used. Add 5 or 6 drops of chloroform to retard 

 bacterial action. This solution may be stable 

 over a long period of time. It should be 

 allowed to age for several days before using in 

 order that the carbon dioxide which may be 

 dissolved in the reagent water can be used up. 



13-48 Normality Determination — The 

 Blank Run. — The normality of the sodium 

 thiosulfate solution must be determined before 

 titrating a series of samples. A correction 

 must be made if oxidizing substances are present 

 in minute quantities in any of the reagents. 

 In order to determine quantitatively the effect 

 of these oxidizing substances on the titrations, 

 a so-called blank run is made. 



This is the first step toward determining 

 the normality of the sodium thiosulfate solu- 

 tion. This blank run must be made each time 

 a new batch of reagents is prepared. Each 

 batch should be rechecked at least weekly or 

 oftener, depending upon the degree of use, and 

 the value entered in oceanographic log sheet C 

 in the manner described in chapter 14. Carry 

 out the blank run in the following way: 



1. Carefully measure out 90 ml. of freshly 

 boiled distilled water, in a 100 ml. graduate, 

 and transfer it to an Erlenmeyer flask. 



2. Add 2 ml. of concentrated hvdrochloric 

 acid (HCl). 



3. Add 1 ml. of alkaline iodide reagent 

 (NaOH-KI). 



4. Add 1 ml. of manganous chloride reagent 

 (MnCb). 



5. After thoroughly mixing, add 1 ml. of the 

 starch indicator solution. 



If no blue color appears, the reagents are 

 free of oxidizing substances. If a blue color 

 appears, the approximate concentration of the 

 oxidizing substances may be ascertained by 

 titrating the solution with the sodium thio- 

 sulfate until it becomes colorless. The burette 

 reading of the sodium thiosulfate is recorded on 

 the log sheet. This is the value V„. 



The value of Vi, should be less than 0.10 ml. 

 If more than this is required to decolorize the 

 solution, then it must be determined which re- 

 agent contains the oxidizing substances. When 

 found the reagent must be purified or replaced 

 with that which is pure. 



If present, the oxidizing substance is most 

 probably an iodate in the alkaline iodide solu- 

 tion. In such case it is well to make up a new 

 solution using potassium iodide that has been 

 purified by recrystallization. This is done by 

 dissolving potassium iodide in water, then add- 

 ing 2 ml. of sulfuric acid and concentrating the 

 solution by boiling. Iodate reacts with iodite 

 in the acid solution, and free iodine is removed 



from the solution by this process. The chemical 

 reactions are: 



I03--f8I- + 6H+-^3l3- + 3H20 



I3-^I2+I-. 



Allow the solution to cool and then reheat over 

 a low flame until the water is evaporated off. 

 Recrystallization will take place as the water is 

 driven off. This step may be carried out in a 

 drying oven set at 105° C. This boiling and 

 recrystallization should be carried out in a 

 location relatively free of sunlight. 



It is important that the hydrochloric acid 

 used be chemically pure reagent grade and not 

 contain impurities acting as oxidizing agents. 



Should the manganous chloride contain an 

 oxidizing agent in the form of a ferric ion, it can 

 be removed by the method of recrj'stallization 

 described in 13-17, the iron remaining in the 

 mother liquid. 



13-49 Normality Determination— Standard- 

 ization of tlie Sodium Tliiosulfate Solution. — 

 As previously stated, the sodium thiosulfate 

 is made stable by the addition of a few drops of 

 chloroform. However, slight changes in nor- 

 mality may occur. It is therefore necessary to 

 make at least one standardization run imme- 

 diately before running the titrations for each 

 station. If a number of stations are run at the 

 same time, the standardization runs may be 

 made at the start and again at the finish of the 

 titrations, but care must be taken to see that 

 each log sheet is completely filled out, showing 

 the complete calculations for normality and all 

 other necessary information. 



The standardization of the sodium thiosulfate 

 solution is carried out in the following manner: 



1. Accurately pipette out 10 ml. of the 

 potassium dichromate (K2Cr207) standard of 

 0.0100 N. This should be done with a NOR- 

 MAX 10-ml. volumetric pipette. Transfer to 

 a 125-ml. Erlenmeyer flask. 



2. Add 5 ml. of 15 percent potassium iodide 

 (KI) solution. 



3. Add 2 ml. of concentrated hydrochloric 

 acid (HCl). 



4 Titrate with sodium thiosulfate solution 

 until the yellow color has almost disappeared. 



5. Add 5 drops of starch indicator solution 

 and titrate until the solution is just colorless. 

 This titration should be reproducible within 

 ±0.03 ml. Make at least three runs and 

 average the best results. 



6. Record the burette readings on the C- 

 sheet as the values V2. 



134 



H. O. 607 



