CHAPTER J 



DISSOLVED OXYGEN CONTENT DETERMINATION OF SEA WATER SAMPLES 



J-1 General. — The concentration of dissolved 

 oxygen in sea water may vary from supei-satui'a- 

 tion near the surface, where photosynthetic ac- 

 tivity by the phytoplankton is veiy liigh, to no 

 oxygen in stagnant basins or deep fjords. The 

 values, therefore, may be anything from to 10 

 milliliters or more per liter of sea water. 



The analysis for dissolved oxygen in sea water 

 is important for numerous reasons : It aids in 

 the interpretation of biological processes taking 

 place in the ocean. It is finding increased use 

 in studies of oceanic currents and mixmg proc- 

 esses. And it is sometimes used as an index for 

 detecting malfunctional sampling equipment 

 and en-oneous values. 



Oxygen samples are drawn for a specific 

 analysis method: (1) The (Modified) Wmkler 

 (Macro) Method, (2) the Chesapeake Bay In- 

 stitute technique for the Winkler Method, or 

 (3) the Gas Chromatography Method. 



J-2 Modified Winkler (Macro) Method.— The 



chemical reactions involved in the modified 

 Winkler (Macro) Method are rather complex, 

 and the complete reactions imknown; however, 

 the analysis itself is not difficult to perform if 

 the necessary precautions are taken in prepar- 

 ing the reagents, in cleaning the glassware, 

 and in carrying out the treatment of the 

 samples and the titrations. 



The method requires that the sample be 

 treated with an alkaline manganous solution 

 while it is protected from oxygenation by air. 

 A white precipitate of manganous hydroxide 

 forms first. 



Mn'^"^+20H" ^Mn(0H)2 



This precipitate rapidly turns brown in the 

 presence of dissolved oxygen as it reacts \vith 

 the manganous hydroxide to form a tetravalent 

 manganese compound. 



2Mn(OH)2+02^2MnO(OH)2 



When this solution is acidified to excess in 

 the presence of an iodide, iodine is released 

 quantitatively; i.e., free iodine (more correctly, 

 triiodide ion) is liberated from the iodide which 

 is equivalent to the amount of dissolved 

 oxygen present in the sample : 



MnO(OH)2+4H +3l 



>Mn +l3-f3H20 



This free iodine (or triiodide ion) is titrated 

 with a standardized solution of sodium thio- 

 sulf ate : 



l3+2S20l^3l+S406~" 



J-3 Chemicals Required. — The following 

 chemicals are required for the (Macro) Win- 

 kler method : 



Manganous Chloride (MnCl.-4H,0) (C.P., 



Sodium Hydroxide (NaOH) (C.P., A.R.) 



Sodium Iodide (Nal) (C.P.,A.R.) 



Sulfuric Acid (H,Sa) (C.P., A.R.) 



Starch soluble (purified) 



Sodium Thiosulfate (NaoSsOs-SH^O) (C.P., 

 A.R.) 



Potassium Biiodate (KH(I03)2) or Potas- 

 sium lodate (KIO3) (C.P.,A.R.) 



Sodium Carbonate (Anhydrous) (NaaCOs) 

 (C.P.,A.R.) 



Chromerge cleaning solution 



The various compounds required in this 

 method must be free of oxidizing agents, and 

 every effort must be taken to prevent contami- 

 nation. All contamers must be clean, rinsed in 

 distilled water, and dried in an oven before use. 

 Amber glass jars with a vinyl plastic cap are 

 strongly recommended for the dry chemicals^ 

 Extreme care must be taken when handling 

 chemicals, especially concentrated sulfuric acid 

 and strong sodium hydroxide solution. These 

 materials will cause severe burns and must be 

 removed immediately from the skin and clothes 

 with large quantities of water. Safety glasses 

 or goggles must be worn during the prepara- 

 tion of the alkaline-iodide solution and chromic 

 acid cleaning solution and also when cleaning 

 glassware with cleaning solution. 



J-4 Apparatus Required. — In addition to the 

 apparatus previously listed in Chapter H, the 

 following apparatus is required to carry out 

 oxygen titrations by the Modified Winkler 

 (Macro) Method: 



Bottle, amber glass*, S.T. stopper, capacity : 

 1,000 ml. 



Bottle, dropping, capacity : 125 ml. 



Bottle, polyethylene, screw cap, capacity : 500 

 and 1,000 ml. 



•Heat resistant borosilleate glass Is preferred. 



J-1 



