RICHARDS AND STULL. — BROMINE AND OXALIC ACID. 333 



0.58 and 1.7 respectively. These numbers are so nearly pro- 

 portional as to indicate beyond question a monomolecular reaction, di- 

 rectly dependent upon the concentration of the bromine alone. The 



corresponding values of - - are respectively 2.2 and 2.1, which of course 



express the same relation. For further evidence one may compare the 

 values corresponding to 98 to 184 milligrams of hydrobromic acid, in each 

 of which cases a similar equality of the " constant " is to be noted. 



The simplest interpretation of this result is to ascribe to hydrobromic 

 acid a catalytic retarding action. This would be rather a classification 

 than an explanation, however, — for no one pretends to understand the 

 exact mechanism of catalytic changes. 



In cases like the present one, where some of the side influences are 

 difficult to interpret, every possible means of determining the order of 

 a reaction should be employed. The other chief methods which have 

 been proposed are those due to van't Hoff,* who suggested calculating 

 according to the equation of each of the orders so as to find which 

 answers ; or else studying the reaction with varying dilutions, and com- 

 paring the results ; and the " isolation method,''" which consists in putting 

 in so great an excess of all except one of the reacting substances that 

 it alone changed perceptibly during the reaction, f 



The present case is too complex for the first two methods ; accordingly 

 the last method was tried, but without notable success. When fifteen or 

 twenty times as much hydrobromic acid is added to the solution as will 

 be formed during the reaction, the speed is so repressed that in order to 

 bring about any appreciable reaction, we were compelled to work at 

 comparatively high temperatures. Even at 50° C. the action was ex- 

 ceedingly slow, and a great part of loss of bromine at this temperature was 

 found to be due to its reaction with water according to the equation 

 2HoO + 2Br 2 = 2 + 4HBr. It might be argued that we could have 

 obtained the value of the speed of this last reaction, and by subtraction 

 eliminate the error introduced by it into the apparent bromine-oxalic 

 acid reaction. Such a course, however, would be unsafe, inasmuch as 

 we know nothing of the mutual influence of the two reactions. Very 

 probably the speed of reaction resulting from the action of water and 



* Van't Hoff, Vorlesung-en, I. p. 193 (1898). 



t Sec Ostvvald, Lehrbuch fur allg. Chem., 2, II. 238. Further information con- 

 cerning these matters may be had from the papers by A. A. Noyes, Zeitschr. phys. 

 Chem., 18, 118 (1895), 19, 599 (1896), also T. S. Price, ibid. 27, 479 (1898), etc. 



