Step 5. Drain the water sample in the pipette 

 into a clean 100-ml. beaker, containing a teflon 

 covered magnetic stirring bar. When the pipette 

 is drained, water will remain in the tip of the 

 pipette. After the last drop has fallen, bring the 

 beaker up to the pipette tip and barely touch the 

 tip to a wet portion of the beaker near the sur- 

 face of the liquid. 



Follow the same technique each and every 

 time so the results will be consistent. 



It is important when drawing samples to re- 

 member to always follow the same routine of 

 "always rinse twice — draw — wipe — drain." 



(2) Titrating with the Knudsen Automatic 

 Burette. — Titrating the sample, whether it is 

 standard sea water or an unknown sea water 

 sample, is carried out in precisely the same man- 

 ner. Once the chemist has established his 

 routine, every analysis should be carried out in 

 exactly the same way. The titration time for 

 each of the samples should be approximately 

 the same. 



Step 1. Fill the burette with silver nitrate 

 solution. To do this, close the delivery stopcock, 

 turn the leveling stopcock to the overflow posi- 

 tion, and open the filling stopcock at the bottom 

 of the burette. If the solution does not fill the 

 burette until it overflows, press the rubber pres- 

 sure bulb connected to the silver nitrate bottle. 



Step 2. After the burette has filled and some 

 silver nitrate has entered the overflow flask, 

 close the filling stopcock ; turn the leveling stop- 

 cock to the vent position. The burette is now 

 automatically zeroed and is ready for titration. 

 Inspect the burette carefully for the presence 

 of air bubbles. If air bubbles are present, drain 

 the burette by opening the delivery stopcock 

 and refill. Before the first titrations are run each 

 day or after the solution has been in the burette 

 for an hour or more, always drain completely 

 and refill the burette at least twice. Shake the 

 silver nitrate bottle occasionally to assure a uni- 

 form mixture. 



Step 3. Add 1 ml. of the phenosafranin in- 

 dicator solution to the sample in the beaker. 



Step 4. Place the sample beaker on the 

 magnetic stirrer beneatli the delivery tip of the 

 burette. Turn the magnetic stirrer to a low speed 

 that will not cause splattering. 



Step 5. Open the delivery stopcock wide until 

 the silver nitrate solution drains to the base of 

 the burette bulb (fig. 1-6) ; then, reduce flow 

 and proceed with caution. Increase the speed of 

 the magnetic stirrer to maintain uniform stir- 

 ring as the volume of solution increases in the 

 beaker. The stirring speed should be as fast as 

 possible without causing splattering of the 

 sample up the sides of the beaker. A speed that 

 is just short of that which will cause splattering 

 is considered best. 



Step 6. Once the silver nitrate solution leaves 

 the bulb and starts down the graduated bore of 



Figure 1-6. Titrating a salinity sample. 



the burette, concentrate on the color of the 

 sample and the rate of delivery. Watching the 

 burette scale during the period of titration is 

 considered poor practice. As the silver nitrate 

 solution is added, the liquid in the beaker will 

 change in color from red to pink to lavender. 



Step 7. As the lavender tint increases, reduce 

 the speed of delivery to a drop at a time, then a 

 half drop at a time, until there is no trace of 

 lavender remaining and the solution is a solid 

 blue. 



Step 8. At this point, close the delivery stop- 

 cock (the stirrer continuously running) , and ob- 

 serve the color for 20 seconds. If a lavender 

 color should return, add another half drop of 

 silver nitrate. This is the final color change or 

 end point. 



Step 9. Immediately read the burette scale. 

 If a standard is being run, read the value to the 

 nearest (.005) five-thousandth, and enter the 

 result in the StaTidardization column (A) of the 

 D-Sheet. If a sea water sample is being run, 

 read the burette to the nearest (.01) hmidredth 

 and enter in Burette Reading column. Shut off 

 the stirrer. "Wliile reading the burette, it is con- 

 venient to hold a piece of stiff white cardboard 

 directly behind the burette. As a result, the 

 meniscus can be seen easily and read quickly. 

 The true meniscus is the bottom line of the con- 

 cave surf ace of the solution wlien the observer's 

 eye is level with the surface of the solution (fig. 

 1-7). 



Step 10. Remove the magnet from the beaker 

 with a magnetized pickup rod and discard the 

 titrated sample. Once the titrations are started, 

 it is not necessary to reclean the beaker after 

 each titration unless the titration has run over 

 the end point. 



1-6 



