gen in sea water may vary from supersaturation 

 near the surface, where photosynthetic activity 

 by tlie phytoplankton is very higli, to no oxygen 

 in stagnant basins or deep fjords. The vahies, 

 therefore, may be anything from to 10 miUi- 

 liters or more per Hter of sea water. 



The analysis for dissolved o.xygen 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 mixing proc- 

 esses; and it is sometimes used as an index for 

 detecting malfunctional sampling equipment 

 and erroneous values. 



13-37 The Winkler Method.~In waters 

 which are relatively free of such sul)Stances as 

 nitrites, ferrous salts, and organic matter, the 

 Winkler method may be used for determining 

 dissolved oxygen. These conditions are usually 

 found in most areas of the open ocean. Thomp- 

 son and Robinson (1939) describe modifications 

 wliich should be applied to Winkler's original 

 method (1888) in order to utilize the method for 

 sea water determinations. In waters containing 

 appreciable amounts of nitrites or ferrous salts, 

 the Rideal-Stewart modifications of the Winkler 

 method should be used. The Winkler method 

 is not applicable to sea water containing hydro- 

 gen sulfide or other reducing substances which 

 will react with iodide ions causing the liberation 

 of free iodine. Such conditions usually are 

 found only in stagnant basins, fjords, and cer- 

 tain estuaries. Additional methods of deter- 

 mining dissolved oxygen can be found in the 

 bibliographv at the end of part I. 



While the chemical reactions involved in this 

 analysis are rather complex and the complete 

 reactions unknown, the analysis itself is not 

 difficult to perform if the necessary precautions 

 are taken in preparing the reagents, cleaning 

 the glassware, and carrying out the treatment 

 of the samples and the titrations. 



The Winkler method requires that the sample 

 be treated with an alkaline manganous solution 

 while protected from oxygenation by air. A 

 white precipitate of manganous hydroxide forms 

 first: 



Mn++ + 20H-^Mn(0H).,. 



This precipitate rapidly turns brown in the 

 presence of dissolved oxygen, which reacts with 

 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 



H. O. 607 



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

 triodide ion) is liberated from the iodide which 

 is equivalent to the amount of dissolved o.xygen 

 present in the sample: 



MnO(OH)2-h4H+-K3I--^Mn++-f-l3-+3H20 



This free iodine (or triodide ion) is titrated with 

 a standardized solution of sodium thiosulfate: 



l2 + I-^I,f 



I3" + 2S2O3- - ^31- + S A"" " 



13 38 Chemicals Required. — The following 

 chemicals are required for dissolved oxygen ti- 

 trations. The amounts in parentheses are those 

 required to analyze 10 stations of 24 bottles 

 each. 



Chloroform, C P., A. C. S., in 4-ounce 

 bottle. 



Hvdrochloric Acid, Cone, 12N, specific 

 gravity 1.19, C. P. (1 hter). 



Manganese (ous) Chloride, C. P.. in 1- 

 pound bottle (400-gram bottle). 



Sodium Hydroxide, pellets, C. P., in 1- 

 pound bottle (360-gram bottle). 



Sodium Thiosulfate, crystal, C. P., A. C. S., 

 in 1-pound bottle (2 vials of 3.723 gms. each). 

 Sulfuric Acid, Cone, 36N, specific gravity 

 1.84, As and N2 pure, A. C. S. (320 ml.). 



Starch, potato, soluble, powder, C. P., in 

 )4-pound bottle (3-gram vial). 

 Paraffin. 



Potassium Dichromate, fine crystal, C. P., 

 in vials containing 4.9040 grams (1 vial). 

 Potassium Dichromate, fine crystal, C. P., in 

 1-pound bottle (32-gram bottle). 



Potassium Iodide, crystal, C. P., low in 

 iodates, in 1-pound bottle (150-gram bottle) 

 (1 vial of 15 grams). 



The various compounds required in this 

 method must be free of oxidizing agents and 

 every effort should be made to keep them from 

 becoming contaminated. Extreme care should 

 be exercised in purifying and recrystallizing the 

 various salts, and especially in handling the 

 hydrochloric acid. This acid is very poisonous; 

 its fumes are extremely caustic, and it should 

 be used only where good ventilation is available 

 to draw off the fumes. 



13 39 Apparatus Required. — In addition to 

 that previously listed in sections 13-2 through 

 13-8, the following apparatus is required to 

 carry out dissolved oxygen titrations by the 

 Winkler method. 



Bottle, Dropping, wS. T., ground pipette, 

 Pyrex, 125-ml. capacity. 



131 



