the last drop of sample from the pipette. Follow 

 the same technique for each sample. 



Step 8. Place the flask under the delivery tip 

 of the burette. Start the magnetic stirrer and 

 stir constantly without splashing. Open the 

 burette stopcock and add sodiimi thiosulfate 

 solution imtil the dark yellow color of the sam- 

 ple becomes a pale straw yellow. 



Step 9. Add 1 ml. of starch solution. This 

 will produce a deep blue color. Continue to add 

 the sodium thiosulfate drop by drop imtil the 

 solution becomes colorless. 



Step 10. The instant the sample becomes 

 colorless, close the stopcock, and immediately 

 read the burette to the nearest hundredth of a 

 ml. Record this value in Isi Burette Reading ml. 

 colmnn. 



Step 11. Di-aui the flask well, and rinse with 

 distilled water. Repeat steps 6 through 10. Re- 

 cord the burette reading in 2nd Burette Read- 

 ing ml. column. The first and second burette 

 readings must be within ± 0.03 ml. ; otherwise, 

 run a third sample. 



4. Calculation of Dissolved Oxygen. — After 

 the samples are titrated, calculations for dis- 

 solved oxygen are performed as follows : 



Step 1. Compute the average burette read- 

 ing for each sample and algebraically add the 

 Vb (average) to compute the corrected burette 

 reading. 



Step 2. Compute the dissolved oxygen O2 

 ml/L for each sample by the formula : 



O, ml/L = K • V 



where V = Corrected burette reading and 



K = 11.380/ (V2 - Vb) for a 50 ml. sample or 

 K = 5.690/ (V.-Vb) for a 100-ml. sample. This 

 formula is derived from the equation : 



Oo in ml/L=Vx.i5^x 5.598 X^X ^/^\. 

 B-4 Vs V2— Vb 



where B = volume of water in oxygen sample 

 bottle in ml., i.e., 250 ml., B - 4 = volume of 

 water in sample bottle after treating sample 

 with 2 ml. of MnCl. and 2 ml. of NaOH-Nal, 

 5.598 = a constant representing the ml. of oxy- 

 gen equivalent to 1.0 ml. of normal sodium thio- 

 sulfate solution, and Vs = volume of pipetted 

 sample. 



Step 3. After the chemist has entered his 

 name in the appropriate block of the log sheet, 

 another member of the survey party should 

 check each computation. File the C-Sheet in the 

 oceanographic station folder with the A-Sheet. 



J-9 Securing the Apparatus After Com- 

 pleting the "ntrations. — After the samples of 

 a station have been titrated, or a day's work 

 has been finished, the apparatus must be 

 secured. The pipette and the burette will stay 

 clean longer if they are kept filled when not 

 in use. 



Rinse and fill the pipette with distilled 

 water. Fill the burette with sodium thiosulfate 

 solution and check to see that there are no air 

 bubbles. Apply a pinch clamp to tlie tubing 

 from the sodium thiosulfate solution bottle. 



Clean the table, and wash and dry all beakers, 

 flasks, magnetic stirring bars, and other 

 glassware. 



J-6 



