rinsing by the suction applied. Refill the 

 pipette and, by turning the stopcock to vent 

 position, drain the sample water into a beaker. 

 Discard this water into the waste carboy. 

 Draw a 15-ml. amount from the sample bottle, 

 turn stopcock to off position and wipe the tube 

 of the pipette with a paper wiper after with- 

 drawing the bottle (fig. 13-3). 



This will be the first sample to be titrated. 

 Drain the water into a clean 100-ml. beaker, 

 containing a plastic covered magnet. Im- 

 mediately after the pipette stops draining, there 

 will be about three-fourths of an inch of water 

 remaining in the delivery tip. A few seconds 

 after the last drop has fallen, bring the beaker 

 up to the pipette tip and barely touch the tip 

 to the surface of the liquid. Do not, under 

 any circumstance, try to force more liquid from 

 the pipette for as long as this procedure is 

 followed each and every time, the results will 

 be consistent. 



The beaker should be washed thoroughly 

 before the first titration and dried with a clean 

 cloth. Once the titrations are started, how- 

 ever, it is not necessary to reclean the beakers 

 after every run. The silver chloride remaining 

 stuck to the sides of the beaker alter a titration 

 is chemically neutral. If one overruns the end 

 point the beaker must be recleaned. Never 

 use a beaker in which the sample has exceeded 

 the end point without either (a) cleaning the 

 beaker or (6) adding a little sea water to the 

 sample and titrating again to the end point 

 before discarding both the sample and the 

 burette reading. 



It is important when drawing samples to 

 remember to always follow the same routine of 

 draw — wipe — drain — always rinse twice. 



Add exactly six drops of the potassium 

 chromate indicator solution to the liquid in 

 the beaker. 



13-29 Titrating the Sample With the Auto- 

 matic Burette. — Analysing the sample by ti- 

 tration, whether it is standard sea water or 

 an unknown sea water sample, is carried out 

 in precisely the same manner. Unless other- 

 wise directed, this technique should never vary. 

 Once the chemist has established his routine, 

 every analysis should be carried out in exactly 

 the same way. The overall time to run 

 titrations of water of nearly the same chlorinity 

 should be approximately the same. Good 

 results will not be obtained if 2 analyses of the 

 same sea water sample take 1 and 2 minutes 

 respectively to run. The titration time for 

 each should be within a few seconds of one 

 another. 



124 



r 



r 



V, 



Figure IS-S. Drawing the salinity sample with the 

 pipette. 



To fill the burette with silver nitrate solu- 

 tion, first close the delivery stopcock, then 

 turn the leveling stopcock at the top of the 

 burette to the closed position. Now open the 

 filling stopcock at the bottom of the burette 

 and press the rubber bulb connfcted to the 



H. O. 607 



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