support rods that are rigidly attached to the 

 titration bench. Attach the filter pump, or 

 aspirator, to the salt water tap and connect it 

 to the pipette with rubber tubing. If a salt 

 water tap is not available, a small electric 

 vacuum pump of the portable laboratory type 

 maj' be used in olace of the aspirator. 



Attach the magnetic stirrer below the burette. 

 When it is being used, the top of the stirrer 

 should be covered with a clean piece of white 

 filter paper. This helps detect the end point 

 color change, which is from blue to colorless in 

 oxygen determinations. 



Sea racks to hold three 500-ml. reagent bot- 

 tles, the starch solution dropping bottle, the 

 sample bottles and several Erlenmeyer flasks 

 should be secured to the top of the table. 

 Above them there should be a rack to hold the 

 various small pipettes required for the normality 

 determination and treatment of th? sea water 

 samples before titrating. 



13-41 Reagent Storage Bottles.— The stor- 

 age bottles for the various reagents used in this 

 determination should be of adequate capacity 

 to carry out the work over one complete leg of 

 the cruise. Those required for the storage of 

 the sodium thiosulfate and the sodium hydrox- 

 ide-potassium iodide solutions must be either 

 amber glass or bottles which have been painted 

 black to protect the reagents from light. The 

 sodium thiosulfate bottle should have a capacity 

 of about 3 liters. A convenient size for the 

 manganous chloride and alkaline iodide and the 

 acid storage bottles is about 2 liters. The shelf 

 bottles used for these solutions should be about 

 500 ml. capacity. 



13-42 Manganous Chloride Solution. — The 

 manganous chloride solution is prepared by dis- 

 solving 400 grams of manganous chloride 

 (MnCl2.4H20) in distilled water and making 

 up to 1 liter. The manganous chloride must 

 be free of ferric compounds. 



13-43 Alkaline Iodide Solution. — Dissolve 

 360 grams of sodium hydroxide (NaOH) in 500 

 ml. of distilled water in a Pyre.x beaker, and 

 150 grams of potassium iodide (KI) in 200 ml. 

 of distilled water. Add the potassium iodide 

 solution to the sodium hj'droxide solution and 

 dilute to 1 liter. Store this solution in an amber 

 glass bottle with a rubber stopper. Because of 

 freezing, ground glass stoppers are not recom- 

 mended. It is important that the potassium 

 iodide be free of periodate, as well as peroxide 

 impurities. This solution must be kept out of 

 contact with sunlight. 



13-44 Starch Solution. — Dissolve 3 grams 

 of soluble potato starch in a small amount of 

 cold water and stir into 300 ml. of boiling 



H. O. 607 



350676 0—56 10 



water. Cover with a cover glass and boil 

 vigorously for a few minutes. Allow the solu- 

 tion to cool, and pour off the supernatant liquid 

 into a 125-ml. dropping bottle. Discard the 

 residual portion. The period of stability of 

 this indicator solution varies from a few days 

 to several weeks depending upon local con- 

 ditions. The addition of 2 or 3 drops of 

 chloroform (CHCU) as a preservative will help 

 keep the starch stable for a long time. 



13-45 Potassium Iodide Solution. — A 15- 

 percent solution of potassium iodide is prepared 

 by dissolving 15 grams of C. P. grade potassium 

 iodide in 100 ml. of distilled water. The 

 potassium iodide should be free of oxidizing 

 substances. 



13 46 Potassium Dichromate Solution. — 

 The stock solution of potassium dichromate 

 must be made up ashore as it requires accurate 

 weighing of chemicals with an analytical 

 balance. Powder a small amount of C. P. 

 grade potassium dichromate (K2Cr207) in a 

 mortar, place it in an oven at 130° C, and dry 

 for 30 minutes; cool in a desiccator. Accu- 

 rately weigh out on an analytical balance 

 4.9040 grams of the dried potassium dichromate. 

 Dissolve this in distilled water and dilute to 

 exactly 1 liter in a volumetric flask. After 

 thoroughly mixing by placing a stopper in the 

 flask and shaking vigorously, divide equally 

 between two 500-ml. glass storage bottles. 

 Seal with paraffin until needed. Open only 

 one bottle at a time and reseal after use. This 

 is the stock solution of 0.1 normality. 



To make the standard solution, which is 

 required for the standardization of the sodium 

 thiosulfate solution, carefully measure out 50 

 ml. of the stock solution using a 50 ml. 

 NORMAX delivery pipette. Dilute this solu- 

 tion to exactly 500 ml. in a volumetric flask. 

 Transfer this solution to the storage bottle 

 from which it will be used to standardize the 

 sodium thiosulfate solution. The normality 

 of the potassium dichromate solution will be 

 exactly 0.0100 normal. 



13-47 Sodium Thiosulfate Solution.— The 

 sodium thiosulfate solution is used to titrate 

 the sea water samples. To make it, dissolve 

 3.723 grams of sodium thiosulfate 

 (Na2S203-5H20) in 3 liters of distilled water 

 that has been freshly boiled for about 10 

 minutes to expel carbon dioxide. The nor- 

 mality of this solution is approximately 0.005N. 

 Store the solution in an amber glass bottle 

 out of contact with the carbon dioxide of the 

 air or direct sunlight. If an amber bottle is 

 not available, then one painted in the same 

 manner described in section 13-24 may be 



133 



