116 llErORTS ON THE STATE OF SCIENCE. 



for determining the partial pressure of the reagent, hydrogen chloride. 

 The form of the experimental tube was so modified that, after the change 

 had occurred (or failed to occur), the tube could be opened, attached to 

 a Sprengel pump, and its exact volume measured by collecting the air 

 iillinf it at a given temperature and pressure. The hydrogen chloride 

 was introduced as dilute standardised solutions, known weights of which 

 were used. A series of such experiments has shown that at 13° the 

 hydrogen chloride must be at a pressure of 14 mm. of mercury for a 

 change to take place. 



Transformation of the Niiroamine in Solution. 



Inasmuch as the nitroamine is colourless, whilst the nitroaniline is 

 intensely coloured, the change can be quantitatively followed by com- 

 paring the colour of the solution, during the course of the tx-ansformation, 

 with that of standard solutions of the nitroaniline. The method is 

 exceedingly delicate ; with practice it is easy to detect the difference 

 in tint between 49 and 50 c.c. of a solution of nitroaniline containing 

 0000225 gram in 100 c.c. when the tint of these volumes is compared 

 in columns 12 cm. in length; that is, 00225 milligram of the aniline 

 can be measured. 



The low solubility of the nitroaniline (100 c.c. at 14° dissolve 

 00022 gram) allows only of the use of solutions of the nitroamino 

 containing 0'003 gram per 100 c.c. in these experiments. 



The measurements of the speed of transformation show that the 

 reaction is one cf the first order. The change into the nitroaniline 

 is accompanied by a conversion (reduction) of the nitroamine into 

 diazonium salt to the extent of 30 per cent. This fact only permits of 

 the determination of an apparent value of the velocity coefficient. 



(i) Solvent. —The only solvents which could be used were water, acetic 

 acid, or mixtures of these. Alcohol and acetone, which are .solvents of 

 the catalysts, react with the nitroamine. 



As the proportion of the acetic acid in the solvent is increased 

 the rate of change slowly rises until, with 74 per cent, acetic acid, 

 the rate is double that in pure water. But the use of higher concentra- 

 tions of acetic acid is followed by a rapid rise in the speed of the 

 change, which in glacial acetic acid is several hundred times as great as 

 in water. 



(ii) Nature of the Acid {Catalyst). — The efficacy of acids as catalysts 

 in this transformation is roughly in the order of their activities as 

 measured by other reactions. Nevertheless, chloric and perchloric acids 

 appear to bring about the change twice as rapidly as hydrochloric acid, 

 although they are somewhat less active as catalytic accelerators in 

 hydrolyses, &c. The reduction of the nitroamine to diazo-compound is, 

 however, much less in the case of these acids, a fact which may account 

 for the more rapid formation of nitroaniline. 



Hydrobromio and hydriodic acids reduce the nitroamine ; the former 

 reacts quantitatively according to the equation, 



Ar.NH.NOa + 3HBr= Ar.NaBrg -f- 2H2O, 



the diazonium perbromide crystallising out when acetic acid is the, 

 solvent. Hydriodic acid carries the reduction further. 



