silver nitrate bottle. The solution will flow 

 into th3 burette until a state of equilibrium 

 is reached. Turn the top stopcock to the 

 overflow position and allow the burette to 

 fill. After the burette has filled and some 

 silver nitrate has spilled into the overflow 

 flask, close the filling stopcock at the bottom. 

 Turn the leveling stopcock to the vent position. 

 The burette is now automatically zeroed and 

 is ready for running the titration. 



Inspect the burette carefully for presence 

 of any air bubbles. If air bubbles are present, 

 drain the burette by opening the delivery 

 stopcock and refill carefully until tliey are 

 removed. Before tlie first titrations are run 

 each day or after the solution has stood for 

 more than a short time in the burette exposed 

 to light, always drain completely and refill 

 the burette at least twice. 



To run the titration, place the beaker con- 

 taining the covered magnet and 15-ml. sample 

 with indicator solution on the magnetic stirrer 

 beneath the delivery tip of the burette. Turn 

 on the magnetic stirrer at a low speed that will 

 not cause splattering. Open the delivery stop- 

 cock wide until the silver nitrate solution 

 drains to the base of the burette bulb, and 

 then proceed with caution. Increase the speed 

 of the magnetic stirrer in order to maintain 

 uniform stirring as the volume of solution 

 increases in the beaker. This speed must be 

 great enough to prevent formation of curds in 

 the precipitate of silver chloride yet it must 

 never be great enough to cause 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. 



Once the silver-nitrate solution leaves the 

 bulb and starts down the graduated bore of the 

 burette, the chemist must not again look at 

 the level of the solution until the end point 

 has been reached and the reading is to be made. 

 He must pay strict attention to ths 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 

 lemon yellow to yellow and will show rapidly 

 increasing tints of tomato red. As the red 

 tint increases, reduce the speed of delivery to 

 a drop at a time until the entire sample be- 

 comes a peach color. At this point, called the 

 first rough color change, close the delivery 

 stopcock (the stirrer continuously running), 

 and observe the color for 20 seconds. The 

 color in the beaker should return to a pale 

 yellow. Now add, drop by drop, silver nitrate 



vmtil the liquid in the beaker again begins to 

 produce tints of tomato red with each drop. 

 This usually takes only a few drops. Now 

 reduce the addition of solution to half drops 

 by turning the stopcock very carefully and 

 then closing it as a tiny droplet appears. 

 Transfer the droplet to the sample with the 

 tip of the stirring rod. Keep repeating this 

 until the liquid turns to a solid peach or dirty 

 orange color throughout without being really 

 orange or even red. The liquid must retain 

 this color for 30 seconds of vigorous stirring. 

 This is the final color change or end point. 

 The solution when examined for transparency 

 is not opaque but rather clear. Without 

 losing any time, take the reading of the burette 

 scale. Shut off the stirrer (fig. 13-4). 



Figure 13- 



Titratiiig the sample with the Knudsen 

 burette. 



While reading the burette, it is convenient to 

 hold a piece of stiff white cardboard directly 

 behind the burette. The bore of the burette 

 then appears to be divided into two parts 

 horizontally; one elark and the other light. 

 As a result, the meniscus can be easily seen 

 and quickly read. The true meniscus is that 

 line of the concave surface of the solution when 

 the observer's eye is level with the top surface 

 of the solution. The dark line is found slightly 

 lower than the uppermost line of solution. 

 Read the graduation of the bottom part of this 

 dark arc. Make the reading immediately 

 and repeat it aloud, then record it on the log 

 sheet described in chapter 14. 



Remove the magnet from the beaker with a 

 magnetized pickup rod and pour the titrated 



H. O. 607 



125 



