NITROGEN AND AJ^MONIA 285 



the pipette. The last of the Hqiiid usually follows the tip of the 

 pipette as it is withdrawn along the wall of the tube and can be 

 sucked up near the rim. Add the liquid in the pipette to B. 



5. Repeat the rinsing of the tube with the last third of drop A. 

 With care, the last of A can be removed from the paraffin without 

 drawing air through the pipette tip. Add the liquid to B. 



6. Draw up drop B completely into the pipette and transfer it 

 to the bottom of the paraffined distillation tube (page 169 and Fig. 

 35, page 167). It is essential that the pipette does not touch the 

 side walls of the tube. Accordingly, clamp the pipette vertically 

 and raise up the tube on a pipetting stand so that the centering 

 of the pipette stem will be proper. Deposit the liquid in the bottom 

 of the tube. It is usually permissible to ignore the film of liquid 

 adhering to the walls of the pipette after it has been emptied. 



7. Add 9 (A. 18 N sodium hydroxide directly to the liquid in 

 the bottom of the paraffined tube with the same type of pipette 

 used for the previous steps. Again it is essential to avoid contact 

 between the pipette and the upper walls of the tube. Use the pipette 

 stand for the operation as in the previous step. Immediately after 

 addition of the alkali place a seal of 45 fA. water across the neck 

 of the upper part of the vessel, using a constriction pipette of the 

 type shown in Figure 58, as described on page 174. 



8. Pipette 7 /A. 0.015 iV sulfuric acid into the water seal at 

 once using a constriction pipette (Fig. 57, page 175) which is 

 emptied by constant air pressure adjusted so that the pipette will 

 deliver when the tip touches the water. This arrangement insures 

 greater accuracy. 



9. Stopper the tube with a rubber cap plugged with a drawn-out 

 piece of glass tubing, rather than a piece of glass rod, to prevent 

 displacement of the seal (Fig. 46, page 171). 



10. Immerse the bottom half of the tube in a thermostat at 

 40° for 1.5 hr. 



11. Add 18 fA. alkaline indicator soln. to the receiving seal by 

 means of a constriction pipette which is emptied by constant air 

 pressure. 



12. Titrate with the 0.01 A^ hydrochloric acid to the color of 

 the color standard using a "flea" stirrer. About 1.5 lA. of the acid will 

 be required to bring the blank without absorbed ammonia to the 

 end point. 



