262 TITRIMETRIC METHODS 



connected to a rubber tube that acts as a mercury reservoir. One end 

 of the rubber tube is plugged to retain the mercury, and a screw 

 clamp is used to force the mercury out of, or to draw it into, the 

 reservoir. An air space is left between the mercury and the titration 

 liquid. While lacking some of the refinements of other models, it 

 can be made into a serviceable instrument. 



Scholander Burette. A diagram of this instrument is shown in 

 Figure 89; an all-steel micrometer, with its anvil removed, is used. 

 A burette (.4) for titration, or a burette (B) for calibration of 

 pipettes and syringes may be fitted by a ground-glass joint to the 

 chamber containing the micrometer spindle. The volume of the 

 bulbs in the burettes should approximate the volume that the spindle 

 can displace. A medium-heavy grease is applied to the micrometer 

 spindle, and the spindle chamber is fixed in position by means of a 

 set screw (3) which presses against the steel disc (1) having a 

 recessed punch mark; a lightly greased paper or fiber gasket (2) 

 seals the open ground end of the chamber held against the spindle 

 bearing. The gasket should fit the spindle tightly. With the spindle 

 retracted until flush with the bearing face, the chamber is filled with 

 mercury through the open ground socket, taking care to remove all 

 air bubbles. The air bubbles adhering to the walls can be removed 

 by touching the bubbles with the end of a fine steel wire and leading 

 them out. Bubbles at the ground socket are avoided by placing 

 several drops of water or titration liquid over the mercury in the 

 socket before inserting the upper part of the burette. Extra mercury 

 can be drawn in through the tip, if necessary. The spindle chamber 

 should be made as small as required to just clear the spindle. By 

 keeping the chamber volume small, and with proper handling of the 

 instrument, temperature errors can be reduced to the point where a 

 water j acket is not necessary. 



The micrometer employed has a total spindle excursion of 25 mm. 

 marked off in 2500 scale divisions. Estimations are made to one fifth 

 of a division. Calibration may be carried out by weighing delivered 

 quantities of water, and relating their volumes to the number of 

 scale divisions required to deliver them. The total capacity of the 

 burette can be delivered with an accuracy of 1 part in 6000 to 7000. 

 With the ordinary spindle, volumes can be measured with an over-all 

 accuracy of about 0.1 /xl. By replacing the spindle with a Vie in. drill 

 rod, a refined burette can be constructed capable of measuring 



