator for Strong Acids and Bases. 299 



ammonium hydroxide is about 0"01 normal ; for dimethylamine 

 004 normal and for ammonia l - 8 normal. With sodium carbonate 

 the alkalinity is only perceptible in very strong solutions and with 

 borax or sodium acetate no effect at all can be observed at the 

 ordinary temperature in solutions of any concentration ; on 

 warming the strong solutions however the colour becomes 

 apparent. 



If the above-mentioned reagent is fused at about 120° with 

 urea a colourless base is obtained which immediately becomes 

 blue when treated with acids [Trans. Ghem. Soc, 1903, 187]. 

 Test-papers prepared from an aqueous-alcoholic solution of this 

 base may be used therefore as an indicator for acids. Normal 

 solutions of hydrochloric or nitric acids give a brilliant Prussian 

 blue colour, the intensity becoming less and fading to -a pale 

 sky-blue as the solution becomes more dilute. The limit of 

 concentration in this case at which the colour is just perceptible 

 is for hydrochloric acid about deci-normal. 



Half normal solutions of oxalic acid give a bright sky-blue 

 colour whereas solutions of tartaric acid of the same concentration 

 give no colour ; solutions of boric acid of any concentration give a 

 negative result. The limit for hydrofluoric acid appears to be 

 about 1*3 normal and with acetic acid the colour is not per- 

 ceptible until the solution reaches a strength of 12 or 13 

 normal. 



An acid-indicator of a different character may be prepared by 

 boiling together the first-named reagent with a primary amine, 

 such as /3 naphthylamine, in alcoholic solution. Test-papers 

 prepared with this solution give, at once, an intense green colour 

 with weak acids. The limits of sensitiveness in this case are, 

 with hydrochloric acid, between about 0*005 normal and deci- 

 normal ; with greater concentrations only a yellow colour is 

 produced. 



