VOLUMETRIC GLASSWARE

 This experiment makes use of several types of volumetric glassware, specialized pieces of glassware which are used to measure volumes of liquids very precisely in quantitative laboratory work.  Examples of these types of glassware include graduated cylinders, burettes, pipets, and volumetric flasks.  These are carefully manufactured and expensive items and should be treated with great care.  Ordinary beakers and flasks are not used for accurate volume measurements; the markings on these pieces of glassware are generally only accurate to within 5%.

 Each piece of volumetric glassware is marked with its total volume, the notation TD or TC, and a temperature (usually 20°C).  The marked temperature indicates the temperature at which the apparatus was calibrated.  Since density and volume change with temperature, the volume markings are strictly correct only at the calibration temperature.  The notations TD and TC stand for the phrases "to deliver" and "to contain."  The TD notation, used on burettes and pipets (and some graduated cylinders), means that the apparatus is calibrated to accurately deliver or transfer the stated volume to another container.  The TC notation, used on volumetric flasks and (most) graduated cylinders, means that the markings give an accurate measure of the volume contained, but that pouring the liquid into another container will not necessarily deliver the indicated volume.

 Most solutions form a curved surface when placed in a glass container.  This surface is called a meniscus and, in the case of aqueous solutions, is concave when viewed from above.  To correctly read the level of liquid in a piece of glassware, it is important that your eye be on the same level as the surface of the liquid in order to avoid parallax errors.  The level of the liquid is then read at the bottom of the meniscus (52.7 mL in the pictured example).

The following table gives the allowed accuracy tolerances for a variety of volumetric glassware.


Table I.  Tolerances for Volumetric Ware

                                           Maximum Allowed Error, mL

Volume, mL      Volumetric Flask        Volumetric Pipet           Burette
     5                             -                               0.01                        -
   10                             -                               0.02                        -
   25                          0.03                            0.03                      0.03
   50                          0.05                            0.05                      0.05
  100                         0.08                            0.08                      0.10
  250                         0.10                              -                            -
  500                         0.15                              -                            -
 1000                        0.30                              -                            -

Source:  J. S. Fritz and G. H. Schenk, Quantitative Analytical Chemistry, 3rd ed., Allyn & Bacon, Boston, 1974, p. 560


THE PIPET

Pipets are used to measure and dispense liquids accurately (more so than a graduated cylinder).  The Mohr pipet is graduated so as to allow the dispensing of varied volumes.  The volumetric transfer pipet is made to dispense one fixed volume with high accuracy.  The use of the volumetric pipet is described HERE.

THE VOLUMETRIC FLASK

The volumetric flask is calibrated to contain a fixed amount of solution with high accuracy.  The flask is used in two major ways.  In one technique, a sample of known mass (the solute) is placed in the flask and dissolved.  The solution is then made up to the mark on the flask by adding solvent, giving a solution of precisely known volume containing a precisely known amount of solute.  If the solute is pure ("primary standard" grade) or its percent purity is known, it is then possible to calculate the molarity of the solution (moles of solute per liter of solution).  Click on the following link to see the Dartmouth ChemLab materials on volumetric flasks.  The second use of the volumetric flask is to dilute a solution in a precisely known manner.  This technique involves placing an aliquot (sample of precisely known volume) of a solution of known molarity in the flask, then diluting to the mark with solvent.  The new concentration may then be calculated using the volumes of the original aliquot and the new total volume (equation 1).

                               M1  x  V1  =  M2  x  V2                                                                              (1)

Note that the product of molarity and volume gives the number of moles of solute.  While the amount of solvent, and thus the total volume, changes, the amount of solute remains constant in a dilution process.

THE BURETTE AND TITRATION

You may review titration techniques by reading the Dartmouth ChemLab titration module; click here to view it.