44 



MEASUREMENTS 



receiving container and remove the pipette. Never blow out the last 

 "drop" because this last "drop" may be a milliliter or more. 



The serological pipette (Fig. 4-4c) is graduated to the very tip, and 

 the last drop is to be blown out. Such pipettes are identified by a 

 ground or etched ring at the top. Serological pipettes and measuring 

 pipettes are superficially so similar that special care is necessary to avoid 

 mistakes. 



Biologists use pipettes more frequently than chemists do, probably 

 for several reasons. The quantities of materials transferred are more 

 likely to be about the right size for a pipette, and pipettes are faster than 

 burettes. Ordinarily, more precise control is achieved with a burette, 

 but the variation in biological materials is usually much greater than 

 any error introduced by pipetting. Finally, the chemist is much more 

 likely to handle acids, caustic solutions, or strong poisons, and a pipette 

 always involves the danger of drawing up a mouthful of the liquid. 



Fig. 4-5. Several volumetric flasks. (Courtesy Corning Glass Works.) 



Volumetric flasks (Fig. 4-5) are used primarily for making solutions 

 or for diluting more concentrated solutions. A convenient method for 

 dissolving many materials is to place the weighed quantity of solute in 

 the flask and then to add distilled water to about K' or % of the capacity. 



