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VIII. CHEMISTRY. OF THE BLEED WATER TRACER 



In order to meet the specified conditions of the model tests 

 the bleed water was simulated by sulphuric acid of suitable 

 strength and specific gravity and the ambient water by ordinary 

 tap water to which was added a suitable quantity of sodium car- 

 bonate and a fraction of a gram of brom cresol green indicator 

 per model volume of about 600 liters. This choice of solutions 

 was made in order to reduce as much as possible any chemical reac- 

 tion between the solutions and the gypsum (calcium sulphate) 

 plaster of which the model was made and also to avoid alteration 

 of pH of the ambient water through acidification by atmospheric 

 carbon dioxide. 



Sulphuric acid introduced to simulate the discharge of bleed 

 water at the specified points and rates in the model necessarily 

 produced a yellow color change in the brom cresol green indicator 

 in solution with it. The color of brom cresol green in alkaline 

 solution is a pleasant blue and the color change as the pH rises 

 through the range 4.0 to 5.6 is at first green and then bright 

 yellow. The change is abrupt from blue to green and more gradual 

 from green to yellow. As the sulphuric acid flowed with the cir- 

 culation of the model a certain amount of mixing took place which 

 diluted the sulphuric acid, and at the same time reaction took 

 place with the carbonate ions present in the admixed ambient 

 water. The mixing processes were mainly due to molecular diffu- 

 sion and microtuibulent exchange across the interface between 



