RUBBER TUBING 



SALT WATER 



Figure IS. Automatic-zeroing pipette. 



LEVELING STOPCOCK 



PRESSURE BULB 



Figure 1-4. Knudsen burette. 



it in the overflow flask. Attach the magnetic 

 stirrer to the support rod below the burette so 

 it will be centered under the delivery tip. Place 

 the silver nitrate solution bottle in a sea rack 

 or on a shelf that is high enough to permit 

 gravity flow to fill the burette, if possible; 

 otherwise, connect the pressure bulb. 



It is recommended that (1) a douglinut- 



shaped piece of half-inch-thick foam rubber 

 be cemented to the top of the stirrer to prevent 

 the 100-ml. beaker, used for titration, from 

 sliding, (2) the table be provided with a sea 

 rack to hold the bottle of standard sea water, 

 the dropping bottle of indicator solution, and 

 two or three 100-ml. beakers, (3) the back- 

 ground behind the burette be painted flat white 

 to aid in reading the burette and in interpreting 

 the color of the titration end point, (4) a fluo- 

 rescent titration lamp be rigged behind the 

 magnetic stirrer at a height to pro\nde proper 

 light to the sample." 



1-9 Preparing the Indicator Solution-— 



Phenosafranin is used as the indicator in 

 determining the end point of the titration. The 

 solution is prepared in the following manner: 



Step 1. Place 700 ml. of distilled water in 

 a 1 -liter beaker and add 1 g. of sodium ben- 

 zoate U.S.P. powder; bring solution to a vigor- 

 ous boil. 



Step 2. In a separate beaker, make a starch 

 suspension by mixing 40 g. of a soluble starch 

 in 75 ml. of distilled water. 



Step 3. While stirring, slowly add the cold 

 starch suspension to the boiling sodium ben- 

 zoate solution, and continue to boil for 2 to 5 

 minutes after addition of starch suspension; 

 then, remove from heat and filter while hot 

 through borosilicate glass wool. 



Step 4. To 50 ml. of cold distilled water, add 

 2.5 g. of phenosafranin dye concentrate. Stir 

 thoroughly then slowly add the dye suspension 

 to the not starch solution. Mix well, and when 

 cooled to room temperature dilute to 1,000 ml. 

 volume with distilled water. (1 ml. is required 

 per determination.) 



I-IO Preparing the Silver Nitrate Solu- 

 tion. — The silver nitrate solution is prepared 

 by dissolving 37.11 grams of silver nitrate in 1 

 liter of distilled water. The best results, how- 

 ever, are obtained by making up solutions in 7 

 liter amounts because the smaller the amount 

 prepared the greater the chance of error. 



Step 1. To mix 7 liters, fii-st open a silver 

 nitrate jar of 259.8 g. and carefully pour all the 

 crystals into a clean 1,000-ml. beaker. Place a 

 teflon coated magnetic stirring bar in the beaker. 

 Fill a 2-liter volumetric flask to the mark with 

 distilled water. Pour off a small amount of the 

 water into the jar; empty the jar into the 

 beaker; repeat three or four times. This will in- 

 sure that all silver nitrate is removed from the 

 jar. Next, pour enough water from the flask 

 into the beaker to fill it three-fourths full. Set 

 the beaker on a magnetic stirrer and stir until 

 the silver nitrate crystals are dissolved. Silver 

 nitrate is a corrosive chetnical and should he 

 handled toith care. 



Step 2. Check the solution. If it is cloudy or 

 if it turns whitish, it is contaminated and must 



1-4 



