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Using a Volumetric Flask

Acquiring an exact Sample Volume


• To measure out a exact sample volume, pour the sample liquid to be measured into a volumetric flask up to a level, safely below the graduation mark..
  
  
• Bringing the flask and its contents up to 20^oC in a hot water bath.
  
  
• Using a Pasteur pipette and teat or a plastic eye dropper, bring the volume up to the the graduation mark so that the bottom of the meniscus lines up with the flask's graduation mark.
  
  
• Alternatively the volumetric flask can be overfilled and when the flask is at the correct temperature of 20^oC, the excess sample can be removed with a Pasteur pipette using suction, to the graduation mark.
  
  
Exact Dilutions or Solution preparations


• If using the volumetric flask for dilutions or preparing solutions -
  
  
  • If liquids are involved, add the liquid to be diluted first by using a pipette.
    
    
  • If solids are involved, accurately weigh out the solids and add them to the volumetric flask using a powder funnel.
    
    
  Then proceed as for obtaining an exact sample volume, as above, to add the diluting solution.
  
  
• To ensure thorough mixing, stopper the volumetric flask and invert the flask a number of times.
  
  
Note - volumetric flasks are used for accurate measurements


• Volumetric flasks are calibrated at 20^oC and therefore their contents should always be measured at 20^oC.
  
  
• When using a volumetric flask for dilution or solution preparation, avoid the temptation to save time by overfilling the volumetric flask before resorting to a pasteur pippete or ommitting using a pasteur pipette altogether, to bring the level up to the graduation mark.
  
  If the accuracy is not critical use a measuring cylinder to save time.
  
  
• When bringing the contents of a volumetric flask up to the graduation mark deliver the final quantity directly on to the surface of the liquid not the sides of the flask's wall above the graduation mark.
  
  
• Always mix the contents after achieving the correct temperature and volume.
  
  After mixing, the contents of the flask, the contents will all be of the same concentration, and residues on the sides of the flask or stopper will not alter the concentration.