Smith — Deiermination of Chlorine in Natural Waters. 361 



3.) The aiiioiint of standard sohition necessary to strike the 

 reddish-yellow color, and thus determine definitely the 

 end-reaction. 

 These are real difficulties, and are of importance, each in its 

 own wav, in estimatino^ the small amount of chlorine in normal 

 natural waters. To meet and reduce the eiTors of observation 

 thus introduced, and in general to make the chlorine determina- 

 tion more exact the following method has been adopted at this 

 laboratory : 



A. Apparatus required. 



1.) A long thin 10 c. c. Greiner Zero burette graduated to 



1-10 c. c. 

 2.) Three white porcelain dishes imiform in color, shape 



and depth, and of a capacity of a little over 100 c. c. 



B. Solutions required. 



1.) Standard silver nitrate. The solution to be of such 

 strength that each c. c. will precipitate exactly one mil- 

 ligTam of chlorine, i. e., 1 c. c. = 0.001 CI. 



Made by dissolving 4.8022 grams pure silver nitrate 

 in water. Take a clean, smooth, porcelain crucible, ig- 

 nite thoroughly and weigh. In the crucible place 

 something over 5 grams of crystals of pure silver ni- 

 trate and fuse over a Bunsen flame turned so low that 

 the flame does not play on the wall of the crucihle above 

 the salt. The crystals should melt to a white clear 

 mass without any trace of decomposition along the wall 

 of the crucible. Allow to cool and weigh accurately. 

 The amount of water in which this is to be dissolved 

 can be conveniently reckoned as follows : 



4.8022 : wt. silver nitrate actually taken : : 1000 

 c. c. : vol. water required. 



The fused silver nitrate is dissolved in some cold 

 chlorine-free water prepared by redistillation with sil- 

 ver nitrate as given belo^w under note and made up to 

 above calcalated volume. Prepared in this manner 

 the solution is very accurate but as a precaution the 

 work should be confirmed by titration against the 

 standard sodium chloride solution prepared below. 



