3. ChemAparatus pt 2.

• Measuring volume


  The process of determining the amount of liquid or gas in a container
• Liter (L)


  The unit of volume, defined as one cubic decimeter
• Milliliter (mL)


  One one-thousandth of a liter (0.001 L), used when the liter represents an inconveniently large volume unit
• Microliter (µL)


  10-6 L or 10-3 mL
• Temperature increases


  Volume of liquid increases
• Volumetric measurements must be referred to a standard temperature, often 20°C
• The ambient temperature of most laboratories is usually close enough to 20°C so that there is no need for temperature corrections in volume measurements for aqueous solutions
• The coefficient of expansion for organic liquids may be large enough to require corrections for temperature differences of 1°C or less
• Calculating volume change due to temperature

  V20° = V5° + 0.00025(20 - 5)(40.00) = 40.00 + 0.15 = 40.15 mL
• Pipets


  Devices that permit the transfer of accurately known volumes from one container to another
• Types of pipets


  • Volumetric pipet
  • Measuring pipet
  • Mohr pipet
  • Serological pipet
  • Ostwald-Folin pipet
  • Lambda pipet
  • Eppendorf pipet

  
• Pipets


  • Deliver a single, fixed volume between 0.5 and 200 mL
  • Calibrated in convenient units to permit delivery of any volume up to a maximum capacity ranging from 0.1 to 25 mL
• Burets


  Devices that make it possible to deliver any volume up to the maximum capacity of the device, with greater precision than pipets
• Burets


  • Consist of a calibrated tube to hold titrant plus a valve arrangement to control the flow of titrant
  • The valve is the principal source of differences in precision between burets
• Volumetric flasks


  Manufactured with capacities ranging from 5 mL to 5 L, calibrated to contain (TC) a specified volume when filled to a line etched on the neck
• Volumetric flasks


  • Used for the preparation of standard solutions and for the dilution of samples to a fixed volume prior to taking aliquots with a pipet
  • Some are also calibrated on a to-deliver (TD) basis, with two reference lines on the neck

  
• All volumetric and measuring pipets are first filled to a calibration mark

  
• The manner in which the transfer is completed depends on the particular type of pipet
• Handheld Eppendorf micropipets can deliver adjustable microliter volumes of liquid

  
• Tolerances for Class A volume measuring apparatuses

  • Pipets
  • Burets
  • Volumetric flasks

  
• Bead


  When tubing is deformed, liquid flows past the bead
• Glass stopcock


  Relies on a lubricant between the ground-glass surfaces of stopcock and barrel for a liquid-tight seal
• Glass stopcocks can freeze when in contact with some solutions, notably bases, for long periods
• Glass stopcocks must be thoroughly cleaned after each use
• Teflon valves


  Unaffected by most common reagents and require no lubricant
• Volumetric flasks


  • Manufactured with capacities ranging from 5 mL to 5 L
  • Calibrated to contain (TC) a specified volume when filled to a line etched on the neck
  • Some are also calibrated on a to-deliver (TD) basis, with two reference lines on the neck

  
• Tolerances for Class A Burets

  • Volume, mL
  • Tolerances, mL
  • 5
  • ±0.01
  • 10
  • ±0.02
  • 25
  • ±0.03
  • 50
  • ±0.05
  • 100
  • ±0.20
• Tolerances for Class A Volumetric Flasks

  • Capacity, mL
  • Tolerances, mL
  • 5
  • ±0.02
  • 10
  • ±0.02
  • 25
  • ±0.03
  • 50
  • ±0.05
  • 100
  • ±0.08
  • 250
  • ±0.12
  • 500
  • ±0.20
  • 1000
  • ±0.30
  • 2000
  • ±0.50
• Cleaning volumetric equipment


  1. Brief soaking in warm detergent solution

  2. Thorough rinsing with tap water and then distilled water

  3. Drying is seldom necessary
• Meniscus


  The top surface of a liquid confined in a narrow tube exhibits a marked curvature
• It is common practice to use the bottom of the meniscus as the point of reference in calibrating and using volumetric equipment
• Parallax


  A condition that causes the volume to appear smaller than its actual value if the meniscus is viewed from above, and larger if viewed from below
• Measuring an aliquot


  1. Draw a small volume into the pipet and thoroughly wet the interior surface, repeat 2-3 times

  2. Fill the pipet to a level above the graduation mark, ensuring no bubbles or foam

  3. Touch the tip to the wall of a glass vessel and slowly allow the liquid level to drop until it coincides with the graduation mark

  4. Remove the pipet, tilt it to draw liquid slightly up, and wipe the tip

  5. Place the pipet tip in the receiving vessel and allow the liquid to drain, resting the tip against the inner wall for 10 seconds

  
• Cleaning a buret


  1. Thoroughly clean the tube with detergent and a brush

  2. Rinse thoroughly with tap water and then distilled water, inspect for water breaks

  3. Repeat cleaning if necessary
• Lubricating a glass stopcock

  1. Carefully remove old grease, dry both parts completely

  2. Lightly grease the stopcock, avoiding the area adjacent to the hole

  3. Insert the stopcock into the barrel and rotate vigorously with slight inward pressure

  4. The proper amount of lubricant is used when the contact area appears nearly transparent, the seal is liquid-tight, and no grease has worked into the tip
• Silicone lubricants are not recommended for stopcocks as contamination is difficult to remove
• Fouling of a buret tip with stopcock grease is not a serious matter, and can be removed with organic solvents
• Before returning a buret to service, it should be tested for leakage by filling with water and checking the volume reading does not change
• Filling a buret


  1. Make certain the stopcock is closed

  2. Add 5-10 mL of the titrant, rotate the buret to wet the interior, and allow to drain, repeat 2-3 times

  3. Then fill the buret
• Silicone lubricants are not recommended because contamination by such preparations is difficult—if not impossible—to remove