where : 



Figure 13-9. Titrating the dissolved oxygen sample. 



Drain the flask well and draw a second sample 

 for titration. The second titration should be 

 reproducible within ±0.03 ml. If necessary, 

 disregard poor runs and repeat with 50 ml. of 

 sample. 



At the end of the analyses, the Erlenmeyer 

 flasks used should be washed with soap and 

 water, rinsed, and finally rinsed with distilled 

 water in preparation for the ne.xt station. 



13-53 Calculations for Dissolved Oxygen. — 

 The amount of dissolved oxygen expressed in 

 milliliters per liter (ml. /I.) is determined from 

 the average of the burette readings of the two 

 titrations of each sample by means of the fol- 

 lowing formula: 



O2 ml./l. 56.45A^F. 



This formula is derived from the equation: 



O,ml./l.=-^X5.6X10XiVXF 



thus : 



5=Volume of the sample bottle. This 



is 250 ml. 

 5-2 = Volume of water in the sample bottle 



after addition of the first two 



reagents (1 ml. of MnCU and 1 ml. 



of NaOH-KI). 

 5. 6 = A constant representing ml. of oxygen 



equivalent to one ml. of normal 



sodium thiosulfate solution. 

 10 = Number of ml. of potassium dichro- 



mate added. 

 A^= Normality of the sodium thiosulfate 



solution. 

 F= Number of ml. of sodium thiosulfate 



required for the titration. This is 



the average burette reading. 



02ml. /l.= 



250 

 250-2 



X5.6X10XA^XF 



02ml./l.=?J5x56 NV. 



therefore : 



02ml./l. = 56.45iVF 

 Another way of expressing this is: 



where: 



and 



X=56.45A^ for 100 ml. samples 



2i'=2(56.45)A^ or 112.9A^ for 50 ml. samples. 

 For example: let A?^= 0.0048. 

 Therefore (for 50 ml. samples) : 



7^=112.9(0.0048) 



/!C = 0.5419. 



Once the value K has been determined, then 

 the dissolved oxygen content is easily deter- 

 mined by multiplying each average burette 

 reading {V) by K. 



For example (if F=9.28): 



O2ml./l. = (0.5419)(9.28) 



O2ml./l. = 5.03 



136 



H. O. 607 



