wash and dry all beakers, magnetic stirring 

 bars, pickup, and stirring rods. Do not leave 

 unwashed any apparatus that has been used. 



13-33 Maintenance and Repair of Appara- 

 tus. — The Knudsen titration apparatus requires 

 little maintenance except routine cleaning. 

 The silver nitrate solution causes the rubber 

 tubing to deteriorate after awhile, and small 

 black particles will begin to get into the burette. 

 The tubing should be replaced when there are 

 indications of this. It is almost impossible to 

 repair broken burettes or pipettes. Even when 

 done by experienced glassblowers they must be 

 recalibrated. It is better to use a new piece 

 of apparatus. 



13-34 Reclaiming the Silver Chloride. — 

 After a period of time the waste jar will ac- 

 cumulate a considerable amount of silver 

 chloride precipitate. This silver chloride con- 

 tains a large amount of pure silver which can 

 be reclaimed by smelting and refining com- 

 panies. To prepare this material for shipment, 

 the water should be decanted and the solids 

 removed from the bottle. It may be placed in 

 flat pans and dried in an oven. The dried 

 material should be stowed in glass bottles. 

 Care should be taken when handling as it will 

 stain. Use of rubber gloves is recommended. 

 U. S. Navy survey ships should return the 

 bottled dried material to the Hydrographic 

 Office. 



13-35 A Method for Preparing Substandard 

 Sea Water. — As the silver nitrate solution 

 must be standardized after every sixth sea 

 water sample, and to adjust the solution, a 

 considerable amount of standard sea water is 

 necessary. This water is expensive and some- 

 times difficult to obtain. To stretch use of 

 standard sea water, some laboratories make 

 use of substandard sea water in place of stand- 

 ard sea water. The advantage is that a large 

 volume of sea water of known chlorinity can 

 be made up, standardized against standard 

 sea water, and used in the latter's place while 

 running the routine analyses of sea water 

 samples. 



The easiest way to prepare substandard sea 

 water is to collect several gallons of clear sea 

 water in an area having higher chlorinity than 

 standard sea water, for example — the Sargasso 

 Sea, and diluting it to as near the chlorinity 

 of standard sea water as possible. Filter the 

 collected water at least twice through fine 

 filter paper into clean well-leached brown 

 glass bottles. 



The substandard sea water should have a 

 chlorinity as near 19.39 °/oo as possible. For 

 example, assume that the Sargasso water to be 



diluted to astandard value of 19.39%o has a 

 present chlorinity of 19.60%o. Also assume that 

 8 liters has been collected and doubly filtered. 

 To find how much distilled water to add to this 

 Sargasso water, we set up the equation: 



19.60 

 19.39' 



X 



8X1,000 

 where : 



19.60 is chlorinity of water to be diluted 

 19.39 is desired chlorinity 

 8 X 1,000 is volume of water in milliliters 

 X is volume of water after dilution. 

 Solving the equation, it is found that: 



X=8,087 ml. 



Therefore 87 ml. of pure distilled water must be 

 added to the 8 liters of Sargasso water in order 

 that it will have a chlorinity of 19.39%o- 



Having made up the substandard sea water, 

 it must be well-mixed and the chlorinity again 

 checked. Store it in 1-gallon brown glass 

 bottles and stopper tightly. Label, date, and 

 indicate the chlorinity on each bottle. The 

 chlorinity value should be checked against 

 standard sea water about once a week after 

 thoroughly shaking the bottle. Note each of 

 these checks on the label. 



There are times when substandard water must 

 be made up with water having chlorinity lower 

 than 19.39°/oo. In this case, the watermusthave 

 sodium chloride added to bring it to the required 

 value. For example, assume that the water 

 after filtering twice has a chlorinity of 19.30°/oo, 

 and it is desired to bring it up to the required 

 value. Again assume there are 8 liters of sample. 

 How much sodium chloride must be added? 

 The simplest, although not exact method, is to 

 add the necessary salt in parts per thousand of 

 salinity. Thus: 



19.39 CI %o = 35.03 S 7oo 

 19.30 CI °/oo =34.87 S 7oo 



.16 grams/liter 



As there are 8 liters of sea water to be brought 

 up to approximately the standard chlorinity, 

 then 8X0.16=1.28 grams of sodium chloride 

 must be added to the 8 liters of water. The 

 resulting solution must be shaken vigorously for 

 at least 5 minutes after the last of the salt has 

 been dissolved in order to have a uniform con- 

 centration. Test and store it in the same man- 

 ner as described above. 



13-36 DETERMINATION OF DIS- 

 SOLVED OXYGEN CONTENT OF SEA 

 WATER. — The concentration of dissolved oxy- 



130 



H. O. 607 



