1. Blank Run. — The blank run is made to 

 determine the correction to apply for the 

 amount of lo-liberating oxidizing substances or 

 reductants present as impurities ni the reagents. 

 A blank run should be made after the reagents 

 are prepared to determine that they are satis- 

 factory, and that the titration equipment is 

 functioning properly. In addition, blank rmis 

 are made before each series (maximmn of 24) 

 of oxygen samples is analyzed. To make the 

 blank run perform the following steps : 



Step 1. With the automatic curette assem- 

 bled and supplied with sodiimi thiosulfate solu- 

 tion (See ng. J-2), place a clean, 1-inch, 

 teflon-covered magnetic bar in a clean 125-ml. 

 Erlenmeyer flask. 



Step 2. Pipette 5.0 ml. of potassium iodate 

 (or biiodate) solution into the flask ; then, add 

 90.0 ml. of distilled water. 



Step 3. Add 2.0 ml. of ION H2SO4 and 2.0 

 ml. of NaOH-Nal solution. 



Step 4. Mix solution with the magnetic stir- 

 rer for 1 minute; then add 2.0 ml. of MnCl; 

 reagent and 1.0 ml. of starch solution. This 

 will cause the solution to turn blue. 



Step 5. Zero the burette by turning the 

 three-way stopcock and filling until burette 

 overflows; then, place flask under the delivery 

 tip, open three-way stopcock and titrate imtil 

 the instant the solution becomes colorless. As 

 the endpoint is approached, reduce delivery to 

 drop by drop, then half drops until colorless 

 end point is reached. 



Step 6. Read the burette to a hundredth of a 

 ml., and record the value in Fb {Isf Run) space 

 on the log sheet. Refer to chapter I, paragraph 

 1-12, for burette reading instruction. 



Step 7. Pipette 5.0 ml. of the potassiiun 

 solution into the same flask. Repeat steps 5 and 

 6. Record burette reading in T',, {2d Run) 

 space. Vb's must agree within ±.10 ml. 



If impurities are present in excess of the 

 above limits, new reagents must be prepared. 



Step 8. Calculate Vb by subtracting Vb (2d 

 Run) from Vb (1st Rmi). If Vb is positive, the 

 blank is oxidizing; if negative, reducing. 



2. Standardization. — Standardization is the 

 process of determining the slight changes that 

 occur in the Na2S203 solution. Standardization 

 should be performed before each series (max- 

 imum of 24) of samples is analyzed. After the 

 blank rim, standardize according to the follow- 

 ing directions : 



Step 1. Place a clean, 1-inch teflon-covered 

 magnetic stirring bar in a clean 125-ml. Erlen- 

 meyer flask. 



Step 2. Pipette 10.0 ml. of the potassium 

 solution into the flask, and add 90.0 ml. of dis- 

 tilled water. 



Step 3. Add 2.0 ml. of ION H^SO^ and 2.0 

 ml. of the NaOH — Nal reagent, and mix. 



Step 4. Place the flask under the delivery 

 tip of the burette, and titrate solution with 

 Na2S203 until the (liberated I,) deep yellow 

 colortunis to a pale straw yellow. 



Step 5. Add 1.0 ml. of starch solution. This 

 will cause the solution to turn blue. Continue to 

 add NaoSaOs until the blue color disappears 

 and the solution is just colorless. 



Step 6. Read the burette and record the 

 value in F2 {1st Run) space on the log sheet. 



Step 7. Repeat steps 1 through 6 two times, 

 and record burette readings in Fj {2d Run) 

 and F2 {3d Run) spaces on the log sheet. Ac- 

 ceptable values must agree with ±0.03 ml. Cal- 

 culate average V2. 



3. Titration of the Oxygen Sample.— Oxy- 

 gen samples should be titrated withm 4 hours 

 of the time they are treated. Always follow the 

 same techniques for every sample. 



Step 1. Fill the self -zeroing burette by turn- 

 ing the three-way stopcock of the burette to 

 open until the sodium thiosulfate solution flows 

 out the small spout into the overflow flask. Turn 

 the stopcock to off. Arrange samples to be anal- 

 yzed in order of descending depth. 



Step 2. Record the sample bottle numbers 

 in the Sample Bottle Number column of the log 

 sheet. Check the A-Sheet to match serial num- 

 ber and sample bottle number. 



Step 3. Place a 1-inch, teflon-covered, mag- 

 netic stirring bar in a 125-ml. Erlenmeyer flask. 

 The bar and flask should be rinsed one time 

 with distilled water between samples. 



Step 4. Turn on salt water tap to activate 

 filter pump aspirator. Adjust the vacuum to get 

 gentle aspiration (fig. J-1) . 



Step 5. Shake the sample bottle vigorously. 

 Remove the glass stopper from the bottle. Place 

 the tip of the automatic pipette near the bot- 

 tom of the bottle. Turn the pipette stopcock to 

 the fill position, and slowly draw 15 to 25 ml. 

 of sample. "Withdraw the sample bottle; rinse 

 the pipette and draui. Rinse the pipette once 

 or twice, depending on the volume of sample 

 to be analyzed. Since the sample bottle for the 

 (Macro) Winkler method contains 250 ml. of 

 sample, it is recommended that a 50 ml. volume 

 sample be analyzed; however, in some areas 

 where the dissolved oxygen content of sea water 

 is extremely low, it may be necessary to analyze 

 a 100-ml. volume sample. 



Step 6. Again place the tip of the pipette 

 near the bottom of the sample bottle. Turn the 

 stopcock to the fill position, and slowly fill the 

 pipette, turn the stopcock to off position, remove 

 the sample bottle, and replace the stopper ; then, 

 wipe the pipette tip with a lab tissue. 



Step 7. Hold the flask (from step 3) beneath 

 the pipette tip and with a minimum of splash- 

 ing, drain the contents of the pipette into the 

 flask. After the pipette is drained, slowly with- 

 draw the pipette stem from the water to obtain 



J-5 



