50 UNIVERSITY OF COLORADO 



Add a few drops of KOH, and close the flask with a one-hole rubber 

 stopper, containing a glass tube with stopcock. Boil the solution 

 for several minutes and close the cock. Cool the flask, pour the 

 contents into the tube, add 1 cc. of iodide of potassium and nitrite 

 solution, and 1 cc. dilute H 2 S0 4 (half and half). Place the burette 

 in the stopper of the wide-mouthed glass bottle, turn on the gas, 

 and after 10 minutes run the solution into the bottle, add a few 

 drops of starch and titrate, as before. The quantity required will 

 give the correction for the nitrites in the solution, and also for the 

 reagents used, as the same amount of H 2 S0 4 , and also the same 

 amount of KI, containing potassium nitrite, is used in each case. 



CALCULATIONS. 

 Let 



L = capacity of Thresh's tube, minus reagents used, = 



293—3 = 290 cc. 

 X = milligrams of oxygen per litre in the solution under 



examination. 

 M = correction for nitrites and oxygen in the reagents used. 

 N = hyposulphite of soda used in the final determination. 

 Then, 



(N— M) x .25 x 1000 

 = X. 



In an actual analysis the following results were obtained: 

 N = 10.2 cc. M = 2.8 cc. 



x= (10.2-2.8) X .25X1000 =6 . 3 mg . per litre. 



290 =.0063 grams O per litre. 



PRECAUTIONS. 



In all stages of the analysis care should be taken to prevent 

 the addition of air to the solution. 



The bromine water in the lower part of the burette under the 

 stopcock quickly deteriorates by loss of Br, and should, therefore, 

 be run off before beginning the titration. 



