270 Mr Bevan, A Method of following the Course 



monomolecular course when the water is in large excess. The 

 method adopted by Buchanan was to titrate a specimen of the 

 reacting mixture, at various times after the beginning of the 

 action, in order to determine the acidity of the mixture. Since, 

 as a result of the action, we have two molecules of acid for one at 

 the beginning, these titrations give a measure of the proportion 

 of monochloracetic acid decomposed. It occurred to the author 

 that a determination of the conductivity of the solution in which 

 the action was taking place might be a delicate method of finding 

 the amount of decomposition that had occurred at any instant, 

 and that this method would afford a means of studying the initial 

 stages of the action, a part of the action to which the titration 

 method is by its nature inapplicable. One obvious advantage 

 of such a method is that the action can be allowed to progress 

 steadily and need not be disturbed at all during the course of an 

 experiment. 



The resistance of the solution was measured by the commu- 

 tator method of Fitzpatrick, and this was found to give results of 

 more than sufficient accuracy. The regulation of temperature, 

 with the thermostat used, was not sufficiently delicate to justify 

 the accuracy that could be obtained in the resistance measure- 

 ments. The thermostat was used with a toluol regulator, and the 

 temperature, except in case of accident, did not vary more than 

 one-fifth of a degree centigrade through an experiment lasting 

 some days. The temperature at which the action was conducted 

 was 90° C. 



Assuming that the action is a monomolecular one we should 

 have, if x be the quantity of monochloracetic acid decomposed, 



x = a (1 — e~ u ), 



where a is the initial quantity of monochloracetic acid and k the 

 velocity constant for the reaction. The constant k is independent 

 of the initial concentration provided that the water is in large excess, 

 which was always the case in the present experiments. The pro- 

 portion of monochloracetic acid was never more than 4 grammes 

 to a litre of solution. 



The following table gives the results obtained for an experi- 

 ment in which the action was allowed to progress nearly to an 

 end. The first column gives the time from the beginning of the 

 action, the second the observed resistance, the third the change of 

 conductivity, and the last the value of k obtained from the 

 expression 



k = - log . 



t a — x 



The monochloracetic acid was freshly distilled and dissolved 



