7. Calculate the normality (A^) of the sodium 

 thiosulfate solution by means of the following 

 formula : 



where 



A'^j = Normality of KjCrjOr solution. This 

 is always'O.OlOO. 



Fi = Milliliters of K2Cr207 solution used. 

 This is always 10.0 ral. 



^2= Average milliliters of Na2S207 re- 

 quired to decolorize sample. For 

 example, assume 20.90 ml. 



F(,= Average milliliters correction ob- 

 tained from the blank run. For 

 example, assume 0.07 ml. 

 therefore 



10.0X0.0100 



A^=- 



20.90-0.07 



7V= 0.0048 



The normality of the sodium thiosulfate is used 

 in determining the amount of dissolved oxygen 

 content of the sea water samples from the 

 burette readings of the titrations. The formula 

 is shown in section 13-53. 



13-50 Treating the Sea Wafer Sample.— 

 Instructions for drawing the sea water samples 

 for dissolved oxygen analysis are given in sec- 

 tions 2-26 and 2-28. These must be followed 

 very carefully. Immediately after drawing 

 the samples from the Nansen bottles, they 

 must be taken to the laboratory and the fol- 

 lowing reagents introduced. Use an auto- 

 matic pipette and introduce the reagents well 

 below the surface of the sample: 



1 ml. of manganous chloride (MnClz- 



4H2O) solution 



1 ml. of the alkaline iodide (NaOH-KI) 



solution. 

 Replace the stopper and shake the sample 

 thoroughly with a snapping motion of the 

 wrist. Allow the precipitate to settle for a 

 few minutes. Shake the sample a second 

 time and allow the precipitate to settle again 

 for at least % hour, but not longer than 6 hours. 



The alkaline iodide solution precipitates 

 manganous hydroxide which reacts with the 

 dissolved o.xygen. The precipitate settles to 

 the bottom of the bottle. After sufficient time 

 for this to occur {% hour minimum to 6 hours' 

 maximum), remove the stopper and with an 

 automatic pipette add (well down in the 



sample) 2 ml. of concentrated hydrochloric 

 acid (HCl). Replace the stopper and shake 

 thoroughly until all the precipitate is dis- 

 solved. This is now the "treated sample" 

 and is ready for titration. 



The introduction of the reagents causes a 

 liberation of part of the nitrogen dissolved in 

 the sample of sea water. In addition, the acid 

 may cause a small amount of carbon dioxide 

 (002) to be liberated. As a result, small 

 bubbles of gas may occur in the sample after 

 the precipitate has dissolved. This gas devel- 

 opecl in the sample before titration only affects 

 the results by diminishing the volume of the 

 bottle's contents, and is of no importance as 

 an aliquot (50-ml. or 100-ml. portion) of the 

 sample is used for the titration. 



13 51 Drawing the Sample With the Auto- 

 matic Pipette. — Drawing the sample for an- 

 alysis is carried out, for the most part, in the 

 same manner as described in section 13-28. 

 The exception to the method is that for rins- 

 ing. As the volume (50 to 100 ml.) of the 

 sample to be titrated is considerably greater 

 than that used for salinity, it is necessary to 

 rinse the pipette with the sample only once 

 before drawing the sample. Use only about 

 15 ml. for rinsing. 



After rinsing draw the sample and transfer 

 it to a clean 250-ml. Erlenmeyer flask. 



13-52 Titrating the Sample With the Auto- 

 matic Burette. — Analysing the sample by 

 titration is carried out in precisely the same 

 manner for eveiy sample. Unless otherwise 

 directed, this technique should never vary. 



To fill the burette with sodium thiosulfate 

 solution turn the three-waj' stopcock at the 

 bottom of the Schellbach burette until the 

 solution flows up the bore and overflows out 

 the small spout into the drain cup at the top. 

 Turn to off position and the self-zeroing burette 

 is ready. It is well to fill and drain the burette 

 twice before commencing titrations. 



Insert a clean plastic-coated magnet in the 

 flask and place it on the magnetic stirrer. 

 Titrate with the standardized sodium thio- 

 sulfate solution until the j^ellow color of the 

 sample has almost disappeared. Add six drops 

 of starch indicator solution. This will produce 

 a deep blue color. Continue titration drop by 

 drop until the solution is just colorless. The 

 sample is stirred constantly without splashing 

 throughout the run (fig. 13-9). 



The instant the sample becomes colorless, 

 close the stopcock and immediately read the 

 burette. Record the burette reading in milli- 

 liters on the C-sheet in the manner described in 

 chapter 14. 



H. O. 607 



135 



