S62 Wisconsin Academy of Sciences^ Arts, and Letters. 



From this solution one-half or one-tenth strength 

 standard solutions are easily prepared by proper dilu- 

 tion with chlorine-free water. 

 2.) Standard solution of sodium chloride. Any arbitrary 

 amount mav be taken but it is well to make it of such 

 strength that each c. c. equals one m. g. chlorine. Use 

 c. p. sodium chloride recrystallized from chlorine-free 

 water, dried and gently heated. This solution care- 

 fully prepared serves the double purpose of checking 

 the standard silver nitrate solution and for preparing 

 the standard color sometimes needed for high chlorine 

 waters, as below. 

 3.) A solution cf c. p. neutral potassium chromate. Made 

 by dissolving two grams of c. p. chlorine-free salt in 

 100 c. c. water. 

 Xote: In the j:) reparation of all of the above solutions a 

 water should be used which has been redistilled with silver ni- 

 trate. By so doinff all traces of chlorine will be removed and 

 organic matters liab-e to affect the silver nitrate destroyed. 

 C. Procedure. 



100 c. c. of redistilled chlorine-free water is measured out 

 into two of the porcelain dishes and then 1 c. c. of the chromate 

 solution added to each. The standard silver nitrate solution is 

 run drop by dro}) into one of them until a distinct reddish tint 

 or color is obtained wliich is permanent on standing five min- 

 utes. Using my own burette I have found that .1 c. c. of the 

 silver solution vrhcre 1 c. c. = 0.001 CI, is required to produce 

 this decided color, and this .1 c. c. is the ^'error of observation" 

 ordinarily employed. 100 c. c. of the water under examination 

 is then measured into the third dish, 1 c. c. chromate added, and 

 it is then titrated to the above tint of color exactly, which color 

 must remain permanent after standing five minutes. This end 

 reaction is slow in setting but is exact; the difference in the 

 readings of the burette less the error of observation is the true 

 amount of chlorine in the water. 



