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the sulphuric acid solution and the ambient water. Thus there 

 was a steep gradient of pH through this boundary and the color 

 change of the indicator with pH was at first too abrupt for the 

 individual stages of color to be discerned. As time passed and 

 distance from the source increased the gradient between the acid 

 and ambient alkali usually weakened sufficiently for the indi- 

 vidual color changes to be observed. At length when the acid 

 concentration was no longer sufficient to maintain a pH greater 

 than 4.0 the acid became invisible, or neutralized. As pointed 

 out earlier, mixing processes in the model were insufficient to 

 represent the rate of mixing known from field evidence to exist 

 in the prototype, and, since only mixing would permit the process 

 of chemical neutralization to take place, the acid color of the 

 bleed water tracer was generally maintained too long and too far 

 from the source. 



Generally speaking the concentration of a sulphuric acid 

 and a sodium carbonate solution can be adjusted so that the inter- 

 mediate color change from greenish yellow to green wHl take place 

 when one volume of the acid is thoroughly mixed with any speci- 

 fied number of volumes of carbonate solution. One gram molecular 

 weight of dilute sulphuric acid will be neutralized by one gram 

 molecular weight of sodium carbonate in aqueous solution. Thus 

 if a day's discharge of bleed water is represented by the dis- 

 charge of a certain volume of liquid containing 0.01 gram molecu- 

 lar weight of sulphuric acid, the presence of this acid will be 



