RICHARDS AND STULL. — BROMINE AND OXALIC ACID. 331 



the second series it had been found that the solution might contain any 

 amount from one per cent to five per cent of oxalic acid without any 

 serious variation in the speed. 



Hence the peculiarities exhibited by these curves must be due essen- 

 tially to the presence of varying hydrobromic acid and bromine. The 

 immensely retarding effect of the hydrobromic acid is at once mani- 

 fest, without further calculation, and evidently the greater the amount of 

 hydrobromic acid, the less rapid is the change of rate as the reaction 

 progresses. 



Thus the downward convexity in Series IV is strongly marked, while 

 in the case of Series VI the curve is but slightly bent. Tlie question at 

 once presents itself, — Would the reaction reduce to a monomolecular 

 one, depending upon the concentration of the bromine alone, if the effect 

 of the hydrobromic acid were eliminated ? The results in hand furnish 

 the data for answering this question. 



An approximate estimate of the speed of reaction at any point in any 

 one of these curves may be obtained by placing a straiglit-edge tangent 

 to the carefully drawn curve, and noting the ordinate, or bromine value, 

 which corresponds to a given time. This was done quite independently 

 by each of the authors, and the rates thus found agreed essentially with 

 one another. The averages of several series of such readings are re- 

 corded in the following table, together with the amounts of hydrobromic 

 acid and the bromine present in each case. Tiie time interval was 

 always reduced to five minutes. The various points are designated 

 by. giving the series and the times from the commencement of the 

 reaction, hence they may all be identified upon the diagram, although 

 of course the actual curves from which the readings were taken were 

 drawn upon a much larger scale. The last column contains a function 

 which would be constant if the speed were directly proportional to the 

 concentration of the bromine and inversely proportional to the concen- 

 tration of the hydrobromic acid. (See page 332.) 



In view of the complicated relations concerning the dissociation of 

 the hydrobromic acid, it is surprising that even an approximately con- 

 stant function could be found. Nevertheless, it is obvious that tiie require- 

 ment of the function — is nearly fulfilled. In a case as complicated as 



this the safest method of solution is to eliminate one variable at a time, not 

 attempting to analyze the reaction at once in a single expression. The 

 conditions of the experiment were so planned that such a method of 

 treatment is possible in this case. 



